When your batteries are too weak to power your digital camera (or flash/game device/etc.) that doesn’t mean they’re totally dead. Make a Joule Thief to use every last bit of stored energy.
A Joule Thief is a simple circuit designed to draw the last bit of power out of a battery using a low load device like a low-voltage LED. Instructables user 1Up got the idea from Make magazine and then tinkered with and refined the idea. His design doesn’t include an enclosure but it’s a great start to a larger project–perhaps a “dead” battery powered nightlight?
Make a Joule Thief [Instructables]