Plate Touch Sensor Circuit

  
This system operates on the principle that capacitance loading of an oscillator will lower its frequency. When a foreign body comes into contact with touch plate, the frequency of Ul is lowered.
Plate Touch Sensor Circuit - schematic

This removes the oscillator signal from Ul from U2`s passband, which causes U2 to lose lock, turns off the LED, and causes the collector of Ql to go low.




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