Thanks fungus! Cleared up a few things.I ran a formula and worked out i should use a 2.5k ohm resistor for the TIP120 running a 1W LED. Does this sound in the ballpark?
Bit wary of running a led straight off the power supply - so i saw some cheap 1w led drivers on ebay. They run on 12v. Do you think i could switch one of these with another resistor and an external power supply?
It doesn't have an operating voltage, i was just going to run it at 5v originally from the arduino 5v pin?
This is my LED: http://www.ebay.com.au/itm/1W-Infrared-IR-940NM-High-Power-LED-Bead-Emitter-with-20mm-Star-Base-/221063642644?pt=LH_DefaultDomain_0&hash=item33786b7e14&_uhb=1#ht_4040wt_1163