I've been working on some electronics stuff more recently, and I designed custom PCBs which read the voltage and internal resistance of the 12V NiMH batteries used by robots in FIRST Tech Challenge. It's not always easy to know how charged or healthy the batteries are for a match, or when it's time to get rid of them. They're pretty small and are easy to assemble. Rather that having a screen (which would take precious flash and pins on the ATTiny412), it just uses a WS2812B RGB LED.

I wrote the firmware in Arduino, which was a great learning experience because I typically work with CircuitPython or Go, where I'm less constrained.