Recently, a friend of mine had an issue with his car’s ECU and needed a frequency counting device.
A simple but beautifully accurate PWM frequency meter.
I needed a small, cheap frequency counter. It would be the frequency display of my planed signal generator.
Measure and record mains frequency on the SD card along with the timestamp.
AC Line frequency (50Hz / 60Hz) has small fluctuations in the short term, but is compensated daily resulting in a quite accurate reference.