Mains power source is oscillating at 50Hz (or 60Hz in US). In fact this frequency fluctuates due to various reasons but biggest is the load on power lines. Due to load generators may increase or decrease speed which result in frequency changes. Fowkc had an idea to make mains frequency monitor. For this he’s chosen seven segment display modules and attached them to Attiny84 microcontroller.
The idea of capturing mains AC frequency was taken from previous project where he used AC wall adapter which in the end clamps signal to 5V supply rails. DS3231 RTC chip is used to get precise oscillator which is used as reference for counting pulses.