It's important to wear lights when you're out and about on your bike, but if you're the type who happens to forget to turn yours on (or off in the morning), Instructables user Solarcycle shows us how to build a light that turns on and off automatically.
You'll be building the light system from scratch here, so the parts list is long, but it's not a complicated system. The end result is a light that automatically turns on when it's dark -- which also includes when you go into tunnels or shaded areas. If your favourite bike path includes lots of shady corners, this is a simple way to make yourself seen.
Automatic Shadow Detection Bike Tail Light [Instructables]