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Humidity Detector Based on Quartz Crystal Oscillator

2026-04-21T05:32:16Z

Xukuan: /* Conclusion and Outlook */

== 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 (<math>Na_2SiO_3</math>), 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 ==
=== Materials and Equipment ===
The core components of the humidity detector included a standard quartz crystal with a fundamental resonance frequency of 6 MHz and a 2N2222 BJT. The hygroscopic coating was prepared using a commercially available sodium silicate (<math>Na_2SiO_3</math>) solution (initial density: <math>1.39 g/cm^3</math>; initial concentration: 30%-50% wt) and deionized (DI) water. The experimental setup utilized a standard breadboard, various capacitors and resistors, a hot plate, a micropipette for precise volume deposition, and a sealed test tube for humidity environment control. An oscilloscope was used to capture the waveforms and estimate the oscillation frequency.

=== Circuit Design ===
A two-stage Colpitts oscillator circuit employing 2N2222 transistors was constructed for this experiment. The first stage serves as the primary oscillation core, maintaining fundamental mode oscillation via a 6 MHz quartz crystal and a capacitive feedback network (C1, C2 = 55 pF), with a 1 kΩ resistor (R3) placed between the Q1 emitter and ground for stabilization. To enhance output stability, a second-stage emitter follower was introduced as a buffer. Its input is coupled to the preceding stage via a 10 Ω resistor (R4) to suppress potential parasitic oscillations. The final output signal (OUT) is drawn from the emitter of Q2 through a series-connected 1 kΩ resistor (R5), which remains ungrounded. This specific configuration effectively isolates the core oscillator from the loading effects of external measurement instruments.
[[File:circuit_schematic.png|500px|thumb|center|Schematic diagram of the Colpitts crystal oscillator circuit]]

=== Preparation of the Hygroscopic Sensing Layer ===
To functionalize the quartz crystal for humidity sensing, a sodium silicate (water glass) thin film was deposited onto its surface.

1. '''Dilution:''' 20μL original sodium silicate solution was diluted with 10 mL DI water to achieve a volume ratio of 1:500.

2. '''Drop-casting:''' Using a micropipette, approximately 6 μL of the diluted solution was carefully drop-cast onto the center region of the quartz crystal. The gold electrodes at the periphery were strictly left uncovered to prevent any interference with electrical conduction.

3. '''Curing:''' The coated crystal was placed on a hot plate to heat gradually to a temperature range of 60–80 °C. It was maintained under these conditions for approximately one hour to fully evaporate the solvent. Then it was cooled down natuarally to room temperature, resulting in the formation of 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]]

===Experimental Setup and Measurement Procedure ===
*'''Humidity Sensing Testing:''' Due to the absence of a calibrated humidity chamber, qualitative humidity variations were simulated using a sealed test tube setup. The coated crystal was placed at the top of the test tube. To create environments with increasing moisture, DI water was sequentially added into the test tube in volumes of 10 mL, 20 mL, and 30 mL.

*'''Data Acquisition:''' Because the humidity-induced frequency shift is minute (typically on the order of tens of hertz) relative to the 6 MHz absolute baseline, a high-precision counting method was employed. The output signal from the buffer stage (OUT) was fed into a digital oscilloscope. The oscillation frequency was estimated by recording the waveform over a precise 1-second interval and counting the total number of oscillation cycles within this gate time. Before introducing any moisture, an initial baseline frequency of the dry coated crystal was recorded. Measurements were then taken for each incremental volume of DI water. To minimize random measurement errors and noise fluctuations, the frequency was monitored until the reading stabilized, and this steady-state value was recorded as the final data point for that specific humidity level.

== 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]]

=== 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. The observed frequency shift can be attributed to the mass-loading effect on the quartz crystal: as water is absorbed by the hygroscopic water-glass film, the effective mass on the crystal surface increases, resulting in a decrease in resonance frequency. This sensing mechanism is consistent with the Sauerbrey relation commonly used to describe quartz crystal microbalance behavior. Overall, the results show that a water-glass-coated quartz crystal oscillator is capable of responding to humidity changes. However, the performance of the system is strongly influenced by the coating condition, measurement temperature, and the very small magnitude of the frequency shift relative to the 6 MHz baseline.

