pc5271AY2526wiki - User contributions [en]

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T05:04:40Z

Xiangyi: /* Results and Discussion */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|500px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|500px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|500px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|500px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|500px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube then place the crystal on the top of it. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, The oscillate frequency of the origin crystal in lab environment is 5.981744 MHz, which is the same as when placed at the top of the sealed test tube with 10 mL of DI water. Every additional 10 mL led to an approximate frequency decrease of 25 Hz, when in 20mL, the frequency is 5.981719 MHz; when in 30mL, the frequency is 5.981694 MHz. This trend indicates that the system is sensitive to humidity-induced changes.
[[File:F-Vgraph.png|500px|thumb|center|Frequency-Volume Curve]]

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T05:04:20Z

Xiangyi: /* Sustainability Test */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|500px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube then place the crystal on the top of it. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, The oscillate frequency of the origin crystal in lab environment is 5.981744 MHz, which is the same as when placed at the top of the sealed test tube with 10 mL of DI water. Every additional 10 mL led to an approximate frequency decrease of 25 Hz, when in 20mL, the frequency is 5.981719 MHz; when in 30mL, the frequency is 5.981694 MHz. This trend indicates that the system is sensitive to humidity-induced changes.
[[File:F-Vgraph.png|500px|thumb|center|Frequency-Volume Curve]]

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T05:04:04Z

Xiangyi: /* Humidity Sensing Performance */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube then place the crystal on the top of it. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, The oscillate frequency of the origin crystal in lab environment is 5.981744 MHz, which is the same as when placed at the top of the sealed test tube with 10 mL of DI water. Every additional 10 mL led to an approximate frequency decrease of 25 Hz, when in 20mL, the frequency is 5.981719 MHz; when in 30mL, the frequency is 5.981694 MHz. This trend indicates that the system is sensitive to humidity-induced changes.
[[File:F-Vgraph.png|500px|thumb|center|Frequency-Volume Curve]]

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T05:03:57Z

Xiangyi: /* Humidity Sensing Performance */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube then place the crystal on the top of it. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, The oscillate frequency of the origin crystal in lab environment is 5.981744 MHz, which is the same as when placed at the top of the sealed test tube with 10 mL of DI water. Every additional 10 mL led to an approximate frequency decrease of 25 Hz, when in 20mL, the frequency is 5.981719 MHz; when in 30mL, the frequency is 5.981694 MHz. This trend indicates that the system is sensitive to humidity-induced changes.
[[File:F-Vgraph.png|700px|thumb|center|Frequency-Volume Curve]]

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

File:F-Vgraph.png

2026-04-19T05:02:23Z

Xiangyi: Xiangyi uploaded a new version of File:F-Vgraph.png

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T04:05:42Z

Xiangyi: /* Results and Discussion */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube then place the crystal on the top of it. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, The oscillate frequency of the origin crystal in lab environment is 5.981744 MHz, which is the same as when placed at the top of the sealed test tube with 10 mL of DI water. Every additional 10 mL led to an approximate frequency decrease of 25 Hz, when in 20mL, the frequency is 5.981719 MHz; when in 30mL, the frequency is 5.981694 MHz. This trend indicates that the system is sensitive to humidity-induced changes.
[[File:F-Vgraph.png|200px|thumb|center|Frequency-Volume Curve]]

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T04:04:55Z

Xiangyi: /* Results and Discussion */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube then place the crystal on the top of it. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, The oscillate frequency of the origin crystal in lab environment is 5.981744 MHz, which is the same as when placed at the top of the sealed test tube with 10 mL of DI water. Every additional 10 mL led to an approximate frequency decrease of 25 Hz, when in 20mL, the frequency is 5.981719 MHz; when in 30mL, the frequency is 5.981694 MHz. This trend indicates that the system is sensitive to humidity-induced changes.
[[File:F-Vcurve.png|200px|thumb|center|Outdoor environment testing]]

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

File:F-Vgraph.png

2026-04-19T04:04:05Z

Xiangyi:

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T04:03:36Z

Xiangyi: /* Humidity Sensing Performance */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube then place the crystal on the top of it. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, The oscillate frequency of the origin crystal in lab environment is 5.981744 MHz, which is the same as when placed at the top of the sealed test tube with 10 mL of DI water. Every additional 10 mL led to an approximate frequency decrease of 25 Hz, when in 20mL, the frequency is 5.981719 MHz; when in 30mL, the frequency is 5.981694 MHz. This trend indicates that the system is sensitive to humidity-induced changes.

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T03:50:46Z

Xiangyi: /* Coating */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Hygroscopic Film Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-19T03:50:13Z

Xiangyi: /* Coating */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution (density <math>1.39g/cm^3</math>) was first diluted to an appropriate concentration (origin solution : DI water = 1: 500). Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Main Page

2026-04-18T03:16:37Z

Xiangyi: /* Humidity Detector Based on Quartz Crystal Oscillator */

'''Welcome to the wiki page for the course PC5271: Physics of Sensors "(in AY25/26 Sem 2)!'''

This is the repository where projects are documented. You will need to create an account for editing/creating pages. If you need an account, please contact Christian.

'''Logistics''':
Our '''location is S11-02-04''', time slots for "classes" are '''Tue and Fri 10:00am-12:00noon'''.

'''Deadline for final entry to the wiki: 23 April 2026 23:59'''.

==Projects==

===[[Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control]]===

Group menber: Zhang yiteng, Li Xiaoyue, Peng Jianxi

This project aims to develop a low-cost, repeatable optical sensing system to quantify the quality of Carbon Quantum Dots (CQDs). We synthesize CQDs using a microwave-assisted method with citric acid and urea, and characterize their fluorescence properties using a custom-built setup comprising a UV LED excitation source and a fiber-optic spectrometer. By analyzing spectral metrics such as peak wavelength, intensity, and FWHM, we establish a robust quality control protocol for nanomaterial production.

