1. CIRCUIT
IDEAS
AUTOMATIC LOW-POWER SUNIL K
UMAR
EMERGENCY LIGHT
S.C. DWIVEDI current through diode D5 and limit-
ing resistor R16. By adjusting preset
H
ere is a white-LED-based VR1, the output voltage can be ad-
emergency light that offers justed to deliver the required charg-
the following advantages: ing current.
1. It is highly bright due to the use When the battery gets charged to
of white LEDs. 6.8V, zener diode ZD1 conducts and
2. The light turns on automatically charging current from regulator IC1
when mains supply fails, and turns off finds a path through transistor T1 to Fig. 2: Pin configurations of LM317, BD140
when mains power resumes. ground and it stops charging of the and BC548
3. It has its own battery charger. battery.
When the battery is fully charged, The LED driver section uses a to- off. On the other hand, when mains
charging stops automatically. tal of twelve 10mm white LEDs. All fails, the base of transistor T2 becomes
The circuit comprises two sections: the LEDs are connected in parallel low and it conducts. This makes all
charger power supply and LED driver. with a 100-ohm resistor in series with the LEDs (LED1 through LED12) glow.
The charger power supply section is each. The common-anode junction of The mains power supply, when
available,
charges the bat-
tery and keeps
the LEDs off as
transistor T2 re-
mains cut-off.
During mains
failure, the
charging sec-
tion stops
working and
the battery sup-
ply makes the
LEDs glow.
Assemble
the circuit on a
general-pur-
pose PCB and
enclose in a
cabinet with
Fig. 1: Automatic high intensity LED-based emergency light enough space
for battery and
built around 3-terminal adjustable all the twelve LEDs is connected to switches. Mount the LEDs on the cabi-
regulator IC LM317 (IC1), while the the collector of pnp transistor T2 and net such that they light up the room.
LED driver section is built around the emitter of transistor T2 is directly A hole in the cabinet should be drilled
transistor BD140 (T2). connected to the positive terminal of to connect 230V AC input for the pri-
In the charger power supply sec- 6V battery. The unregulated DC volt- mary of the transformer.
tion, input AC mains is stepped down age, produced at the cathode junc- EFY lab note. We have tested the
by transformer X1 to deliver 9V, 500 tion of diodes D1 and D3, is fed to circuit with twelve 10mm white LEDs.
mA to the bridge rectifier, which com- the base of transistor T2 through a 1- You can use more LEDs provided the
prises diodes D1 through D4. Filter kilo-ohm resistor. total current consumption does not ex-
capacitor C1 eliminates ripples. Un- When mains power is available, the ceed 1.5A. Driver transistor T2 can de-
regulated DC voltage is fed to input base of transistor T2 remains high and liver up to 1.5A with proper heat-sink
pin 3 of IC1 and provides charging T2 does not conduct. Thus LEDs are arrangement.
126 • JANUARY 2008 • ELECTRONICS FOR YOU WWW.EFYMAG.COM