Description:
Referring the figure, we see that the circuit is actually very easy to understand, let’s
evaluate it with the following points:
The transformer, bridge and the capacitor forms a standard Power
supply for the circuit. The circuit is basically made up of a single PNP
transistor, which is used as a switch here.
We know that PNP devices are referenced to positive potentials
and it acts like ground to them. So connecting a positive supply to the base of
a PNP device would mean grounding of its base. Here, as long as mains power is
ON, the positive from the supply reaches the base of the transistor, keeping it
switched off. Therefore the voltage from the battery is not able to reach the
LED bank, keeping it switched off.
In the meantime the battery is charged by the power supply
voltage and it’s charged through the system of trickle charging.
However, as soon as the mains power disrupts, the positive
at the base of the transistor disappears and it gets forward biased through the
10K resistor.
The transistor switches ON, instantly illuminating the LEDs.
Initially all the diodes are included in the voltage path,
and are gradually bypassed one by one as the LED gets dimmer.
Parts List
R1 = 10K,
C1 = 100uF/25V,
D1, D2 = 1N4007,
D3---D6 = 1N5408,
T1 = BD140
T1 = BD140
Tr1 = 0-9V, 500mA,
LEDs = white, hi-efficiency, 5mm,
S1 = switch with three changeover contacts.