===[[Inductive Sensors of Ultra-high Sensitivity Based on Nonlinear Exceptional Point]]===
Team members: Yuan Siyu; Zhu Ziyang; Wang Peikun; Li Xunyu

We are building two coupled oscillating circuits: one that naturally loses energy (lossy) and one that gains energy (active) using a specific amplifier that saturates at high amplitudes. When tuning these two circuits to a nonlinear Exceptional Point (NEP), the system becomes extremely sensitive to small perturbations in inductance, following a steep cubic-root response curve, while remaining resistant to noise.

'''CK:''' We likely have all the parts for this, but let us know the frequency so we can find the proper amplifier and circuit board.

'''SY:''' Thanks for your confirmation. The operating frequency is around 70-80 kHz.

'''CK:''' Have!

===[[EA Spectroscopy as a series of sensors: Investigating the Impact of Solvent Type on Mobility in Organic Diodes]]===
Team members: Li Jinhan; Liu Chenyang

We will use EA spectroscopy, which will include optical sensors, electrical sensors, and lock-in amplifiers, among other components as a highly sensitive, non-destructive optical sensing platform to measure the internal electric field modulation response of organic diodes under operating conditions, and to quantitatively extract carrier mobility based on this measurement. By systematically controlling the thin film preparation temperature and comparing the EA response characteristics of different samples, the project aims to reveal the influence of film preparation temperature on device mobility and its potential physical origins.

===[[Optical Sensor of Magnetic Dynamics: A Balanced-Detection MOKE Magnetometer]]===
Team members: LI Junxiang; Patricia Breanne Tan Sy

We will use a laser-based magneto-optical Kerr effect setup featuring a high-sensitivity differential photodiode array to measure the Kerr rotation angle induced by surface magnetism. This system serves as a versatile optical platform to investigate how external perturbations such as magnetic fields or radiation source alter the magnetic ordering of materials, allowing for the quantitative extraction of the magneto-optical coupling coefficients of various thin films.

===[[Precision Measurement of Material and Optical Properties Using Interferometry]]===
Team members: Yang SangUk; Zhang ShunYang; Xu Zifang

We will be constructing an interferometer and use it as a tool for precision measurement. One primary objective is determination of the refractive index of solution of different salt concentration by analyzing the resulting shift interference fringes.

===[[Precision Thermocouple Based Temperature Measurement System]]===
Team members: Sree Ranjani Krishnan; Nisha Ganesh ; Burra Srikari

We will design, build, and validate a precision thermocouple-based temperature measurement system using the Seebeck effect. The system will convert the extremely small thermoelectric voltage generated by a thermocouple into accurate, real-time temperature data. Since the output voltage is really small we will be using an instrumentation amplifier to amplify the output voltage and use an Arduino to digitalize the results.
Materials needed: K-type thermocouple/Thermophile;Arduino

===[[Surface EMG Sensor for Muscle Activity Measurement: AFE Design and Signal Processing]]===
Team members: Liu Chenxi; Wang Jingyi; Zhong Baoqi; Hong Jialuo; Zhang Lishang;

Electromyography (EMG) measures the electrical activity generated by skeletal muscles and is widely used in biomedical sensing, rehabilitation, and human–machine interfaces. The electrical signals produced by muscle fibers are typically in the microvolt to millivolt range and are easily corrupted by noise and motion artifacts, making proper signal conditioning essential. In this project, we design and implement an analog front-end (AFE) for surface EMG acquisition, including an instrumentation amplifier, band-pass filtering, and a 50 Hz notch filter to suppress power-line interference. The conditioned signal is then observed and recorded using an oscilloscope for further analysis of muscle activity in both the time and frequency domains.

===[[Humidity Detector Based on Quartz Crystal Oscillator]]===

Team members: Ma Xiangyi; Li Xukuan; Zhang Yixuan; Zhu Rongqi

This project aims to develop a humidity sensor based on a quartz crystal oscillator. The group will first construct the oscillator circuit on a breadboard. They will then fabricate the sensing device by depositing water-absorbing materials onto the quartz crystal. Humidity detection will be achieved by measuring the frequency change of the circuit caused by the mass variation on the crystal surface.

===[[Alcohol Sensor Based on Gas-Sensitive Resistive Materials]]===
Team members: Lyu Jiaxin; Yue Yucheng; Zhang ningxin; Zhong Yihui

This project aims to develop a low-cost alcohol sensing system based on gas-sensitive resistive materials. The presence of alcohol vapor induces changes in electrical resistance, which are measured and analyzed. The sensor is calibrated under different alcohol concentrations, and key parameters such as sensitivity and response time are evaluated to demonstrate reliable alcohol detection.

===[[Rotational Speed Measurement System Based on Hall-Effect Sensor]]===
Team members: Cui Yufan; Guo Chentong; He Qifang; Wang Xingyuan

This project aims to design, construct, and characterize a Hall-effect-based rotational speed measurement system. The system operates by monitoring the periodic magnetic field produced by magnets attached to a rotating disk. A Hall sensor converts this magnetic variation into an electrical signal, whose period is measured using an oscilloscope. From this measured period, the rotational speed can be calculated and used to assess the performance of the experimental setup.

