Here’s the Joule thief. It’s like an open PCB torch that will take either an AA or AAA battery in the holder and use whatever power is left in the battery to light a white LED. The amazing thing about this circuit is that it will run right down to about 0.35V if left running continuously, and will often provide a week of continuous low level light from a battery that would normally be considered dead. The level of light is initially quite high but gradually reduces as the battery voltage goes lower. However, it can still be used for reading in a dark room, even when the battery is almost completely drained. (via)