M.Joshi wrote in
:


Hello,

Does anyone know if there are any websites that have circuit diagrams
for a mains warning alarm that sounds when the mains is off?

We occasionally find that the circuit breaker trips for a circuit in
the kitchen and if left unnoticed, the food in the freezer could thaw
out.

Thanks.

Simple:
Neon bulb connected to mains illuminates CDS cell or photodiode.
Aformentioned light sensor biases trasnistor off, fixed trasnistor
biases base on. Trasnistor controls beeper module. LV side powered by
couple AA cells, as required for the beeper.

Two:
Small plugin trasnformer:
Trickle charges rechargeable battery.
Biases PNP trasnistor off, resistor to pull PNP trasnistor on.
Trnasistor controls beeper, powered by rechargeable battery.