Automatic Low-Power Emergancy Light

Portable, simple and inexpensive unit, Useful for personal use

Here is a white-LED-based emergency light that offers the following advantages. 1-It is highly bright due to the use of white LEDs. 2-The light turns on automatically when mains supply fails, and turns off when mains power resumes. 3-It has its own battery charger. When the battery is fully charged, charging stops automatically. The charger power supply section is built around 3-terminal adjustable regulator IC LM317 (IC1), while the LED driver section is built around transistor BD140 (Q2).

In the charger power supply section, an input AC main is stepped down by T1 to deliver 9V, 500mA to the bridge rectifier, which comprises diodes D1 through D4. Filter capacitor C1 eliminates ripples. Unregulated DC voltage is fed to input pin 3 of IC1 and provides charging current through D5 and limiting resistor R15. By adjusting preset P1, the output voltage can be adjusted to deliver the required charging current. When the battery gets charged to 6.8V, D6 conducts and charging current from IC1 finds a path throughQT1 to ground and it stops charging of the battery. When mains power is available, the base of Q2 remains high and Q2 does not conduct. Thus LEDs are off.

On the other hand, when mains fails, the base of Q2 becomes low and it conducts. This makes all the LEDs glow. The mains power supply, when available, charges the battery and keeps the LEDs off as Q2 remains cut-off. During mains failure, the charging section stops working and the B1 supply makes the LEDs glow. Assemble the circuit on a general-purpose PCB and enclose in a cabinet with enough space for battery and switches. We have tested the circuit with twelve 10mm white LEDs. You can use more LEDs provided the total current consumption does not exceed 1.5A. Driver transistor Q2 can deliver up to 1.5A with proper heat-sink arrangement.

Circuit diagram:
Parts:

P1 = 2.2K
R1-R12 = 100R-1/2W
R13 = 1K-1/2W
R14 = 180R-1/2W
R15 = 16R/5W
R16 = 1.2K
C1 = 1000uF-25V
D1-D5 = 1N4007
D6 = 6.8V-0.5W Zener
D7-D18 = 10mm- White LEDs
Q1 = BC548
Q2 = BD140
B1 = 6V-4.5Ah Battery
IC1 = LM317
T1 = 9Vac-Transformer