This circuit uses a sensor made of a small piece of etched PC board and a simple SCR circuit to detect rain and sound a buzzer. The SCR could also be used to activate a relay, turn on a lamp, or send a signal to a security system.
R1(1)1K 1/4 W Resistor R2(1)680 Ohm 1/4 W Resistor D1(1)1N4001 Silicon Diode BZ1(1)12V Buzzer S1(1)SPST Switch SCR1(1)C106B1 SCR106CY SENSOR(1)See Notes MISC(1)Board, Wire, Case, PC Board (For Sensor)
The sensor is a small piece of PC board etched to the pattern showen in the schematic. The traces should be very close to each other, but never touching. A large spiral pattern would also work.