Biking at night is dangerous. Seriously. If you've ever taken a night ride, you've probably experience a few close encounters with a set of headlights. Which is why you use bike lights—to make yourself more visible. Problem is, unlike modern car lights, bike lights don't turn themselves off when you're done riding.
To solve this irritating issue, miceuz of WeMakeThings modified his bike light with a simple circuit that makes it turn off if the bike stays stationary for more than 30 seconds.
The circuit is just a small piece of copper clad board with a spring attached. The spring completes a timer circuit that charges the capacitor to turn on the LED.
When the bike is in motion, the movement causes the spring to bounce around, and when the circuit is closed, the MOSFET charges the capacitor, which keeps the LED lit up. As long as the movement causes the spring to close the circuit at least once per minute, the LED stays lit.
When the bike stops, the spring ceases movement and no longer closes the circuit, allowing the capacitor to lose its charge and turn off the LED. A cheap and easy fix for a common (and annoying) problem.
Check out miceuz's blog post for more details and a rough sketch of the circuit diagram.