|Originally posted by sonichead
...I found a really cool green one that's clear until you give it current, then goes green, also it's low current and 5mm!
Those ones are cool. I used a blue one for a guitar amp a while ago. There's a nice texture to the LED when you look at it. Unfortunately, it has a 5V forward voltage, so it will probably go out before the battery's dead. What's the forward voltage on the one you found, and where did you find it?
Since LEDs are spec'd with forward voltage, you just need a number lower than your minimum voltage. For a single 9V battery, which will drop to 4-5V before it's dead, you need a forward voltage less than 4V. Significantly less, hopefully. Most LEDs have a forward voltage around 2V, which is "right" for just about any battery-powered circuit you'll find around here.