==Resources==
===Books and links===
* A good textbook on the Physics of Sensors is Jacob Fraden: Handbook of Mondern Sensors, Springer, ISBN 978-3-319-19302-1 or [https://link.springer.com/book/10.1007/978-3-319-19303-8 doi:10.1007/978-3-319-19303-8]. There shoud be an e-book available through the NUS library at https://linc.nus.edu.sg/record=b3554643
* Another good textbook: John B.Bentley: Principles of Measurement Systems, 4th Edition, Pearson, ISBN: 0-13-043028-5 or https://linc.nus.edu.sg/record=b2458243 in our library.

===Software===
* Various Python extensions. [https://www.python.org Python] is a very powerful free programming language that runs on just about any computer platform. It is open source and completely free.
* [https://www.gnuplot.info Gnuplot]: A free and very mature data display tool that works on just about any platform used that produces excellent publication-grade eps and pdf figures. Can be also used in scripts. Open source and completely free.
* Matlab: Very common, good toolset also for formal mathematics, good graphics. Expensive. We may have a site license, but I am not sure how painful it is for us to get a license for this course. Ask if interested.
* Mathematica: More common among theroetical physicists, very good in formal maths, now with better numerics. Graphs are ok but can be a pain to make looking good. As with Matlab, we do have a campus license. Ask if interested.

===Apps===
Common mobile phones these days are equipped with an amazing toolchest of sensors. There are a few apps that allow you to access them directly, and turn your phone into a powerful sensor. Here some suggestions:

* Physics Toolbox sensor suite on [https://play.google.com/store/apps/details?id=com.chrystianvieyra.physicstoolboxsuite&hl=en_SG Google play store] or [https://apps.apple.com/us/app/physics-toolbox-sensor-suite/id1128914250 Apple App store].

===Data sheets===
A number of components might be useful for several groups. Some common data sheets are here:
* Photodiodes:
** Generic Silicon pin Photodiode type [[Media:Bpw34.pdf|BPW34]]
** Fast photodiodes (Silicon PIN, small area): [[Media:S5971_etc_kpin1025e.pdf|S5971/S5972/S5973]]
* Photogates:
** reflective, with mounting holes: [[Media:TTElectronics-OPB704WZ.pdf|OPB704.WZ]]
** transmissive, no mounting holes: [[Media:Vishay_TCST1103.pdf|TCST1103]]
* PT 100 Temperature sensors based on platinum wire: [[Media:PT100_TABLA_R_T.pdf|Calibration table]]
* Thermistor type [[Media:Thermistor B57861S.pdf|B57861S]] (R0=10kΩ, B=3988Kelvin). Search for [https://en.wikipedia.org/wiki/Steinhart-Hart_equation Steinhart-Hart equation]. See [[Thermistor]] page here as well.
* Humidity sensor
** Sensirion device the reference unit: [[media:Sensirion SHT30-DIS.pdf|SHT30/31]]
* Thermopile detectors:
** [[Media:Thermopile_G-TPCO-035 TS418-1N426.pdf|G-TPCO-035 / TS418-1N426]]: Thermopile detector with a built-in optical bandpass filter for light around 4μm wavelength for CO2 absorption
** [[Media:Thermopile_TS305_TCPO-033.pdf|TPCO-033 / TS305]]: Thermopile detector with wideband sensitivity 5um-25um
** [[Media:Thermopile_G-TPCO-019_TS105-10L5.5MM.pdf|G-TPCO-019 /TS105-10L5.5MM]]: Thermopile detector with wideband sensitivity 5um-25um and silicon lens (field of view: 10 degree)
* Resistor color codes are explained [https://en.wikipedia.org/wiki/Electronic_color_code here]

* Magnetic field sensors:
** Fluxgate magnetometer [[media:Data-sheet FLC-100.pdf|FCL100]]
** Hall switch 1 (SOT-23 casing): [[media:INFineon_TLE49681KXTSA1.pdf|TLE49681]]
** Hall switch 2 (TO-92 casing): [[media:DiodesInc_AH9246-P-8.pdf|AH9246]]
** Linear Hall sensor (to come)
* Lasers
** Red laser diode [[media:HL6501MG.pdf|HL6501MG]]
* Generic amplifiers
** Instrumentation amplifiers: [[media:Ad8221.pdf|AD8221]] or [[media:AD8226.pdf|AD8226]]
** Conventional operational amplifiers: Precision: [[media:OP27.pdf | OP27]], General purpose: [[media:OP07.pdf | OP07]]
** JFET op-amp, reasonably fast: [https://www.ti.com/document-viewer/tl071/datasheet TL071]
** Transimpedance amplifiers for photodetectors: [[media:AD8015.pdf | AD8015]]

==Some wiki reference materials==
* [https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:FAQ MediaWiki FAQ]
* [[Writing mathematical expressions]]
* [[Uploading images and files]]

==Old wikis==
You can find entries to the wiki from [https://pc5271.org/PC5271_AY2425S2 AY2024/25 Sem 2] and [https://pc5271.org/PC5271_AY2324S2 AY2023/24 Sem 2].

