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Step 4Led mounting and wiring
I used 88 infrared leds I ordered surplus online. Their maximum output was 10mW @ 940 nm. Each led needed 100mA at 1.45V so i wired sets of 8 in series along with a 5 ohm resistor. I wired these sets in paralell across the 12V rail of an old computer power supply I had lying around. Since the total current draw is over an amp, you can't get by with a wall wart.
Pour yourself a glass of whiskey and fire up the soldering iron...it will take a while to wire up 88 leds.
Pour yourself a glass of whiskey and fire up the soldering iron...it will take a while to wire up 88 leds.
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PS: im sorry for the lame paint-made schematic.
PPS:I know, I made a short story long. Who cares? xD
and from experience IR LED's usualy work best at about 1.5v and i think they are about 50mA
so if you linked them all up in paralell you would be looking at a 1.5v 2500mA (2.5A) supply to power all 50 LED's
http://www.dealextreme.com/details.dx/sku.2399
$2.93 for 20 (as of the time of this posting)
my radioshack didn't have a 5 ohm resistor either but you can get the equivalent by wiring two 10 ohm resistors in parallel. what is the current rating for your leds?
you can plug the values in to this web page
http://www.quickar.com/noqbestledcalc.htm
and it will compute what resistor you need.
here is another example of how to perform the computation
for a single led:
http://www.liquidcrystaltechnologies.com/LED_Resistor.htm
hope this helps.
now, the question is what size resistor to put in series with the leds. this is where we have to use ohms law which states that VOLTAGE = CURRENT*RESISTANCE. for our resistor, we want VOLTAGE=0.8v. we also know the current since we wamt each led to have 100mA = 0.1Amps of current then our equation reads 0.8 = 0.1*RESISTANCE so we need RESISTANCE= 8ohms. so you want to have an 8ohm resistor in series with each string of 8leds in order to get 100mA of current per led. suppose instead you want to run the leds at 200mA (which is probably ok based on the specification of max 250mA max) then you have 0.8 = 0.2*R so choose a resistor with R=4ohms.
last step is, what if you don't have a 4ohm resistor? you can combine multiple resistors to get the right resistance. if you wire two resistors in series, the resistances add up. so two 5ohm resistors in series gives a 10ohm resistance. if you put identical resistors in paralell then current can flow thru both of them and so the resistance drops. two 10ohm resistors in paralell gives 5ohms of resistance.
if you have a multi-menter (a good investment), you can check the resistance of the resistors to make sure you are doing the right thing. also, once you wire up the resistor and led string, you can use the meter to measure the current to make sure it is around 200mA. hope this helps