09-04-2012, 12:48 PM
STRIP LED LAMP
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Strip LEDs are available in different
colours powered by direct
current (DC) source. These LEDs
are available as surface mount devices
with current limiting resistors. Usually
there are 300 LEDs in a 5-metre strip.
The strip can be cut into pieces so that
the bits having three or four LEDs can
be used with 12V DC source. The circuit
given here uses the strip LEDs to
make an automatic white LED lighting
source.
The automatic working of the circuit
is based on the light-sensing property
of the light-dependent resistor
(LDR). Operational amplifier CA3140
(IC1) is used as a comparator with
two potential dividers in its inverting
and non-inverting inputs. LDR1 and
resistor R3 form one potential divider
that provides a variable voltage at the
inverting input pin 2 of IC1. Second
potential divider comprises resistors
R4 and R5, which provide half of the
supply voltage (6V) to the non-inverting
pin 3 of IC1.
However, at night the light incident
on LDR1 is low and its resistance is
high. The voltage at inverting input of
the comparator decreases, making it
lower than the voltage at non-inverting
input. This makes IC1 output high.
Transistor T1 goes into saturation,
thus connecting cathodes of LEDs to
ground. All the LEDs in the strip turn
on and remain that way till morning.
Assemble the circuit on a generalpurpose
PCB and enclose it in a suitable
shock-proof case. Strip LEDs
are available in ribbon-shaped form.
Use 5cm bits (two bits) having three
LEDs each. The strip can be cut at
supply-contact points. Strip LEDs
are arranged on a flexible belt with
double-sided adhesive on the back
side, so it can be glued to any surface.
Connect the LED strip in the circuit
with correct polarity.