13-11-2012, 04:13 PM
AUTOMATIC EMERGENCY LIGHT
AUTOMATIC EMERGENCY LIGHT.pdf (Size: 44.96 KB / Downloads: 27)
This emergency light has the following two advantages:
1. It turns on automatically when the mains power fails, so you need not search it in the
dark.
2. Its battery starts charging as soon as the mains resumes.
Operation of the circuit is quite straight forward. Mains supply is stepped down by
transformer X1, rectified by a full-wave rectifier comprising diodes D1 and D2, filtered by
capacitor C1 and fed to relay coil RL1 The relay energises to connect the battery to the
charging circuit through its normally-opened (N/O) contacts. Freewheeling diode D3 acts as
a spike buster for the relay.
The charging circuit is built around npn transistor BD139 (T1). The transformer output is fed
to the collector of transistor T1, which provides a fixed bias voltage of 6.8V to charge the
battery.When the battery is fully charged, the battery voltage becomes equal to the
breakdown voltage of the zener diode (ZD1). Zener diode ZD1 conducts to provide an
alternative path for the current to ground and battery charging stops.
When mains fails, relay RL1 de-energises. The battery now gets connected to the white LED
array (comprising LED1 through LED6) through current-limiting resistor R2. The LEDs glow
to light up the room. To increase the brightness in your room, you can increase the number
of white LEDs after reducing the value of resistor R2 and also use a reflector assembly.