50Hz AC is used as the timebase for a seconds display for the digital clock. The pulses are fed into a divide by 5 and a divide by 10 counter then into the counters driving the 7-segment LEDs. The switch select between the 50 pulse/second drive (up position) and the 1 pulse per second (down position). The latter is correct for the clock simulation but very slow to watch in real time.
Inspired by "EEVblog #801 - How To Design A Digital Clock", https://youtu.be/5kGhoRuhlxM
