Editing Humidity Detector Based on Quartz Crystal Oscillator (section)

== 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.