Humidity Detector Based on Quartz Crystal Oscillator

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.

Colpitts oscillator circuit on breadboard

Waveform Diagram without coating

Coating

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

Dropped solution on the quartz crystal

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.