Air humidity measuring instrument circuit

(1) MS01 waterfall sensor MS01 humidity sensor is made of silicon powder mixed with a small amount of alkali metal oxide and sintered. Its resistance value
changes with the relative humidity of the surrounding atmosphere, so people are accustomed to calling it MS01 type humidity resistor. Its resistance value
decreases as the relative humidity of the surrounding atmosphere increases, so it is a characteristic humidity-sensitive resistor. The text symbol is RH. The air
humidity measuring instrument circuit assembled with MS01 humidity sensor is shown in Figure 5-34.
  (2) Working principle In Figure 5-34, VT1,
, rr2 form a self-excited multivibrator. When working, the two
tubes take turns to be saturated and cut off. By appropriately selecting R2, island
or CJ, c2, the set of VTZ
A 1-10Hz very low frequency AC (square wave) signal is generated on the electrode and
is sent to the lower end of the humidity sensor RH through R6 and RP.
    When the selector switch SA is turned to "Measurement",
the alternating current from the VT2 collector will flow through R6, RP,
RH, SA, bridge rectifiers VD1~VD4 and is
a full-wave rectifier, regardless of which input terminal A or B
If it is negative, it can rectify AC current into DC current.
    The rectified DC current, after being filtered by the wind and filter capacitors C3 and cI, becomes a stable DC current and is added to the microampere meter PA.
Since the relative humidity of the air is different, the resistance of the humidity sensor RH is different, and the current value flowing through the meter PA after rectification is also different
. Therefore, the air humidity can be read intuitively through the PA scale.