I'm in the process of building a brake light for my bike, which I would say is 85% complete now.
In Sweden there is a popular type of old military bikes called Kronan (see attached picture) that have a metal cylinder intended to house a small bicycle pump. That's where I'm going to put the battery+brake light assembly.
However, I just realized that maybe I could use this to help me dark winter mornings, when I'm looking for my bike at the bus station. Is there any good communications system that is pretty much always on, yet doesn't require much energy?
Either I have a separate small device with a button for when I want to turn on the brake light. I would like to be able to use my smartphone for it. But I guess that limits it to Bluetooth which, as far as I know, isn't very energy saving.
My idea is that I get off the bus in the morning, turn on the light by remote, find it, turn off the light, and then bike to university (with the accelerometer functionality turning on the light when I'm braking).