Grain moisture detection circuit

It uses a 555 time base integrated circuit Ar and a capacitive humidity sensor C as the moisture detection
element. C is actually made of two metal plates with a certain area, which are parallel and separated by a certain distance. When
grain with a certain humidity is placed in it, the multivibrator composed of 555 time base integrated circuit A will
emit different oscillation frequencies according to the humidity of the grain, and turn it into a DC voltage, which is measured by a DC voltmeter. PV (scale changed from voltage to
humidity) is indicated. As long as the capacitance of the flat capacitive humidity sensor changes by 10 t,F, the DC voltmeter will
indicate a change of about 2V.
    In the figure, RP is the zero-adjusting potentiometer, that is, when no grain is loaded into the humidity sensor , adjust RP so that the indication of the voltmeter
PV is ov.