Rain Detector using SCR 106CY This circuit uses a sensor made of a small piece of circuit board and recorded a simple SCR circuit to detect rain and sound of a bell. The SCR could also be used to activate a relay, light a lamp, or send a signal to a security system. 1. 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. 2. Make sure to use a loud buzzer.