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:10:52Z

Xiangyi: /* Results and Discussion */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution was first diluted to an appropriate concentration. Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

== Conclusion ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:10:05Z

Xiangyi: /* Introduction */

== Introduction ==
=== Background and Theoretical Inspiration ===
Accurate humidity monitoring is a fundamental requirement in numerous industrial, meteorological, and scientific applications. While traditional humidity sensors largely rely on capacitive or resistive mechanisms, they often exhibit limitations regarding response time, hysteresis, and long-term stability. To address the demand for high-precision, real-time sensing, mass-sensitive acoustic wave devices have garnered significant attention.<ref> X. Ding, X. Chen, N. Li, et al., “A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor,” Sensors and Actuators B: Chemical, vol. 266, pp.534–542, 2018.</ref>

The theoretical foundation of our project is inspired by the Quartz Crystal Microbalance (QCM) principle. At its core, this principle relies on the mass-loading effect described by the Sauerbrey equation <ref> G. Sauerbrey, “Verwendung von Schwingquarzen zur W¨agung d¨unner Schichten und zur Mikrow¨agung,” Zeitschrift f¨ur Physik, vol. 155, no. 2, pp. 206–222, 1959.</ref>

<math>
\Delta f = - \frac{2 f_0^2}{A \sqrt{\rho_q \mu_q}} \Delta m
</math>

Where a change in the mass (<math>\Delta m</math>) attached to the surface of a piezoelectric quartz crystal results in a directly proportional, negative shift in its resonant frequency (<math>\Delta f</math>). While commercial QCM systems utilize highly specialized and often expensive internal circuitry, the fundamental physical phenomenon—that a crystal's oscillation frequency drops as it gets heavier—serves as the conceptual basis for our humidity detector.

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate ($Na_2SiO_3$), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.

=== Core Architecture: The Colpitts Crystal Oscillator ===
The central engineering challenge of this project lies in designing a robust electronic interface capable of driving the quartz crystal and precisely tracking its frequency changes. Rather than relying on commercial QCM equipment, this project focuses on the implementation of a custom-built Colpitts crystal oscillator <ref> A. Alassi, M. Benammar, and D. Brett, ``Quartz Crystal Microbalance Electronic Interfacing Systems: A Review,'' \textit{Sensors}, vol. 17, no. 12, 2799, 2017.</ref>.

The Colpitts topology is highly regarded for its exceptional frequency stability and low phase noise, making it an ideal choice for sensor applications where minute frequency deviations must be detected. Our circuit utilizes a bipolar junction transistor (specifically the 2N2222 BJT) as the active gain element to sustain oscillation. A 6 MHz quartz crystal is integrated into the feedback loop. In this configuration, the crystal acts as a highly selective inductive element that dictates the oscillation frequency.

A critical aspect of the circuit design is the careful selection of feedback capacitors and biasing resistors to ensure reliable startup and to suppress any high-frequency parasitic oscillations that the 2N2222 might otherwise introduce due to its wide bandwidth. By ensuring the circuit oscillates cleanly at the crystal's fundamental 6 MHz mode, the system can reliably translate the mass variations from the water glass coating into a stable, measurable output signal.

=== Project Objectives and Scope ===
The primary objective of this project is to construct and characterize a functional, low-cost humidity detector driven by a custom Colpitts oscillator. The specific scope of work includes:

1. Design and hardware implementation of a 6 MHz Colpitts crystal oscillator using a 2N2222 transistor, ensuring stable and parasitic-free operation.
2. Application of a uniform sodium silicate (water glass) sensing layer onto the quartz crystal.
3. Calibration of the sensor by exposing it to environments with varying relative humidity and recording the corresponding frequency shifts.
4. Analysis of the sensor's performance metrics, including sensitivity, linearity, and response time.

Through this approach, the project aims to demonstrate that a fundamental electronic oscillator circuit, combined with a simple hygroscopic coating, can effectively replicate the core functionality of complex mass-sensitive humidity detectors.

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution was first diluted to an appropriate concentration. Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:06:32Z

Xiangyi: /* Background and Theoretical Inspiration */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:05:17Z

Xiangyi: /* Introduction */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:03:43Z

Xiangyi: /* Introduction */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:03:20Z

Xiangyi: /* Background and Theoretical Inspiration */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:02:25Z

Xiangyi: /* Background and Theoretical Inspiration */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T03:01:11Z

Xiangyi: /* Introduction and background */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:56:55Z

Xiangyi: /* Results and Discussion */

== Introduction and background ==

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution was first diluted to an appropriate concentration. Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.

=== Conclusion and Outlook ===
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:56:10Z

Xiangyi: /* Results and Discussion */

== Introduction and background ==

== Methods ==

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution was first diluted to an appropriate concentration. Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:55:19Z

Xiangyi: /* Results and Discussion */

== Results and Discussion ==
=== Building Colpitts oscillator circuit ===
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:Circuit.jpeg|200px|thumb|center|Colpitts oscillator circuit on breadboard]]
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]

=== Coating ===
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution was first diluted to an appropriate concentration. Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
[[File:Solution-on-crystal.jpeg|200px|thumb|center|Dropped solution on the quartz crystal]]
[[File:Hot-plate.jpeg|200px|thumb|center|Heating on the hot plate]]

=== Sustainability Test ===
After film deposition, the stability of the oscillator was evaluated under different environmental conditions. Measurements were conducted in a laboratory environment (22 °C, ~40% relative humidity) and an outdoor environment (30 °C, ~75% relative humidity). In both cases, the oscillator was operated continuously for approximately one hour, during which stable oscillation with negligible frequency drift was observed, indicating good operational stability.
[[File:Out-of-room-testing.jpeg|200px|thumb|center|Outdoor environment testing]]

=== Humidity Sensing Performance ===
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles.
The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.
The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:53:48Z

Xiangyi: /* Results and Discussion */

File:Out-of-room-testing.jpeg

2026-04-18T02:52:49Z

Xiangyi:

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:51:48Z

Xiangyi: /* Results and Discussion */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:50:51Z

Xiangyi: /* Coating */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:50:02Z

Xiangyi: /* Building Colpitts oscillator circuit */

File:Circuit.jpeg

2026-04-18T02:49:09Z

Xiangyi:

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:48:46Z

Xiangyi: /* Results and Discussion */

File:Hot-plate.jpeg

2026-04-18T02:48:05Z

Xiangyi:

