nube, Uno: how 2 drive hi pwr IR led safely?

[danz]

I just got an Arduino Uno board, which I want to configure to function as a remote control; I intend to use a Radio Shack "High-Output Infrared LED" as its control output, 100mA forward current, forward voltage 1.2V.

The LED will be pulsed with remote control codes at 38 kHz carrier.

I don't know how to figure proper current limit resistor value to avoid damage to Uno board.

1. How can I initially make SURE not to pull damaging level of current from Uno board when driving that IR LED (ie, how2 figure proper current limit resistor value)?

2. How can I subsequently drive LED toward MAX amount that Uno can provide,(for effective distance), again without damaging Uno?

3. If it turns out Uno max output is insufficient for distance control, how would I use transistor to drive LED brighter, toward its max?

I don't have an amp-meter, but since LED will be being pulsed, I don't think it would even help?

dan

[stephanie]

I'd drive it through a fast switching transistor, like a 2N2222 or something along those lines. That will isolate your Uno from the 100mA current. As for the limiting resistor to protect the LED, there's lots of online LED calculators out there, just do a search. You plug in the various parameters -- supply voltage 5v (from the uno), led voltage 1.2v, current 100mA, and it will tell you what size resistor you need.

Have a look at the schematic for the TV-B-Gone, that does essentially the same thing, only with an ATTiny instead of an Arduino as the uC.

Cheers!

[danz]

Thanks! And I just found the LED portion of the ladyada Arduino tutorial, can't believe I didn't look there before posting!

Would you or anyone have any idea how to allow for the fact that the LED isn't being asked to light constantly, but is pulsing? In other words, I presume that if I gave the LED a smaller than proper resistor, and pulsed it quickly on/off, that it would be "protected" against burning out by not being on all the time, but would that same protection apply to the current SOURCE? And if so, is there a way to compute acceptable R value for that, taking duty cycle and pulsing duration into account?