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This is only a preview of the March 1995 issue of Silicon Chip. You can view 30 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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An IR illuminator for
cameras & night viewers
What ever would you use an infrared (IR)
illuminator for? To see in the infrared region,
that’s what for. More precisely, an IR illuminator
can be used with CCD video cameras & with IR
night viewers such as the model described in the
September 1994 issue of SILICON CHIP.
By BRANCO JUSTIC
This IR illuminator provides an
output of up to 1.4 watts at 880
nanometres. Most CCD cameras will
respond, to some extent at least, to
infrared light. The CCD modules themselves are quite responsive to infrared
light but many cameras include an
infrared filter. This is done so that
pictures taken in low light conditions
do not have unnatural highlights (to
our eyes) due to the pickup of infrared
light.
There are two easy ways to check the
IR response of your CCD video camera.
First, set it up in a darkened room and
then use a torch with red cellophane
over the glass. The camera should
then produce a useable picture of the
room. Second, try the same thing but
with illumination now provided by
the infrared remote control for your
TV, VCR or other appliance. This is
also a good way of checking that your
IR remote control is working.
Applications
Now that we have established that
CCD cameras can work with IR light,
why would you want to do it? The
most important appli
cation is for
security. You could light a building,
room, or a yard with infrared light and
any miscreant would have no way of
knowing that his actions were being
monitored by a video camera.
You could also use an IR illuminator
and CCD video camera for watching
wildlife. Perhaps you have possums or
other nocturnal visitors in your backyard or at your campsite. Now you can
video them without any disturbance
to their behaviour.
March 1995 69
D1
MR856
L1
200uH
0.47
5
4
PARTS LIST
+26.5-29.5V
+9-12V
100k
100
47
47
A
1
100
16VW
IC1
LM2577
2
4.3k
1.5k
1k
0V
K
A
LED40
LED20
K
LED41
K
A
VR1
1k
A
LED21
K
3
0.47
47
A
LED1
2 PC boards (see text)
1 200µH inductor (L1)
1 1kΩ pot (VR1)
A
LED60
K
K
CIRCULAR PCB
1
5
A
K
INFRA-RED ILLUMINATOR
Fig.1: the circuit uses a switched mode power supply based on IC1 to step up
the battery voltage to 26.5-29.5V. This rail then drives an array of 60 IR LEDs
via 47Ω current limiting resistors.
Another application is for monitoring patients in sickrooms or in hospital. They can then be permanently
watched without having their sleep
disturbed.
Some CCD camera modules intended for security applications come
with inbuilt IR LEDs for illumination
but generally they would only be
sufficient for close-up work. The IR
illuminator to be described here is
much brighter.
It is also an ideal IR source for most
first generation IR night viewers. For
example, just one single fibre optic
tube from the three stage viewer design published in the September 1994
issue of SILICON CHIP would produce
good results when illuminated with
this IR source.
One experiment involved combining the unit with a CCD camera
supplied by Oatley Electronics. This
setup provided good vision on a monitor of a vehicle parked about 50 metres
away in very low ambient light.
The circuit
As you can see from the circuit
diagram of Fig.1, the illuminator is
basically a closely packed array of 60
IR LEDs. There are three series strings
of 20 LEDs fed via a 47Ω resistor
The power supply section employs
a switched mode power supply which
is used to step up the voltage of the
battery to a regulated output voltage
adjustable over a range of about
26.5-30V. The battery voltage can be
9-12V without any need to change
the circuit.
An economical way of obtaining
a 12V battery for this unit would be
to connect two 6V lantern batteries
in series. These can be obtained for
around $4 each, or less.