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:47:15Z

Xiangyi: /* Building Colpitts oscillator circuit */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:46:50Z

Xiangyi: /* Results and Discussion */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:46:38Z

Xiangyi: /* Results and Discussion */

File:Solution-on-crystal.jpeg

2026-04-18T02:46:06Z

Xiangyi:

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:44:50Z

Xiangyi: /* Results and Discussion */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:44:04Z

Xiangyi: /* Results and Discussion */

== Results and Discussion ==
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
[[File:5271-1.jpg|200px|thumb|center|Waveform Diagram without coating]]
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution was first diluted to an appropriate concentration. Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:43:05Z

Xiangyi: /* Results and Discussion */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:42:51Z

Xiangyi: /* Results and Discussion */

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-18T02:41:31Z

Xiangyi: Created page with "== Results and Discussion == A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty. File:5271-1.jpg"

File:5271-1.jpg

2026-04-18T02:40:47Z

Xiangyi: waveform diagram without coating

== Summary ==
waveform diagram without coating

Main Page

2026-04-17T13:56:27Z

Xiangyi: /* Humidity Detector Based on Quartz Crystal Oscillator */

'''Welcome to the wiki page for the course PC5271: Physics of Sensors "(in AY25/26 Sem 2)!'''

This is the repository where projects are documented. You will need to create an account for editing/creating pages. If you need an account, please contact Christian.

'''Logistics''':
Our '''location is S11-02-04''', time slots for "classes" are '''Tue and Fri 10:00am-12:00noon'''.

'''Deadline for final entry to the wiki: 23 April 2026 23:59'''.

==Projects==

===[[Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control]]===

Group menber: Zhang yiteng, Li Xiaoyue, Peng Jianxi

This project aims to develop a low-cost, repeatable optical sensing system to quantify the quality of Carbon Quantum Dots (CQDs). We synthesize CQDs using a microwave-assisted method with citric acid and urea, and characterize their fluorescence properties using a custom-built setup comprising a UV LED excitation source and a fiber-optic spectrometer. By analyzing spectral metrics such as peak wavelength, intensity, and FWHM, we establish a robust quality control protocol for nanomaterial production.

===[[Inductive Sensors of Ultra-high Sensitivity Based on Nonlinear Exceptional Point]]===
Team members: Yuan Siyu; Zhu Ziyang; Wang Peikun; Li Xunyu

We are building two coupled oscillating circuits: one that naturally loses energy (lossy) and one that gains energy (active) using a specific amplifier that saturates at high amplitudes. When tuning these two circuits to a nonlinear Exceptional Point (NEP), the system becomes extremely sensitive to small perturbations in inductance, following a steep cubic-root response curve, while remaining resistant to noise.

'''CK:''' We likely have all the parts for this, but let us know the frequency so we can find the proper amplifier and circuit board.

'''SY:''' Thanks for your confirmation. The operating frequency is around 70-80 kHz.

'''CK:''' Have!

===[[EA Spectroscopy as a series of sensors: Investigating the Impact of Solvent Type on Mobility in Organic Diodes]]===
Team members: Li Jinhan; Liu Chenyang

We will use EA spectroscopy, which will include optical sensors, electrical sensors, and lock-in amplifiers, among other components as a highly sensitive, non-destructive optical sensing platform to measure the internal electric field modulation response of organic diodes under operating conditions, and to quantitatively extract carrier mobility based on this measurement. By systematically controlling the thin film preparation temperature and comparing the EA response characteristics of different samples, the project aims to reveal the influence of film preparation temperature on device mobility and its potential physical origins.

===[[Optical Sensor of Magnetic Dynamics: A Balanced-Detection MOKE Magnetometer]]===
Team members: LI Junxiang; Patricia Breanne Tan Sy

We will use a laser-based magneto-optical Kerr effect setup featuring a high-sensitivity differential photodiode array to measure the Kerr rotation angle induced by surface magnetism. This system serves as a versatile optical platform to investigate how external perturbations such as magnetic fields or radiation source alter the magnetic ordering of materials, allowing for the quantitative extraction of the magneto-optical coupling coefficients of various thin films.

===[[Precision Measurement of Material and Optical Properties Using Interferometry]]===
Team members: Yang SangUk; Zhang ShunYang; Xu Zifang

We will be constructing an interferometer and use it as a tool for precision measurement. One primary objective is determination of the refractive index of solution of different salt concentration by analyzing the resulting shift interference fringes.

===[[Precision Thermocouple Based Temperature Measurement System]]===
Team members: Sree Ranjani Krishnan; Nisha Ganesh ; Burra Srikari

We will design, build, and validate a precision thermocouple-based temperature measurement system using the Seebeck effect. The system will convert the extremely small thermoelectric voltage generated by a thermocouple into accurate, real-time temperature data. Since the output voltage is really small we will be using an instrumentation amplifier to amplify the output voltage and use an Arduino to digitalize the results.
Materials needed: K-type thermocouple/Thermophile;Arduino

===[[Surface EMG Sensor for Muscle Activity Measurement: AFE Design and Signal Processing]]===
Team members: Liu Chenxi; Wang Jingyi; Zhong Baoqi; Hong Jialuo; Zhang Lishang;

Electromyography (EMG) measures the electrical activity generated by skeletal muscles and is widely used in biomedical sensing, rehabilitation, and human–machine interfaces. The electrical signals produced by muscle fibers are typically in the microvolt to millivolt range and are easily corrupted by noise and motion artifacts, making proper signal conditioning essential. In this project, we design and implement an analog front-end (AFE) for surface EMG acquisition, including an instrumentation amplifier, band-pass filtering, and a 50 Hz notch filter to suppress power-line interference. The conditioned signal is then observed and recorded using an oscilloscope for further analysis of muscle activity in both the time and frequency domains.

