i wonder how feasible it would be to make one without a microcontroller? 555 timer with a potentiometer to adjust the led blink speed. dial + arrow on the potentiometer to read the rpms.

i’m not sure what the error tolerances would be like for the blink rate after making the circuit, so maybe calibrate the dial from a known rpm source?

i saw something like this at a music shop. it was a little keychain sized guitar tuner (looked like one of the little led flashlight deals). i don’t play guitar, so i haven’t used it, but i’m guessing you’d select a note on it somehow, it would blink at the same frequency as that note would in “average*” air, hold it near the guitar string and tune until the string looks stationary.

*i’m guessing different air temperatures / humidities would alter slightly the frequency at which a note sounds “in tune”?

anyway, i like this kind of project. simple idea, polished execution, many applications.