The circuit shown here will alert you every time the mains power come back by indicating you through a melody alarm. The alarm runs for few seconds and then automatically goes off. The alarm running time depends on the 100uF capacitor and by increasing or decreasing the value of that capacitor you can increasing or decrease the alarm length.

The circuit is using two ICs. The first IC used is a NE555 timer IC that is working as a timer. The second IC used in the circuit is a UM66 melody IC that will generate melody in the 8 ohms speaker through a 2N3904 transistor. For speaker you can use any 8 ohms 200mW to 1W value. For increasing the output sound use BD139 transistor in the place of 2N3904.

Note: Many points of this circuit are at AC mains voltage. It could give you lethal shock if you are not careful. If you don't know much about working with AC line voltages do not attempt to construct this circuit.

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Mains power goes off some times due to some technical issues but in some countries load shedding occurs on frequent basis. So some people use uninterruptible power supplies (UPS) and some use generators to run their appliances until the mains voltage comeback.
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Mains Power Come Back Alarm Circuit Diagram