This is a pretty cool device that is designed to steal every last drop of energy from a power source such as an old battery and use it to light the LED. The led I used is from a grow-light I had that got killed by a power surge. Some of the LEDs from the grow-light were still good so I used one of them. I made this joule thief a few years ago and it has been running off of a small lithium battery and a solar cell ever since. It hangs from a string in my indoor greenhouse lighting plants on the bottom shelf.