The current consumption of the circuit is only 450uA in standby due to which the batteries will last longer. For example three 1.5V AA size alkaline batteries will last for several months.

The circuit is very portable and can be placed in a small enclosure with the batteries in side. For antenna use a thin hard 4 inches wire and place it inside the enclosure to not come physically in contact with any mains connection. The sensitivity of the circuit can be adjusted with the 1M variable resistor.

The circuit is based on two ICs and can be divided in to two sections. The first section is an EMF detector that is build around the LM358 IC. And the second section is a beeper circuit built around the NE555 IC. The sensitivity of the circuit can be adjusted with the 1M variable resistor. The 100K variable resistor is used to adjust the speed of the flashing LED and beeps. The operating voltage of the circuit is 4.5V to 12V DC.
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Here is a very useful project of a contact less mains failure alarm circuit. The circuit wirelessly detects the absence of AC mains voltage in the lines and alarms. A beeper circuit is used as an alarm that will continuously produce beep beep sound until the voltage in the mains back.

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Contact Less Mains Failure Alarm Circuit Diagram