how hard can I push an IR LED?

Ok, I have one of these: http://www.radioshack.com/product/index.jsp?productId=2062565. 1.2V, 100ma rating, but not much in the way of other specs available.

I'm trying to build a replacement control for a remote. The remote that I am replacing has no identifying marks about the LED in it. It's a darker blue bulb than the Radioshack one I have, and it appears to have just a 6.8 ohm resistor in series (running off 3x AAA batteries, so I assume that means it's getting a LOT of power thru it. If it's the same 1.2v as mine, that'd be like 600ma, right?)

I currently have mine running off the 5V rail of the arduino via a transistor. It's got about 50 ohm resistance on it (what I had handy...). It works, but the range is way lower than the original remote.

I looked at both systems via my cell camera. The original is MUCH brighter than mine.

So, how hard can I really push these LEDs before I'd burn them out? Since it's a remote, it's just being used for brief bursts of signal.

(secondly, how much wattage do you really need in the resistor to go with that? The 6.8 ohm in the original remote is a beefy thing - online calculators say it'd need to be 2W to put 500ma thru the LED. But would a lesser resistor really burn out in such short bursts of power?)

You really need a datasheet to tell you the max peak current is... then you can figure out a good resistor. As you pointed out, it is probably 100mA continuous.

Yeah, well, there’s not much datasheet available on the radioshack leds. I do have some ordered that I suspect are the same as the RS one, but with more info available. They’re 100ma continuous, 2A pulsed. I’ve tried running the RS one that I have at 1A and it seems fine. Still not as bright on the camera (or as good of range) as the original remote though.

Wonder if the original is a 850nm…

If you have a DMM, why not try to figure out the current on the remote?

I got my LEDs that I actually know specs on… they’re 1.2V, 940nm, 100ma continuous, 1 or 2A (forget at the moment, but high) pulse.

So, I’ve got a 12V 1A power supply hooked to my arduino board. I’m then using a transistor to switch the LEDs on and off, connecting them to the 5V rail of the board.

I’ve tried a variety of resistors and LED setups. I’m currently working with 3 of the LEDs in series, with a 1ohm 1/2w resistor. Still doesn’t seem to get very bright, although these are working far better than the original radio shack one.

How much current can I pull from that 5V on the arduino? Various places online seem to give me different info… to get max power out of the LEDs, would a different arrangement be better (use 3 transistors to switch 3 LEDs in parallel or something?

I haven’t noticed anything getting hot on the board or my components, but I also haven’t run it very long.

  1. Keep in mind that series circuits, even with resistive loads, cause all the lights to be dim. Parallel is what you want, however, be aware that 12V is a bit too much for your LEDs... the fact that you have them in series allows their voltage drop to take care of it.

  2. Arduino 5V pin is wired directly to the output of the 5V regulator, which can handle 800mA max. This should be accurate, as I am referencing the part number found on the arduino schematic.

baum

Thanks... so 3 in parallel would be better than in series. I tried first just putting 1 wiht a 15R resistor (making ~250ma to the LED). Range seems about the same as the 3 I had in series before, but less area covered (I had the three bent in different directions).

I'm thinking then I'll run 3 15R and LED sets in parallel to get the wide coverage again. This is all off the 5V of the arduino, so I don't think the 12V AC adapter I'm using is a factor, other than the voltage regulator has to work a bit extra to drop it to 5V. 3 of the LEDs in parallel should then pull ~750ma, but again, it's only in tiny bursts, not constant.

When I get a chance, I'll wire that up and try it out. I'm hoping to be able to bounce the signal off the white ceiling and cover the whole area from the controller (I'm making a remote control for a model train layout). Worst case, I'll run a wire out from the controller and mount the LEDs on the ceiling pointed down. That'd undoubtedly work better, but more hassle and makes the controller less moveable.