Red LEDs - in the range 1.8-2.8 volts would do. Two rechargeable AAs or AAAs would be good being about 2.4 volts in series. Ordinary batteries are more like 3.0 volts in series - suggest you try a series resister with your LED - try 100 ohm.
I'm running a red LED off 1.7 volts. It just glows but that's all I want - it's to show me, in the dark, if I have the tracker switched on:-
http://www.iceinspace.com.au/forum/a...0&d=1177740460
If you are playing with white or blue LEDs, they need a higher voltage to operate - more like 3-4 volts as I recall. Look up the Jaycar site (
www.jaycar.com.au), find LEDS - they have specs for most LEDS - look for the Vmax or Vtyp values to give you an idea. In my experience LEDS are pretty robust (unless you push 12 Volts onto a red LED!)