Humidity Detector Based on Quartz Crystal Oscillator - Revision history

Xukuan: /* Conclusion and Outlook */

2026-04-21T05:32:16Z

Conclusion and Outlook

← Older revision		Revision as of 13:32, 21 April 2026
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	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.		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.		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 ==		== Reference ==

119.234.89.127: /* Conclusion and Outlook */

2026-04-20T12:24:23Z

Conclusion and Outlook

← Older revision		Revision as of 20:24, 20 April 2026
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	[[File:F-Vgraph.png\|500px\|thumb\|center\|Frequency-Volume Curve]]		[[File:F-Vgraph.png\|500px\|thumb\|center\|Frequency-Volume Curve]]

	=== Conclusion and Outlook ===		== 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.		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.

Rongqi: /* Experimental Setup and Measurement Procedure */

2026-04-20T12:17:18Z

Experimental Setup and Measurement Procedure

← Older revision		Revision as of 20:17, 20 April 2026
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	*'''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.		*'''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 relative to the 6 MHz absolute baseline, a counting method was employed. The oscillation frequency was estimated by recording the waveform ~~on the oscilloscope~~ over a 1-second interval and counting the total number of oscillation cycles. Measurements were taken ~~and recorded~~ for each incremental volume of DI water.		*'''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 ==		== Results and Discussion ==

Rongqi: /* Hygroscopic Film Coating */

2026-04-20T12:10:13Z

Hygroscopic Film Coating

← Older revision		Revision as of 20:10, 20 April 2026
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	[[File:Circuit.jpeg\|500px\|thumb\|center\|Colpitts oscillator circuit on breadboard]]		[[File:Circuit.jpeg\|500px\|thumb\|center\|Colpitts oscillator circuit on breadboard]]
	[[File:5271-1.jpg\|500px\|thumb\|center\|Waveform Diagram without coating]]		[[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 ===		=== Sustainability Test ===

Rongqi: /* Circuit Design and Fabrication */

2026-04-20T11:59:31Z

Circuit Design and Fabrication

← Older revision		Revision as of 19:59, 20 April 2026
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	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.		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 ~~and Fabrication~~ ===		=== Circuit Design ===
	A Colpitts ~~crystal~~ oscillator circuit was constructed ~~on a breadboard to drive the quartz crystal and precisely monitor its frequency shifts, powered by a 5 V DC power supply~~. The ~~2N2222 BJT was integrated~~ as the ~~active gain element to sustain~~ oscillation~~. The~~ quartz crystal ~~was~~ placed in the ~~feedback loop~~, ~~acting~~ as a ~~highly selective inductive element~~. ~~Feedback capacitors and biasing resistors were strategically selected and tuned~~ to ~~ensure reliable circuit startup and~~ to suppress ~~any high-frequency~~ parasitic oscillations ~~typical~~ of the ~~wide-bandwidth 2N2222 transistor~~.		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]]		[[File:circuit_schematic.png\|500px\|thumb\|center\|Schematic diagram of the Colpitts crystal oscillator circuit]]

Rongqi: /* Preparation of the Hygroscopic Sensing Layer */

2026-04-20T11:49:54Z

Preparation of the Hygroscopic Sensing Layer

← Older revision		Revision as of 19:49, 20 April 2026
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	To functionalize the quartz crystal for humidity sensing, a sodium silicate (water glass) thin film was deposited onto its surface.		To functionalize the quartz crystal for humidity sensing, a sodium silicate (water glass) thin film was deposited onto its surface.

	1. '''Dilution:''' ~~The~~ original sodium silicate solution was diluted with DI water to achieve a volume ratio of 1:500 ~~(original solution to DI water)~~.		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.		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.

Rongqi: /* Circuit Design and Fabrication */

2026-04-20T11:47:18Z

Circuit Design and Fabrication

← Older revision		Revision as of 19:47, 20 April 2026
Line 38:		Line 38:

	=== Circuit Design and Fabrication ===		=== Circuit Design and Fabrication ===
	A Colpitts crystal oscillator circuit was constructed on a breadboard to drive the quartz crystal and precisely monitor its frequency shifts. The 2N2222 BJT was integrated as the active gain element to sustain oscillation. The quartz crystal was placed in the feedback loop, acting as a highly selective inductive element. Feedback capacitors and biasing resistors were strategically selected and tuned to ensure reliable circuit startup and to suppress any high-frequency parasitic oscillations typical of the wide-bandwidth 2N2222 transistor.		A Colpitts crystal oscillator circuit was constructed on a breadboard to drive the quartz crystal and precisely monitor its frequency shifts, powered by a 5 V DC power supply. The 2N2222 BJT was integrated as the active gain element to sustain oscillation. The quartz crystal was placed in the feedback loop, acting as a highly selective inductive element. Feedback capacitors and biasing resistors were strategically selected and tuned to ensure reliable circuit startup and to suppress any high-frequency parasitic oscillations typical of the wide-bandwidth 2N2222 transistor.
	[[File:circuit_schematic.png\|500px\|thumb\|center\|Schematic diagram of the Colpitts crystal oscillator circuit]]		[[File:circuit_schematic.png\|500px\|thumb\|center\|Schematic diagram of the Colpitts crystal oscillator circuit]]

Rongqi: /* Methods */

2026-04-20T11:43:05Z

Methods

← Older revision		Revision as of 19:43, 20 April 2026
Line 51:		Line 51:
	[[File:Solution-on-crystal.jpeg\|500px\|thumb\|center\|Dropped solution on the quartz crystal]]		[[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]]		[[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 relative to the 6 MHz absolute baseline, a counting method was employed. The oscillation frequency was estimated by recording the waveform on the oscilloscope over a 1-second interval and counting the total number of oscillation cycles. Measurements were taken and recorded for each incremental volume of DI water.

	== Results and Discussion ==		== Results and Discussion ==

Rongqi: /* Preparation of the Hygroscopic Sensing Layer */

2026-04-20T11:38:23Z

Preparation of the Hygroscopic Sensing Layer

← Older revision		Revision as of 19:38, 20 April 2026
Line 49:		Line 49:

	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.		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]]

	== Results and Discussion ==		== Results and Discussion ==

Rongqi: /* Preparation of the Hygroscopic Sensing Layer */

2026-04-20T11:37:01Z

Preparation of the Hygroscopic Sensing Layer

← Older revision		Revision as of 19:37, 20 April 2026
Line 48:		Line 48:
	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.		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 ~~covered by an inverted dish and~~ 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.		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.

	== Results and Discussion ==		== Results and Discussion ==