onkelond's photostream

Obviously you'll need a LEGO minifig to mod, I selected a stormtrooper since the helmet makes it easy to hide the electronics.
The other items you'll need are:
30 cm of 0.15 mm isolated copper wire.
A 1k ohm SMD resistor.
A general purpose swiching SMD transistor.
A red SMD LED.
A small ferrite bead, 6 mm outer diameter.
A minature push button.
A pice of thin copper foil for battry terminals (I got this pice from a PCB).
A pice of thick paper to mount behind the LED in the helmet.
and finaly a small pice of flexible dark red plastic film to go in front of the LED.

Here you see the assembled joul thief.
The ferrit bead has been woundup with 0.15 mm copper wire. In the center of the bead you see the resistor and transister connected.
The two wires sticking out to the left is where the LED goes. The wires going up and down is where the push button and battery is connected

See my flickr page for more views on how I've performed this mod.
Here the minifig has been mounted with pressure applied to the push button to turn the LED on.

Here you see how the neck has been drilled out so that the battery and push button in the upper body can be connected with the electronics in the helmet

Here you see how the back has been prepared for insertion of a miniature push button in the top center of the upper body.