above: 950mW infared LED array! The LEDs get quite warm during use

Having a Sony TR940 Hi8 camcorder with "Nightshot"  capabilities for
low light taping is handy. The Nightshot feature seems to remove an IR
filter from infront of the CCD allowing the great sensitivity to IR inherent
in the CCD to be used. Still, some light is needed for this to work and
sometimes there is simply not enough. 

So I decided to prototype an infared LED array as a light source for taping
in dark or complete darkness. The first unit consisted of three Radio Shack
high output IR LEDs. Total optical output was around 45mW.  This worked for
upclose work on in room with white walls. Adding three more LEDs help a bit
but much work could still be done. 

After brief research I was able to locate the brightest IR LED I was able to
locate. It was the HSDL-4220 from HP/Agilent Technologies. Optical output is
rated at an average of 38mW at 100mA forward current. The optical
output/forward current curve was linear as well. running 100 of these LEDs
at derated an output of 30mW would give a total output of 3W of infared
radiation. 

I ordered 105 from Newark Electronics for about $70 and made a test array of
32 LEDs in two parallel circuits on 16 each with a 10ohm resistor per leg. 
This was powered by 24V. The average forward current was about 85mA and rose
as the LEDs warmed up. I was able to illuminate entire rooms with this
setup. 

The next step is building a constant current regulator and finding housing
for all 100 LEDs. The constant current regulator will be a LM317 with a 15
ohm resistor across the out and sense lead. This will give 83mA of
currnet.

02-18-2000