The washrooms at my boarding school had the lights on all day. Energy-wise this was absurd, and I’d been sitting with a pile of Arduino Pro Micros, PIR sensors, and relays during vacations, looking for something to put them to use on. I proposed to the school that I retrofit the washrooms for motion-activated lighting. They agreed.
The build
Every module was a Pro Micro wired to a PIR sensor and a relay that sat in series with the washroom light. When someone walked in, the PIR triggered; the relay closed; the light came on; a configurable delay kept it on for a few minutes after the last motion event. Nothing exotic — the same logic that any off-the-shelf motion-light product runs — but built from discrete parts because that was what I had.
The constraint that made it interesting
Soldering irons weren’t allowed in the boarding dorms. So the build happened in hidden corners and stretched vacations, and every module had to be assembled into final form before I could bring it onto campus. That also meant there was no iteration in the field — if a module failed after install, I had to pull it, take it home on a vacation, fix it, and return it. The designs got conservative about failure modes in a way that would have been overkill for a normal hobby project.
Status
Installed. Ran. My first piece of engineering that existed in the world outside my head, used by people who didn’t know me.