WHAT HAPPENS AT your place if there’s a sudden "blackout" or mains power failure? It’s a familiar story – if it’s at night, you’re left floundering in the darkness, searching for some candles or your torch. And if you do find the torch, it’s more than likely that the batteries have gone flat.

Fig.1: the circuit uses transistors Q1 & Q2 and 555 timer IC1 to detect when the mains fails. When it does, pin 3 of IC1 switches high and Q4 turns on and connects an SLA battery to the emergency lights. Zener diode ZD1 and transistor Q3 trigger IC1 and turn the lights off again to prevent over-discharge if the battery voltage drops below 11.6V.

This project means that you should never have to search around in the darkness during a blackout again. As soon as the mains power fails, it automatically turns on the power for some 12V emergency lights within a second or two. It then keeps them operating until either the mains power is restored or its internal 12V sealed lead-acid (SLA) battery is discharged to the safe minimum level.

Basically, the project is designed to be used in conjunction with a small 12V/1A automatic SLA battery charger, such as the Powertech MB-3526 unit sold by Jaycar stores and dealers. This unit normally keeps the internal SLA battery at full charge and we use this project to monitor the charging voltage so that it can determine when there is a mains failure.

That’s how it knows when to switch on your 12V emergency lights.