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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.
This circuit can be used as an alarm for power supplies in the
range of 5V to 15V.
To calibrate the circuit, first connect the power supply (5 to
15V) then vary the potentiometer VR1 until the buzzer goes from on
to off.
Whenever the supply fails, resistor R2 pulls the base of
transistor low and saturates it, turning the buzzer ON.
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