Description.

Here is a simple mains failure alarm circuit that produces an alarm whenever the mains supply fails.Lot of such circuits are available , but the peculiarity of this circuit is that it requires no back up power source like a battery to power the alarm when the mains is absent.

When there is mains supply the transistor Q1(BC558) will be OFF and the capacitor C1 will be charged.When the mains supply fails the transistor Q1 becomes ON and the capacitor C1 discharges through the Q1 to drive the buzzer to produce an alarm.The capacitor C2 is the filter capacitor for the bridge.Diode D2 prevents the discharge of the C2 when mains fails.If D2 is not there ,the alarms will remain silent for a time capacitor C2 to fully discharge after the power failure.

Circuit diagram with Parts list. 

Notes. 

• T1  can be  a 230V primary   6V secondary  500mA  transformer.
• B1 can be a 1A bridge.You can make the same using four 1N 4007 diodes.
• All capacitors are rated 25V.
• You can use any general purpose PNP transistor (like BC158,BC177  etc) as Q1.