Originally Posted by LeeR

Note: If you are going to use 12V source, get the 12V models of the bayonet base LEDS. While the experiment proved safe, the 6V models of LEDs are overdriven at 12V and may not last very long. The worst aspect of this is that they will draw more current, since their limiting resistor is sized to provide nominal operating current at 6V, and not at 12V.

As an example, I used a Radio Shack LED in my Elctron Bean controller, which needed a limiting resistor. For 6V, I used a 100 Ohm resistor (110 was best choice, but I only had 100 -- well within tolerance). For 12V, I would have needed a 470 ohm. This was based on a forward voltage of 3.2-3.8 V, and suggested operating current of 20mA.

I'd use the bayonet-base LED lamps in the future, but if you want to play with LEDs and resistors, check out www.ledcalc.com