One important factor is the thickness of the water-glass coating. In principle, a thicker sodium silicate film can absorb more water from the environment and therefore produce a larger mass-loading effect, leading to a greater frequency shift. However, this advantage is accompanied by a limitation: as the film becomes thicker, the additional mass and mechanical damping imposed on the quartz crystal also increase. If the coating is too thick, the oscillation amplitude may decrease and the oscillator may become more difficult to sustain. In the present work, the sensing layer was prepared by drop-casting diluted water glass onto the crystal surface. Although this method is simple and practical, it does not guarantee a perfectly uniform coating thickness. As a result, local nonuniformity of the film may contribute to measurement variation. Therefore, the effect of coating thickness can be understood as a trade-off: a film that is too thin may produce only a weak humidity response, while a film that is too thick may degrade oscillator performance. Optimizing the thickness and uniformity of the sensing layer is thus important for improving both sensitivity and reliability.

Temperature is another critical factor in this experiment. Under normal testing conditions, the oscillator response was generally stable, indicating that the circuit and coated crystal were capable of reliable operation for qualitative sensing. However, a clear difference was observed between cold-water and hot-water testing. When cold water was used, the frequency change remained small but stable. In contrast, when hot water at around 60 °C was used, the oscillation could no longer be measured reliably, and the waveform became nearly flat. This behavior suggests that the hot-water condition did not simply enhance the humidity response, but instead pushed the sensor beyond its stable operating range. A possible explanation is localized vapor condensation near the quartz crystal or surrounding circuit. Under such conditions, the sensor surface may no longer behave as a lightly mass-loaded rigid film, but rather as a liquid-coupled or highly viscoelastic layer. This would strongly increase damping of the quartz oscillation, reduce the resonance quality factor, and prevent reliable frequency detection. In addition, hot-water testing introduces coupled effects of humidity and temperature drift, making the result unsuitable for direct quantitative interpretation. Therefore, temperature should be regarded as a significant disturbance factor in the present setup, especially under extreme vapor conditions.

The stability of the oscillator was also found to depend on the circuit wiring layout. In particular, longer breadboard connections produced a less stable signal, while shortening the wire length improved the stability of the oscillation. This can be explained by the additional parasitic capacitance, inductance, and noise coupling introduced by long leads. Since crystal oscillators are highly sensitive to their surrounding load and layout, these parasitic effects can disturb the oscillation condition and reduce measurement reliability. This observation highlights the importance of compact circuit construction in high-frequency oscillator-based sensing systems.

A further limitation of the system is the very small magnitude of the sensing signal. The quartz crystal oscillates at approximately 6 MHz, whereas the humidity-induced frequency shift is only on the order of tens to around one hundred hertz. Although this shift is meaningful and consistent with the expected mass-loading effect, it represents only a very small fractional change compared with the carrier frequency. Consequently, the measurement is highly sensitive to instrumental resolution, baseline drift, temperature fluctuation, and waveform noise. This means that the difficulty lies not in the absence of a sensing response, but in the challenge of accurately detecting a very small change superimposed on a much larger base frequency. For this reason, a high-resolution frequency counter or a more stable digital frequency measurement system would be more appropriate than relying mainly on simple oscilloscope observation.

== Reference ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-20T01:17:19Z

Xukuan: /* Conclusion and Outlook */

== 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. The observed frequency shift can be attributed to the mass-loading effect on the quartz crystal: as water is absorbed by the hygroscopic water-glass film, the effective mass on the crystal surface increases, resulting in a decrease in resonance frequency. This sensing mechanism is consistent with the Sauerbrey relation commonly used to describe quartz crystal microbalance behavior. Overall, the results show that a water-glass-coated quartz crystal oscillator is capable of responding to humidity changes. However, the performance of the system is strongly influenced by the coating condition, measurement temperature, and the very small magnitude of the frequency shift relative to the 6 MHz baseline.