===[[Humidity Detector Based on Quartz Crystal Oscillator]]===

Group menber: Ma Xiangyi; Li Xukuan; Zhang Yixuan; Zhu Rongqi

This project aims to develop a humidity sensor based on a quartz crystal oscillator. The group will first construct the oscillator circuit on a breadboard. They will then fabricate the sensing device by depositing water-absorbing materials onto the quartz crystal. Humidity detection will be achieved by measuring the frequency change of the circuit caused by the mass variation on the crystal surface.

===[[Alcohol Sensor Based on Gas-Sensitive Resistive Materials]]===
Team members: Lyu Jiaxin; Yue Yucheng; Zhang ningxin; Zhong Yihui

This project aims to develop a low-cost alcohol sensing system based on gas-sensitive resistive materials. The presence of alcohol vapor induces changes in electrical resistance, which are measured and analyzed. The sensor is calibrated under different alcohol concentrations, and key parameters such as sensitivity and response time are evaluated to demonstrate reliable alcohol detection.

===[[Rotational Speed Measurement System Based on Hall-Effect Sensor]]===
Team members: Cui Yufan; Guo Chentong; He Qifang; Wang Xingyuan

This project aims to design, construct, and characterize a Hall-effect-based rotational speed measurement system. The system operates by monitoring the periodic magnetic field produced by magnets attached to a rotating disk. A Hall sensor converts this magnetic variation into an electrical signal, whose period is measured using an oscilloscope. From this measured period, the rotational speed can be calculated and used to assess the performance of the experimental setup.

==Resources==
===Books and links===
* A good textbook on the Physics of Sensors is Jacob Fraden: Handbook of Mondern Sensors, Springer, ISBN 978-3-319-19302-1 or [https://link.springer.com/book/10.1007/978-3-319-19303-8 doi:10.1007/978-3-319-19303-8]. There shoud be an e-book available through the NUS library at https://linc.nus.edu.sg/record=b3554643
* Another good textbook: John B.Bentley: Principles of Measurement Systems, 4th Edition, Pearson, ISBN: 0-13-043028-5 or https://linc.nus.edu.sg/record=b2458243 in our library.

===Software===
* Various Python extensions. [https://www.python.org Python] is a very powerful free programming language that runs on just about any computer platform. It is open source and completely free.
* [https://www.gnuplot.info Gnuplot]: A free and very mature data display tool that works on just about any platform used that produces excellent publication-grade eps and pdf figures. Can be also used in scripts. Open source and completely free.
* Matlab: Very common, good toolset also for formal mathematics, good graphics. Expensive. We may have a site license, but I am not sure how painful it is for us to get a license for this course. Ask if interested.
* Mathematica: More common among theroetical physicists, very good in formal maths, now with better numerics. Graphs are ok but can be a pain to make looking good. As with Matlab, we do have a campus license. Ask if interested.

===Apps===
Common mobile phones these days are equipped with an amazing toolchest of sensors. There are a few apps that allow you to access them directly, and turn your phone into a powerful sensor. Here some suggestions:

* Physics Toolbox sensor suite on [https://play.google.com/store/apps/details?id=com.chrystianvieyra.physicstoolboxsuite&hl=en_SG Google play store] or [https://apps.apple.com/us/app/physics-toolbox-sensor-suite/id1128914250 Apple App store].

===Data sheets===
A number of components might be useful for several groups. Some common data sheets are here:
* Photodiodes:
** Generic Silicon pin Photodiode type [[Media:Bpw34.pdf|BPW34]]
** Fast photodiodes (Silicon PIN, small area): [[Media:S5971_etc_kpin1025e.pdf|S5971/S5972/S5973]]
* Photogates:
** reflective, with mounting holes: [[Media:TTElectronics-OPB704WZ.pdf|OPB704.WZ]]
** transmissive, no mounting holes: [[Media:Vishay_TCST1103.pdf|TCST1103]]
* PT 100 Temperature sensors based on platinum wire: [[Media:PT100_TABLA_R_T.pdf|Calibration table]]
* Thermistor type [[Media:Thermistor B57861S.pdf|B57861S]] (R0=10kΩ, B=3988Kelvin). Search for [https://en.wikipedia.org/wiki/Steinhart-Hart_equation Steinhart-Hart equation]. See [[Thermistor]] page here as well.
* Humidity sensor
** Sensirion device the reference unit: [[media:Sensirion SHT30-DIS.pdf|SHT30/31]]
* Thermopile detectors:
** [[Media:Thermopile_G-TPCO-035 TS418-1N426.pdf|G-TPCO-035 / TS418-1N426]]: Thermopile detector with a built-in optical bandpass filter for light around 4μm wavelength for CO2 absorption
** [[Media:Thermopile_TS305_TCPO-033.pdf|TPCO-033 / TS305]]: Thermopile detector with wideband sensitivity 5um-25um
** [[Media:Thermopile_G-TPCO-019_TS105-10L5.5MM.pdf|G-TPCO-019 /TS105-10L5.5MM]]: Thermopile detector with wideband sensitivity 5um-25um and silicon lens (field of view: 10 degree)
* Resistor color codes are explained [https://en.wikipedia.org/wiki/Electronic_color_code here]

