ELECTRONICS PROJECTS Vol. 21182

Most of the power-supply failureindicator circuits need a sepa-rate power-supply for them-

selves. But the alarm circuit presentedhere needs no additional supply source. Itemploys an electrolytic capacitor to storeadequate charge, to feed power to thealarm circuit which sounds an alarm fora reasonable duration when the mainssupply fails.

During the presence of mains powersupply, the rectified mains voltage isstepped down to a required low level. Azener is used to limit thefiltered voltage to 15-volt level.Mains presence is indicated byan LED. The low-level DC isused for charging capacitor C3and reverse biasing switchingtransistor T1. Thus, transistorT1 remains cut-off as long asthe mains supply is present. Assoon as the mains power fails,the charge stored in the capaci-tor acts as a power-supplysource for transistor T1. Since,in the absence of mains supply,the base of transistor is pulled

‘low’ via resistor R8, it conducts andsounds the buzzer (alarm) to give awarning of the power-failure.

With the value of C3 as shown, a good-quality buzzer would sound for about aminute. By increasing or decreasing thevalue of capacitor C3, this time can bealtered to serve one’s need.

Assembly is quite easy. The values ofthe components are not critical. If thealarm circuit is powered from any exter-nal DC power-supply source, the mains-supply section up to points ‘P’ and ‘M’ can

be omitted from the circuit. Followingpoints may be noted:

1. At a higher DC voltage level, tran-sistor T1 (BC558) may pass some collec-tor-to-emitter leakage current, causing acontinuous murmuring sound from thebuzzer. In that case, replace it with somelow-gain transistor.

2. Piezo buzzer must be a continuoustone version, with built-in oscillator.

To save space, one may use five small-sized 1000μF capacitors (in parallel) inplace of bulky high-value capacitor C3.

POWER-SUPPLY FAILURE ALARM