Humidity Detector Based on Quartz Crystal Oscillator
Introduction and background
Methods
Results and Discussion
Building Colpitts oscillator circuit
A Colpitts oscillator circuit was successfully constructed on a breadboard, and stable oscillation was achieved. The designed oscillation frequency was approximately 6 MHz. For the uncoated quartz crystal, the measured oscillation frequency was 5.9786 MHz, which is consistent with the expected value within experimental uncertainty.
Coating
To enable humidity sensing, a hygroscopic film was deposited on the quartz crystal using a sodium silicate (water glass) solution. The original solution was first diluted to an appropriate concentration. Subsequently, approximately 6 μL of the diluted solution was drop-cast onto the center region of the quartz crystal, while the gold electrodes at the periphery were intentionally left uncovered to avoid interference with electrical conduction. The coated crystal was then placed on a hot plate and heated gradually to 60–80 °C, where it was maintained for approximately one hour to evaporate the solvent and form a solid hygroscopic thin film.
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.
Humidity Sensing Performance
The humidity sensing performance was then investigated. Due to the lack of a calibrated humidity chamber, a qualitative humidity variation was introduced by placing different volumes of deionized (DI) water inside a sealed test tube. The oscillator frequency was monitored under these conditions. Given that the relative frequency shift is small compared to the absolute frequency (~6 MHz), the frequency was estimated by recording the waveform over a 1-second interval and counting the number of oscillation cycles. The results show a clear dependence of oscillation frequency on the amount of water present. Starting from 10 mL of DI water, every additional 10 mL led to an approximate frequency decrease of 20 Hz. This trend indicates that the system is sensitive to humidity-induced changes.
Conclusion and Outlook
This work demonstrates the feasibility of using a quartz crystal oscillator as a humidity sensor. Future improvements could include quantitative calibration of frequency shift versus relative humidity using a controlled humidity chamber, enabling precise and reproducible measurements. Owing to the high sensitivity of quartz crystal oscillators to mass changes, further optimization of the sensing layer and measurement setup may allow deployment in applications requiring high-resolution humidity monitoring. The observed frequency shift can be attributed to the mass loading effect on the quartz crystal, where absorbed water in the hygroscopic film increases the effective mass, leading to a reduction in resonance frequency. This mechanism is consistent with the Sauerbrey relation for quartz crystal microbalances.
