14-01-2013, 02:29 PM
AUTOMATIC LOW-POWER EMERGENCY LIGHT
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 circuit comprises two sections:
charger power supply and LED driver.
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 (T2)).
In the charger power supply
section, input AC mains is stepped
down by transformer X1 to deliver
9V, 500 mA 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 diode D5
and limiting resistor R16. By adjusting
preset VR1, the output voltage can be
adjusted to deliver the required charging
current.
When the battery gets charged to
6.8V, zener diode ZD1 conducts and
charging current from regulator IC1
finds a path through transistor T1 to
ground and it stops charging of the
battery.
The LED driver section uses a total
of twelve 10mm white LEDs. All the
LEDs are connected in parallel with a
100-ohm resistor in series with each.
The common-anode junction of all
the twelve LEDs is connected to the
collector of pnp transistor T2 and the
emitter of transistor T2 is directly connected
to the positive terminal of 6V
battery. The unregulated DC voltage,
produced at the cathode junction of
diodes D1 and D3, is fed to the base
of transistor T2 through a 1-kilo-ohm
resistor.
When mains power is available,
the base of transistor T2 remains high
and T2 does not conduct. Thus LEDs
are off. On the other hand, when
mains fails, the base of transistor T2
becomes low and it conducts. This
makes all the LEDs (LED1 through
LED12) glow.
The mains power supply, when
a v a i l a b l e ,
cha rge s the
ba t t e ry and
keeps the LEDs
off as transistor
T2 remains
cut-off. During
mains failure,
the charging
section stops
working and
the battery supply
makes the
LEDs glow.
As s emb l e
the circuit on
a general-purpose
PCB and
enclose in a
cabinet with
enough space
for battery and
switches. Mount the LEDs on the cabinet
such that they light up the room. A
hole in the cabinet should be drilled to
connect 230V AC input for the primary
of the transformer.
EFY lab note. 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 T2 can
deliver up to 1.5A with proper heat sink
arrangement
exceed 1.5A. Driver transistor T2 can
deliver up to 1.5A with proper heatsink
arrangement