IR LED Driver Circuit

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi Everyone,

Background: I've been driving an IR LED with one of the digital I/O pins just fine. The LED's specs are 1.2V at 100mA. The duty cycle on the LED is never greater than 50%. I had been driving the LED from the I/O pin through a current limiting resistor of 100 ohms. It worked fine but my range was only about 4 feet.

Currently: Given the max current output of an I/O pin at only 40mA, I decided to drive the LED using a 2N2222A NPN transistor to increase its output. I connected the I/O pin to the base through a 1K ohm resistor. I then attached the emitter to Gnd and the collector to the LED (no current limiting resistor due to the duty cycle ratings on the LED and wanting to crank up the output). I just used this circuit to drive relays on my last project without problems.

Problem: The range has actually decreased. While it does work, it seems inconsistent and only works at about a 1 foot range. HELP - any thoughts on what I've done wrong? I've check and double checked to ensure I've wired it correctly - there's hardly enough components to screw it up (but then again, I've been known to do stupid...er things). :slight_smile:

Thanks for the consideration,

Mark~

2

Are you over-driving the infra red diode?
I am reminded of some leds I once cooked when I used the wrong resister. They went dim then didn't work any more.

3

Hi Mouse - hmm, I've got that sick feeling.

I'll put a resistor in series with the LED to see if that works (and possibly replace the LED too). Good (and obvious) suggestion. Look for an update from me later tonight (hopefully).

Mark~

4

quick trick for testing an IR led. Point a digital camera at it and look at the display. Turn the LED on and off and you should notice it light up on screen.