This circuit can be used to glow a bulb or turn a alarm when the phone rings.Using this circuit nuissence of telephone ring at night can be avoided.
When the telephone rings the line voltage rises to 72 volts.At this time the LED in the opto-coupler glows and the transistor conducts. Due to this the transistor Q1 conducts.This makes the relay ON.The load connected to the relay whether bulb or
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