* Magnetic field sensors:
** Fluxgate magnetometer [[media:Data-sheet FLC-100.pdf|FCL100]]
** Hall switch 1 (SOT-23 casing): [[media:INFineon_TLE49681KXTSA1.pdf|TLE49681]]
** Hall switch 2 (TO-92 casing): [[media:DiodesInc_AH9246-P-8.pdf|AH9246]]
** Linear Hall sensor (to come)
* Lasers
** Red laser diode [[media:HL6501MG.pdf|HL6501MG]]
* Generic amplifiers
** Instrumentation amplifiers: [[media:Ad8221.pdf|AD8221]] or [[media:AD8226.pdf|AD8226]]
** Conventional operational amplifiers: Precision: [[media:OP27.pdf | OP27]], General purpose: [[media:OP07.pdf | OP07]]
** JFET op-amp, reasonably fast: [https://www.ti.com/document-viewer/tl071/datasheet TL071]
** Transimpedance amplifiers for photodetectors: [[media:AD8015.pdf | AD8015]]

==Some wiki reference materials==
* [https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:FAQ MediaWiki FAQ]
* [[Writing mathematical expressions]]
* [[Uploading images and files]]

==Old wikis==
You can find entries to the wiki from [https://pc5271.org/PC5271_AY2425S2 AY2024/25 Sem 2] and [https://pc5271.org/PC5271_AY2324S2 AY2023/24 Sem 2].

Main Page

2026-03-10T08:20:58Z

Xiangyi: /* Humidity detector based on quartz crystal oscillator */

'''Welcome to the wiki page for the course PC5271: Physics of Sensors "(in AY25/26 Sem 2)!'''

This is the repository where projects are documented. You will need to create an account for editing/creating pages. If you need an account, please contact Christian.

'''Logistics''':
Our '''location is S11-02-04''', time slots for "classes" are '''Tue and Fri 10:00am-12:00noon'''.

==Projects==

===[[Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control]]===

Group menber: Zhang yiteng, Li Xiaoyue, Peng Jianxi

This project aims to develop a low-cost, repeatable optical sensing system to quantify the quality of Carbon Quantum Dots (CQDs). We synthesize CQDs using a microwave-assisted method with citric acid and urea, and characterize their fluorescence properties using a custom-built setup comprising a UV LED excitation source and a fiber-optic spectrometer. By analyzing spectral metrics such as peak wavelength, intensity, and FWHM, we establish a robust quality control protocol for nanomaterial production.

===[[Inductive Sensors of Ultra-high Sensitivity Based on Nonlinear Exceptional Point]]===
Team members: Yuan Siyu; Zhu Ziyang; Wang Peikun; Li Xunyu

We are building two coupled oscillating circuits: one that naturally loses energy (lossy) and one that gains energy (active) using a specific amplifier that saturates at high amplitudes. When tuning these two circuits to a nonlinear Exceptional Point (NEP), the system becomes extremely sensitive to small perturbations in inductance, following a steep cubic-root response curve, while remaining resistant to noise.

'''CK:''' We likely have all the parts for this, but let us know the frequency so we can find the proper amplifier and circuit board.

'''SY:''' Thanks for your confirmation. The operating frequency is around 70-80 kHz.

'''CK:''' Have!

===[[EA Spectroscopy as a series of sensors: Investigating the Impact of Film-Processing Temperature on Mobility in Organic Diodes]]===
Team members: Li Jinhan; Liu Chenyang

We will use EA spectroscopy, which will include optical sensors, electrical sensors, and lock-in amplifiers, among other components as a highly sensitive, non-destructive optical sensing platform to measure the internal electric field modulation response of organic diodes under operating conditions, and to quantitatively extract carrier mobility based on this measurement. By systematically controlling the thin film preparation temperature and comparing the EA response characteristics of different samples, the project aims to reveal the influence of film preparation temperature on device mobility and its potential physical origins.

===[[Optical Sensor of Magnetic Dynamics: A Balanced-Detection MOKE Magnetometer]]===
Team members: LI Junxiang; Patricia Breanne Tan Sy

We will use a laser-based magneto-optical Kerr effect setup featuring a high-sensitivity differential photodiode array to measure the Kerr rotation angle induced by surface magnetism. This system serves as a versatile optical platform to investigate how external perturbations such as magnetic fields or radiation source alter the magnetic ordering of materials, allowing for the quantitative extraction of the magneto-optical coupling coefficients of various thin films.

===[[Precision Measurement of Material and Optical Properties Using Interferometry]]===
Team members: Yang SangUk; Zhang ShunYang; Xu Zifang

We will be constructing an interferometer and use it as a tool for precision measurement. One primary objective is determination of the refractive index of various gases by analyzing the resulting shift interference fringes.
===[[Precision Thermocouple Based Temperature Measurement System]]===
Team members: Sree Ranjani Krishnan; Nisha Ganesh ; Burra Srikari

We will design, build, and validate a precision thermocouple-based temperature measurement system using the Seebeck effect. The system will convert the extremely small thermoelectric voltage generated by a thermocouple into accurate, real-time temperature data. Since the output voltage is really small we will be using an instrumentation amplifier to amplify the output voltage and use an Arduino to digitalize the results.
Materials needed: K-type thermocouple/Thermophile;Arduino

===[[Surface EMG Sensor for Muscle Activity Measurement: AFE Design and Signal Processing]]===
Team members: Liu Chenxi; Wang Jingyi; Zhong Baoqi; Hong Jialuo; Zhang Lishang;

Electromyography (EMG) measures the electrical activity generated by skeletal muscles and is widely used in biomedical sensing, rehabilitation, and human–machine interfaces. The electrical signals produced by muscle fibers are typically in the microvolt to millivolt range and are easily corrupted by noise and motion artifacts, making proper signal conditioning essential. In this project, we design and implement an analog front-end (AFE) for surface EMG acquisition, including an instrumentation amplifier, band-pass filtering, and a 50 Hz notch filter to suppress power-line interference. The conditioned signal is then observed and recorded using an oscilloscope for further analysis of muscle activity in both the time and frequency domains.