One important factor is the thickness of the water-glass coating. In principle, a thicker sodium silicate film can absorb more water from the environment and therefore produce a larger mass-loading effect, leading to a greater frequency shift. However, this advantage is accompanied by a limitation: as the film becomes thicker, the additional mass and mechanical damping imposed on the quartz crystal also increase. If the coating is too thick, the oscillation amplitude may decrease and the oscillator may become more difficult to sustain. In the present work, the sensing layer was prepared by drop-casting diluted water glass onto the crystal surface. Although this method is simple and practical, it does not guarantee a perfectly uniform coating thickness. As a result, local nonuniformity of the film may contribute to measurement variation. Therefore, the effect of coating thickness can be understood as a trade-off: a film that is too thin may produce only a weak humidity response, while a film that is too thick may degrade oscillator performance. Optimizing the thickness and uniformity of the sensing layer is thus important for improving both sensitivity and reliability.

Temperature is another critical factor in this experiment. Under normal testing conditions, the oscillator response was generally stable, indicating that the circuit and coated crystal were capable of reliable operation for qualitative sensing. However, a clear difference was observed between cold-water and hot-water testing. When cold water was used, the frequency change remained small but stable. In contrast, when hot water at around 60 °C was used, the oscillation could no longer be measured reliably, and the waveform became nearly flat. This behavior suggests that the hot-water condition did not simply enhance the humidity response, but instead pushed the sensor beyond its stable operating range. A possible explanation is localized vapor condensation near the quartz crystal or surrounding circuit. Under such conditions, the sensor surface may no longer behave as a lightly mass-loaded rigid film, but rather as a liquid-coupled or highly viscoelastic layer. This would strongly increase damping of the quartz oscillation, reduce the resonance quality factor, and prevent reliable frequency detection. In addition, hot-water testing introduces coupled effects of humidity and temperature drift, making the result unsuitable for direct quantitative interpretation. Therefore, temperature should be regarded as a significant disturbance factor in the present setup, especially under extreme vapor conditions.

A further limitation of the system is the very small magnitude of the sensing signal. The quartz crystal oscillates at approximately 6 MHz, whereas the humidity-induced frequency shift is only on the order of tens to around one hundred hertz. Although this shift is meaningful and consistent with the expected mass-loading effect, it represents only a very small fractional change compared with the carrier frequency. Consequently, the measurement is highly sensitive to instrumental resolution, baseline drift, temperature fluctuation, and waveform noise. This means that the difficulty lies not in the absence of a sensing response, but in the challenge of accurately detecting a very small change superimposed on a much larger base frequency. For this reason, a high-resolution frequency counter or a more stable digital frequency measurement system would be more appropriate than relying mainly on simple oscilloscope observation.

