Step 5: Picking a re-used old crappy bike light & removing its guts!
i recommend using AA batteries so that you can have full brightness and still get decent battery life, however it is getting hard to find AA bike light cases these days. if you use AAA, you will get 4-5 hours for a rear-only light, 2 to 2.5 hours for the dual light. so the AAA build-option includes resistors to reduce the brightness a bit and increase battery life, but if you are ok with the short life you can still get full brightness with AAA's.
you will need a bike light that has an easy pop-off cover exposing the battery connections - virtually all of them are like this.
start by opening it up and removing the guts - including desoldering the battery wires.
now you can see how much space is inside the case and figure out where you will be mounting the control circuit you will be building. all my cases were too small to fit the circuit, so probably yours will be too. that doesn't make it any harder to build, it just means we'll be gluing the circuit to the outside of the case where you can show it to your friends more easily.