Originally Posted by rusty_2000

Me and a mate set up brake lights on his bike when we were kids. The difference was that it was off, then when you braked, the light when on. I'm sure it could be modified.

The design made a circuit thruogh the light, which incorporated the brake lever. When the brakes were applied, the movement completed the circuit and the light went on. Real simple stuff, but worked pretty well.

No practical use, but when you are the only 9 year old with brake lights on your bike, you look pretty cool (well, sorta!)