Ferrite is another Germanium Joule
Thief. Both of these artbots have a magnet on the bottom of the
battery holder and is used to attach to a disc of steel.
On the top of the head is a CDS photo resistor that causes the leds to
only come on in the dark.
Link to PDF of the schematic
showing the concept of adding a photo senser to the Joule Thief.