Rain warning beeper

  
One small spot of rain on the sense pad of this bleeper will start this audio warning. It can also be operated by rising water. The circuit has two transistors, with feedback via capacitor Cl, but Trl cannot operate as long as the moisture sense pad is dry.
Rain warning beeper - schematic

When the pad conducts, Trl and Tr2 form an audio oscillatory circuit, the pitch depends somewhat on the resistance.




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