Main Power off Alarm

Sometimes it’s useful to have a simple alert when the main electrical power goes off. This circuit automatically activates a beeping alarm when the mains supply fails, providing an audible warning for about 1 to 3 minutes after power loss.

The duration of the beeping sound is determined by the value of the timing capacitor, which can range from 1000µF to 4700µF. Larger capacitance gives a longer alarm period.

For the input stage:

• Use a 220KΩ resistor if the mains voltage is 220V AC.
• Use a 100KΩ resistor if the mains voltage is 110V AC.

This compact and reliable circuit is ideal for home safety, equipment monitoring, or as an educational demonstration of power-failure detection.