I'm working on a project. The Salient part here is a drive-shaft connected to a bearing, onto which a simple circuit is mounted. I want to switch the circuit on every $π$ and off for the other $π$ of a full $2π$. In other words, I want it on for every half rotation. I want to do this mechanically because non-mechanical solution seem like they might be hard to sync. Also, the solution needs to interfere with the machine's operation as little as possible. It is a complex machine that will be subjected to high moisture and so forth, so ideally, the switching system would entirely co-rotate with the shaft.
I don't know if there are any 'ready made solutions,' or not, but i'd be thankful to be pointed in the right direction (towards something plausible), or told I should wing it and create something from scratch (presumably because there isn't any solution that can simply be bought at a hardware shop).