A Joule Thief is a simple circuit which can use "dead" batteries to run an LED light or other things. I show you how to make one here: http://www.thedoityourselfworld.com/articles/article.php?id=11461
All you need to make a Joule Thief is a 1K Ohm resistor, a 2N2222 or other PNP transistor, a white LED light, a torroid form and some wire.
The Joule Thief is able to power an LED light way below the voltage when it is considered dead and useless for other projects.
This particular Joule Thief can use a dead 1.5 volt AA or AAA flashlight battery down to 0.37 volts. This is quite amazing considering that a white LED requires about 3.2 volts to light up.
The Joule Thief steps up the voltage while reducing current in order to drive the LED.
Now with this circuit you can use up those old flashlight batteries.
But dont throw them away just yet. I have a project coming up for those fully drained batteries so stay tuned.