Being seen when you're biking is incredibly important, and we've shown off many ways to add signals to your bike. This method is easy, runs for ages on a single nine-volt battery, and is a perfect primer for electronics projects.
The finished product is extremely long-lasting (the circuit is completely off when not in use, so the battery doesn't slowly discharge), it's small and light so it doesn't get in the way while you bike, and it's super-bright.
Instead of an Arduino or another microcontroller, Tom decided to wire this up from scratch, with a collection of wires, resistors, diodes and capacitors. Check out the full build -- including the parts list and the circuit diagrams you'll need to replicate it yourself -- at the link below.