AC Circuits

Simple Power Failure Alarm

Beep once when power fails. 12 volt relay is used as a switch. When power is available, relay pulls and capacitor charges via diode D1 and NO and Common contacts of relay. When power fails, relay turns off and capacitor discharges via Common and NC contacts and buzzer. Buzzer gives a short beep at that time. If more timing for beep is needed, connect one more 4700 capacitor parallel to C1. 12 Volt DC for the circuit should be from a 12 Volt adapter or transformer power supply.