Rain alarm-door bell

  
With SI open the circuit functions as a doorbell. With SI closed, rain falling on the sensor will turn on Ql, triggering Q2 and the thyristor and activating the bell, R4 provides the holding for the thyristor while Dl prevents any damage to the thyristor from back EMF in the bell coil. The sensor can be made from 3 square inches of copper clad board with a razor cut down the center
Rain alarm-door bell - schematic

Cl prevents any mains pickup in the sensor leads.




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