A little buzzer/led you can plug into an épée to detect hits without a scoring box, there's no grounding so it still triggers on hitting the opponents weapon but was more than good enough for practices.
It was very simple, using a 555 timer and took almost no time to make, but it was among the best returns of effort I put in vs value I got out of it. It also stands out as one of my few personal projects that was something physical since most of them are only software now, and more tools than things purely for fun.