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Power Supply Failure Alarm 2011-12-29

Most of the power supply failure indicator circuits need a separate power supply for themselves. But the alarm circuit presented here needs no additional supply source. It employs an electrolytic capacitor to store adequate charge, to feed power to the alarm circuit which sounds an alarm for a reasonable duration when the supply fails.


Whenever the supply fails, resistor R1 pulls the base of transistor low and saturates it, turning the buzzer ON.




  • This circuit can be used as an alarm for power supplies in the range of 5V to 15V.
  • The piezo buzzer needs to be an active type (not a piezo diaphragm).
  • The purpose of the diode is to prevent the charge on the electrolytic being passed to any other part of the circuit when the power fails.
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