The Joule Thief is a device designed for two purposes:

1) To suck any and all traces of juice from your "dead" batteries

2) To make it possible for an LED with a voltage requirement of higher than 1.5 volts to run off of a single 1.5 (or less) volt battery! For example, run a white LED from a single AA battery!

There are other circuits out, for example, some are more efficient (but have many parts and are very expensive. Others that have a fewer parts, but are not very efficient or reliable.

This circuit was built to be a perfect balance of both worlds. Efficiency is around 70% with only 7 parts.

You can order the kit here: Joule Thief