== Reference ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-20T01:17:04Z

Xukuan: /* Conclusion and Outlook */

== 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. The observed frequency shift can be attributed to the mass-loading effect on the quartz crystal: as water is absorbed by the hygroscopic water-glass film, the effective mass on the crystal surface increases, resulting in a decrease in resonance frequency. This sensing mechanism is consistent with the Sauerbrey relation commonly used to describe quartz crystal microbalance behavior. Overall, the results show that a water-glass-coated quartz crystal oscillator is capable of responding to humidity changes. However, the performance of the system is strongly influenced by the coating condition, measurement temperature, and the very small magnitude of the frequency shift relative to the 6 MHz baseline.\\
One important factor is the thickness of the water-glass coating. In principle, a thicker sodium silicate film can absorb more water from the environment and therefore produce a larger mass-loading effect, leading to a greater frequency shift. However, this advantage is accompanied by a limitation: as the film becomes thicker, the additional mass and mechanical damping imposed on the quartz crystal also increase. If the coating is too thick, the oscillation amplitude may decrease and the oscillator may become more difficult to sustain. In the present work, the sensing layer was prepared by drop-casting diluted water glass onto the crystal surface. Although this method is simple and practical, it does not guarantee a perfectly uniform coating thickness. As a result, local nonuniformity of the film may contribute to measurement variation. Therefore, the effect of coating thickness can be understood as a trade-off: a film that is too thin may produce only a weak humidity response, while a film that is too thick may degrade oscillator performance. Optimizing the thickness and uniformity of the sensing layer is thus important for improving both sensitivity and reliability.\\
Temperature is another critical factor in this experiment. Under normal testing conditions, the oscillator response was generally stable, indicating that the circuit and coated crystal were capable of reliable operation for qualitative sensing. However, a clear difference was observed between cold-water and hot-water testing. When cold water was used, the frequency change remained small but stable. In contrast, when hot water at around 60 °C was used, the oscillation could no longer be measured reliably, and the waveform became nearly flat. This behavior suggests that the hot-water condition did not simply enhance the humidity response, but instead pushed the sensor beyond its stable operating range. A possible explanation is localized vapor condensation near the quartz crystal or surrounding circuit. Under such conditions, the sensor surface may no longer behave as a lightly mass-loaded rigid film, but rather as a liquid-coupled or highly viscoelastic layer. This would strongly increase damping of the quartz oscillation, reduce the resonance quality factor, and prevent reliable frequency detection. In addition, hot-water testing introduces coupled effects of humidity and temperature drift, making the result unsuitable for direct quantitative interpretation. Therefore, temperature should be regarded as a significant disturbance factor in the present setup, especially under extreme vapor conditions.\\
A further limitation of the system is the very small magnitude of the sensing signal. The quartz crystal oscillates at approximately 6 MHz, whereas the humidity-induced frequency shift is only on the order of tens to around one hundred hertz. Although this shift is meaningful and consistent with the expected mass-loading effect, it represents only a very small fractional change compared with the carrier frequency. Consequently, the measurement is highly sensitive to instrumental resolution, baseline drift, temperature fluctuation, and waveform noise. This means that the difficulty lies not in the absence of a sensing response, but in the challenge of accurately detecting a very small change superimposed on a much larger base frequency. For this reason, a high-resolution frequency counter or a more stable digital frequency measurement system would be more appropriate than relying mainly on simple oscilloscope observation.

== Reference ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-20T01:16:49Z

Xukuan: /* Conclusion and Outlook */

== 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. The observed frequency shift can be attributed to the mass-loading effect on the quartz crystal: as water is absorbed by the hygroscopic water-glass film, the effective mass on the crystal surface increases, resulting in a decrease in resonance frequency. This sensing mechanism is consistent with the Sauerbrey relation commonly used to describe quartz crystal microbalance behavior. Overall, the results show that a water-glass-coated quartz crystal oscillator is capable of responding to humidity changes. However, the performance of the system is strongly influenced by the coating condition, measurement temperature, and the very small magnitude of the frequency shift relative to the 6 MHz baseline.
One important factor is the thickness of the water-glass coating. In principle, a thicker sodium silicate film can absorb more water from the environment and therefore produce a larger mass-loading effect, leading to a greater frequency shift. However, this advantage is accompanied by a limitation: as the film becomes thicker, the additional mass and mechanical damping imposed on the quartz crystal also increase. If the coating is too thick, the oscillation amplitude may decrease and the oscillator may become more difficult to sustain. In the present work, the sensing layer was prepared by drop-casting diluted water glass onto the crystal surface. Although this method is simple and practical, it does not guarantee a perfectly uniform coating thickness. As a result, local nonuniformity of the film may contribute to measurement variation. Therefore, the effect of coating thickness can be understood as a trade-off: a film that is too thin may produce only a weak humidity response, while a film that is too thick may degrade oscillator performance. Optimizing the thickness and uniformity of the sensing layer is thus important for improving both sensitivity and reliability.
Temperature is another critical factor in this experiment. Under normal testing conditions, the oscillator response was generally stable, indicating that the circuit and coated crystal were capable of reliable operation for qualitative sensing. However, a clear difference was observed between cold-water and hot-water testing. When cold water was used, the frequency change remained small but stable. In contrast, when hot water at around 60 °C was used, the oscillation could no longer be measured reliably, and the waveform became nearly flat. This behavior suggests that the hot-water condition did not simply enhance the humidity response, but instead pushed the sensor beyond its stable operating range. A possible explanation is localized vapor condensation near the quartz crystal or surrounding circuit. Under such conditions, the sensor surface may no longer behave as a lightly mass-loaded rigid film, but rather as a liquid-coupled or highly viscoelastic layer. This would strongly increase damping of the quartz oscillation, reduce the resonance quality factor, and prevent reliable frequency detection. In addition, hot-water testing introduces coupled effects of humidity and temperature drift, making the result unsuitable for direct quantitative interpretation. Therefore, temperature should be regarded as a significant disturbance factor in the present setup, especially under extreme vapor conditions.
A further limitation of the system is the very small magnitude of the sensing signal. The quartz crystal oscillates at approximately 6 MHz, whereas the humidity-induced frequency shift is only on the order of tens to around one hundred hertz. Although this shift is meaningful and consistent with the expected mass-loading effect, it represents only a very small fractional change compared with the carrier frequency. Consequently, the measurement is highly sensitive to instrumental resolution, baseline drift, temperature fluctuation, and waveform noise. This means that the difficulty lies not in the absence of a sensing response, but in the challenge of accurately detecting a very small change superimposed on a much larger base frequency. For this reason, a high-resolution frequency counter or a more stable digital frequency measurement system would be more appropriate than relying mainly on simple oscilloscope observation.

