This automatic bicycle light from recycled and found parts is about the most eco-friendly DIY bike light in the world. It uses an LED placed inside a plastic bottle cap, which activates when a magnet mounted on the wheel passes a sensor, similar to how a bicycle computer works.

If the LED is bright, it will shine through the cap, so choose an orange or red cap for a tail light.

[youtube:https://www.youtube.com/watch?v=iZGU5v8QSlw]