Infrared Illuminator

Infrared (IR) illuminators are
widely used to improve the imagecapturing
quality of security cameras
fitted in dark zones. Just like our
eyes, cameras also can’t record movements
movements
in dark.
However, unlike
our eyes,
most of the latest
cameras can
capture infrared
light.
I n a n I R
I l l u m i n a t o r ,
many infrared
IR LEDs are grouped together to throw
good amount of IR light. Typically,
LEDs output at 470 nm (blue region),
525 nm (green region) and 625 nm
(red region). IR LEDs produce longer
wavelengths, 880 nm and 940 nm being
the common ones. Most CCD cameras
are a little more sensitive to 880 nm,
although when these LEDs are used
for security applications, some individuals
can detect a very dim red glow
from them. The 940nm LED radiations
are completely invisible to the eye.
Some of these LEDs are clear, while
others are tinted with pale shades of
dark gray, blue or even black. They
come in various configurations and
radiation patterns, but 5mm types with
15- to 40-degree patterns are the most
popular.
Typically, IR LEDs run at around
1.3 to 1.7 volts, depending on the
LED current (typically 10 to 30 mA).
However, this may vary with the type
and manufacturer. Practically, IR illuminators
may have 6 or 60 to 100 or
more LEDs, depending on the output
needed.
The circuit (refer Fig. 1) can be
divided into three parts: ambient light
sensor, relay driver and IR LEDs. The
ambient light sensor is built around
multiturn linear potmeter VR1 and
light-dependent resistor LDR1. The
relay driver section is built around
transistors T1 through T3. The IR LEDs
section is built around LED1 through
LED40.
The light sensor circuit is a simple
transistor switch with the base of the
Darlington pair (formed by T1 and
T2) connected to a voltage divider.
Variable resistor VR1 and the 10mm
encapsulated LDR are used to sense
the ambient light. As light falls on the
surface of LDR1, its resistance changes.
The amount of minimum light needed
to actuate the relay through driver
transistor T3 can be varied by adjusting
VR1. Diode 1N4001 eliminates
any back voltage when the relay
de-energises. Switch S1 is the mains
power on/off switch and switch S2 is
added to bypass the ambient light detection
function.
Relay RL1 energises only when
the ambient light level falls below a
threshold value set by VR1, i.e., when
it’s dark. Normally-opened (N/O) contacts
of the relay ground path to the IR
LEDs (LED1 through LED40) to make
them glow. The blue LED (LED41) indicates
the circuit activity. When there
is ambient light and you want to use the
illuminator, switch S2 ‘on.’ All the LEDs
(LED1 through LED40) glow to fulfil
your requirement.
Assemble the circuit on a generalpurpose
PCB and enclose in a suitable
cabinet. The IR LEDs assembly is very
important. A set of 40 (5×8) 5mm infrared
LEDs (IR LED1 through IR LED40)
with independent current-limiting
resistors (R3 through R10) per string
is used. This section is powered by
the input DC supply through the relay
contacts.
Mount IR LEDs on the general-purpose
PCB board such that these make
three circles. After soldering, carefully
cut the outside of the circuit board in
a round shape and fit it in a suitable
metal/plastic cabinet. If available, adda suitable reflector sheet for the IR
LED bank. Finally, fit the LDR bank
on top of the enclosure with switches,
indicator-sensitivity-control pot and
power input socket. Fig. 2 shows the
infrared illuminator unit.To make the circuit actuate the relay
when the intensity of ambient light
is less than the preset light level, throw
light on LDR1 and then slowly adjust
the potentiometer until LED1 lights up
and the relay energises.