Moisture Detector Circuit

  
The moisture detector uses two transistors and a piezoelectric transducer to sound an alarm tone when water is present. Transistor Ql forms a crystal-controlled oscillator, using a portion of piezoelectric transducer XDCwhich contains two piezoelectric crystal regionsas the crystal. The transducer has three separate leads. One lead goes to each of the crystals, and the third lead is common to both. The smaller internal crystal region sets the frequency of operation and the larger element is driven by Ql (when it is biased on) to provide the loud tone output.
Moisture Detector Circuit - schematic

To turn the pnp transistor Ql (used as an oscillator) on pnp transistor Q2 (used here as a switch) must be on. To turn it on with the biasing that is normally connected, you would only need to connect a resistor from the collector of Q2 to the base, which gives the base a negative (-) bias. The resistor used is the water that is to be detected. That turns Q2 on, which, in turn, turns on Ql. The result when water touches the probe is that the transducer emits a loud sound.




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