Oscar built a DIY bike rear light to keep him safe and wrote a post on his blog detailing it’s assembly:
In this project I will be using a microcontroller to allow for switching between the various functions I programmed. The entire system is built on the backbone of an ATtiny4313 chip with a few other parts totalling at about $35. It might seem a little expensive, but this build covers a lot of ground. You not only get safety-centric feature, you also get a handy USB charging facility and programmable LED display.