“This Arduino based digital clock is synchronized by the 60Hz power line. It has a simple and inexpensive common anode 4 digit 7 segment display which shows hours and minutes. It uses a cross over detector to detect when the incoming 60Hz sine wave crosses the zero voltage point and derives a 60 Hz square wave.
Over short time periods the frequency of the incoming sine wave from the power line may vary very slightly due to load, but over long periods of time it averages to 60Hz very precisely. We can take advantage of this to derive a timing source to synchronize our clock.”