Joule thief

If you have partially drained batteries, you may be interested in this idea. It can power LEDs from almost drained batteries, although with a bit of thought I am sure you can think of other uses.

This photo is on Wikipedia. The basic schematic is simple.

See https://en.wikipedia.org/wiki/Joule_thief .

Joule Thief circuit

This month's Practical Wireless makes reference to something called the Joule Thief circuit which allows a very nearly exhausted single cell battery to run a white LED. The article is in G3RJV's regular column. The nifty circuit only needs 4 parts. For a video describing how to build this, see this YouTube item. Another useful resource is http://www.emanator.demon.co.uk/bigclive/joule.htm from which this image was taken.