Reelights, http://www.reelight.com/ Most Basic they flash the LED with every time
the magnet and sensor rotate past each other , as the wheel Rotates..

further up the offerings, they add capacitors to make a steady on, light.

happen to like lights that the motion itself makes the power for the light.



personally, I invested in a Dynohub and LED head and taillights.. German made stuff.