== Reference ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-20T01:14:09Z

Xukuan: /* Conclusion and Outlook */

== 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.
The results demonstrate that a water-glass-coated quartz crystal oscillator can respond to humidity change, but the performance of the system is strongly influenced by coating condition, measurement temperature, and the very small magnitude of the frequency shift relative to the 6 MHz baseline.
One important factor is the thickness of the water-glass coating. In principle, a thicker sodium silicate film can absorb more water from the environment and therefore produce a larger mass-loading effect on the quartz crystal, which may increase the frequency shift. However, this benefit is accompanied by a major disadvantage: as the coating becomes thicker, the additional mass and mechanical damping imposed on the crystal also increase. If the film is too thick, the oscillation amplitude may decrease and the oscillator may become less stable or more difficult to sustain. In the present work, the sensing layer was prepared by drop-casting diluted water glass onto the crystal surface, which is a simple method but does not guarantee perfectly uniform thickness. As a result, local nonuniformity of the coating may also contribute to measurement variation and instability. Therefore, the effect of coating thickness is a trade-off: a film that is too thin may give only a weak humidity response, whereas a film that is too thick may degrade oscillator performance.
Temperature is another critical factor in this experiment. In the current setup, humidity was changed by using a column containing different volumes of water, and a clear difference was observed between cold-water and hot-water testing. When cold water was used, the frequency change was relatively small but stable. In contrast, when hot water around 60 °C was used, the oscillator became highly unstable, the frequency could no longer be reliably detected, and the output curve became nearly flat. The instability observed under the hot-water condition may be caused by localized vapor condensation near the quartz crystal or surrounding circuit. In this case, the sensor surface may no longer behave as a lightly mass-loaded rigid film, but rather as a liquid-coupled or highly viscoelastic layer. Such loading strongly increases damping of the quartz oscillation, reduces the resonance quality factor, and may prevent reliable oscillation, causing the measured frequency to become undetectable and the output waveform to appear flat and unstable. In addition, hot-water testing may introduce coupled effects of humidity, temperature drift, making the result unsuitable for direct quantitative interpretation. Therefore, temperature should be regarded as a significant disturbance factor in this work rather than a negligible background effect.
A further limitation of the system is the small magnitude of the sensing signal. The quartz crystal oscillates at approximately 6 MHz, whereas the humidity-induced frequency change is only on the order of tens to around one hundred hertz. Although this shift is meaningful and consistent with the expected mass-loading effect, it is extremely small compared with the absolute carrier frequency. A high-resolution frequency counter or a more stable digital frequency measurement system would be more appropriate than relying mainly on simple oscilloscope observation.

== Reference ==

Humidity Detector Based on Quartz Crystal Oscillator

2026-04-20T01:06:20Z

Xukuan: /* Conclusion */

Main Page

2026-03-10T02:51:09Z

Xukuan: /* 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 kindly 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].

Main Page

2026-03-10T02:50:26Z

Xukuan: /* 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 kindly 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].

Main Page

2026-03-10T02:37:29Z

Xukuan: /* Projects */

'''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.

==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].