Each IR LED drops a voltage of
approximately 1.33V when it is conducting, thus each string of 20 LEDs
requires a minimum of 26.6V. The
Semiconductors
1 LM2577T-ADJ step-up voltage
regulator (IC1)
1 MR856 or PL01 fast recovery
diode (D1)
60 IR383 880nm IR diodes
(LED1-60)
Capacitors
2 100µF 35VW electrolytic
2 0.47µF 50V monolythic
Resistors (0.25W 5%)
1 100kΩ
1 1kΩ
1 4.3kΩ
3 47Ω 1W
1 1.5kΩ
Where to buy a kit
The complete kit for this project,
including the two PC boards is
priced at $60 plus $4 for postage
& packing. The LEDs are available
separately at 10 for $9.00.The kit is
available from Oatley Electronics,
PO Box 89, Oatley, NSW 2223.
Phone (02) 579 4985 or fax (02)
570 7910.
current in each string is equal to (Vo26.6)/47Ω. The current in each string
is, therefore, adjustable from about 8
to 72mA. Since there are three strings,
the maximum total power delivered by
the step-up inverter is approximately
6.5W.
The voltage regulator employs a
National Semiconductor LM2577T.
This device can be used to step up
input voltages in the range of 3.5-40V
to output voltages up to 60V.
The IR LEDs used in this project
(IR383) have a very high quantum
efficiency. They are specified as having an output of 30mW <at> 100mA at a
wavelength of 880 nanometres. The
maximum continuous current for
these is 100mA but they can be pulsed
at currents up to 1.2A. The diodes supplied in the kit have a radiation angle
of 12° but they are also available in a
60° version (IR333).
Construction
This view shows the final version of the
IR Illuminator. Note the small heatsink
attached to IC1.
70 Silicon Chip
Two PC boards are required for this
project. There is a small board for the
step-up circuit and one for the 60-LED
array. The photos accompanying this
LED1-60
100uF
0V
0.47 0.47
200uH
+9-12V
D1
100k
IC1
LM2577
(ON HEATSINK)
1.5k
Fig.2: install the parts on
the two PC boards as shown
in this wiring diagram. The
200µH inductor is supplied
ready-wound.
100uF
A
A
A
A
A
A
A
A
A
A
A
A
47
A
A
A
A
A
47
A
A
A
A
A
47
1k
4.3k
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
VR1
article show the two boards neatly
mount
ed in a short length of PVC
tubing but while this is quite an attractive package, we found it doesn’t
work well in practice because both
the LM2577T switching regulator and
the LEDs themselves dissipate quite a
respectable amount of heat.
However, provided the regulator
is fitted with a small heatsink and is
not mounted in the same housing as
the LEDs, the circuit will function
satisfactorily.
On the other hand, if the whole unit
is packed into a short length of tubing
as shown, and no heatsink is fitted, the
current drawn from the battery will
gradually rise and the regulator’s temperature will rise to the point where it
switches itself off. So you have been
warned – don’t pack it tightly into a
small space and make sure both the
regulator and the LEDs are reasonably
well ventilated.
Assembly of the boards is quite
straightforward. Install all the components on the regulator board first. Note
that a 100µF electrolytic capacitor
must be connected across the battery
inputs to the board. This capacitor is
not shown on the
screen print overlay
on this board alA
though it is shown
on Fig.2.
A
A
When the reguA
A
A
lator board is complete, power it up
A
A
A
and check the DC
A
A
A
output voltage. The
voltage should be
A
A
A
able to be varied
from about 29.3V to
A
A
A
26.6V. The LM2577
A
A
IC runs at close to
50kHz and if you
A
are able to examine
the switching waveform on an oscilloscope, you will find
that the duty cycle
varies depending
on the input voltage
and the setting of
the pot, VR1.
Now assemble the LED board and
make sure you connect each LED in the
right way around. The longer lead on
each LED is the cathode, marked “K”
on the PC board.
When complete, connect both PC
boards together and power up. Unfortunately, you can’t immediately
tell whether the LEDs are emitting
but after a short while you can easily
tell –they radiate heat! As a final
check, fire up your video camera
in a completely dark room – it will
show the illuminator lighting it up
SC
brightly.
The prototype PC boards were built into a short length of PVC tubing but note that this will lead to overheating
problems unless the unit is used only in brief bursts (see text). A heatsink should also be fitted to IC1.
March 1995 71
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