===[[Humidity Detector Based on Quartz Crystal Oscillator]]===

Group menber: Ma Xiangyi; Li Xukuan; Zhang Yixuan; Zhu Rongqi

This project aims to develop a humidity sensor based on a quartz crystal oscillator. The group will first construct the oscillator circuit on a breadboard. They will then fabricate the sensing device by depositing water-absorbing materials onto the quartz crystal. Humidity detection will be achieved by measuring the frequency change of the circuit caused by the mass variation on the crystal surface.

'''To Prof.''' Several materials are available for use as the water-absorbing layer, such as PVA, polyimide, graphene oxide, and silica gel. We are currently unsure which material would be the most suitable for our device. Could you provide us with some suggestions?

==Resources==
===Books and links===
* A good textbook on the Physics of Sensors is Jacob Fraden: Handbook of Mondern Sensors, Springer, ISBN 978-3-319-19302-1 or [https://link.springer.com/book/10.1007/978-3-319-19303-8 doi:10.1007/978-3-319-19303-8]. There shoud be an e-book available through the NUS library at https://linc.nus.edu.sg/record=b3554643
* Another good textbook: John B.Bentley: Principles of Measurement Systems, 4th Edition, Pearson, ISBN: 0-13-043028-5 or https://linc.nus.edu.sg/record=b2458243 in our library.

===Software===
* Various Python extensions. [https://www.python.org Python] is a very powerful free programming language that runs on just about any computer platform. It is open source and completely free.
* [https://www.gnuplot.info Gnuplot]: A free and very mature data display tool that works on just about any platform used that produces excellent publication-grade eps and pdf figures. Can be also used in scripts. Open source and completely free.
* Matlab: Very common, good toolset also for formal mathematics, good graphics. Expensive. We may have a site license, but I am not sure how painful it is for us to get a license for this course. Ask if interested.
* Mathematica: More common among theroetical physicists, very good in formal maths, now with better numerics. Graphs are ok but can be a pain to make looking good. As with Matlab, we do have a campus license. Ask if interested.

===Apps===
Common mobile phones these days are equipped with an amazing toolchest of sensors. There are a few apps that allow you to access them directly, and turn your phone into a powerful sensor. Here some suggestions:

* Physics Toolbox sensor suite on [https://play.google.com/store/apps/details?id=com.chrystianvieyra.physicstoolboxsuite&hl=en_SG Google play store] or [https://apps.apple.com/us/app/physics-toolbox-sensor-suite/id1128914250 Apple App store].

===Data sheets===
A number of components might be useful for several groups. Some common data sheets are here:
* Photodiodes:
** Generic Silicon pin Photodiode type [[Media:Bpw34.pdf|BPW34]]
** Fast photodiodes (Silicon PIN, small area): [[Media:S5971_etc_kpin1025e.pdf|S5971/S5972/S5973]]
* Photogates:
** (to come)
* PT 100 Temperature sensors based on platinum wire: [[Media:PT100_TABLA_R_T.pdf|Calibration table]]
* Thermistor type [[Media:Thermistor B57861S.pdf|B57861S]] (R0=10kΩ, B=3988Kelvin). Search for [https://en.wikipedia.org/wiki/Steinhart-Hart_equation Steinhart-Hart equation]. See [[Thermistor]] page here as well.
* Humidity sensor
** Sensirion device the reference unit: [[media:Sensirion SHT30-DIS.pdf|SHT30/31]]
* Thermopile detectors:
** [[Media:Thermopile_G-TPCO-035 TS418-1N426.pdf|G-TPCO-035 / TS418-1N426]]: Thermopile detector with a built-in optical bandpass filter for light around 4μm wavelength for CO2 absorption
** [[Media:Thermopile_TS305_TCPO-033.pdf|TPCO-033 / TS305]]: Thermopile detector with wideband sensitivity 5um-25um
** [[Media:Thermopile_G-TPCO-019_TS105-10L5.5MM.pdf|G-TPCO-019 /TS105-10L5.5MM]]: Thermopile detector with wideband sensitivity 5um-25um and silicon lens (field of view: 10 degree)
* Resistor color codes are explained [https://en.wikipedia.org/wiki/Electronic_color_code here]

* Magnetic field sensors:
** Fluxgate magnetometer [[media:Data-sheet FLC-100.pdf|FCL100]]
** Hall switch 1 (SOT-23 casing): [[media:INFineon_TLE49681KXTSA1.pdf|TLE49681]]
** Hall switch 2 (TO-92 casing): [[media:DiodesInc_AH9246-P-8.pdf|AH9246]]
** Linear Hall sensor (to come)
* Lasers
** Red laser diode [[media:HL6501MG.pdf|HL6501MG]]
* Generic amplifiers
** Instrumentation amplifiers: [[media:Ad8221.pdf|AD8221]] or [[media:AD8226.pdf|AD8226]]
** Conventional operational amplifiers: Precision: [[media:OP27.pdf | OP27]], General purpose: [[media:OP07.pdf | OP07]]
** JFET op-amp, reasonably fast: [https://www.ti.com/document-viewer/tl071/datasheet TL071]
** Transimpedance amplifiers for photodetectors: [[media:AD8015.pdf | AD8015]]

==Some wiki reference materials==
* [https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:FAQ MediaWiki FAQ]
* [[Writing mathematical expressions]]
* [[Uploading images and files]]

==Old wikis==
You can find entries to the wiki from [https://pc5271.org/PC5271_AY2425S2 AY2024/25 Sem 2] and [https://pc5271.org/PC5271_AY2324S2 AY2023/24 Sem 2].