One of the air humidity measurement circuits

Humidity control circuit is a self-control circuit that detects or controls the humidity of air, soil, and objects. Commonly used hygroscopic resistors (such as
MS01 type, MS04 type, SM-1 type, YSH type, etc.), when the surrounding relative humidity changes between 30% and 90%, the resistance value
will change from thousands of ohms to hundreds of ohms. ), capacitive humidity sensors, etc. are used as moisture detection components.
    The circuit is shown in Figure 11-89. It uses MS01 type humidity resistor RS as the moisture detection element.
A self-excited multivibrator is composed of transistors VTi and VTz. The collector of VT2 outputs a square wave whose frequency is determined by the resistor-capacitor circuit R3 and C. Determined by R4 and C2
, appropriate selection of the values ​​of these components can make the oscillation frequency 1-10Hz to meet
the needs of the MS01 type humidity resistor. When the humidity increases, the RS resistance decreases.
    Turn the switch SA to the measurement position, and
the humidity value in the air can be measured by the DC ammeter PA (changed from microampere scale to humidity scale). When SA is moved to the calibration position and the meter PA has an indication, it means the circuit is normal.