Most of us would reach for a microcontroller and call it done, so it is refreshing to see a clock that gets its timekeeping pulse by dividing the 60Hz from the wall down to 1Hz with nothing but 8-bit counters and AND gates. The maker built this one as a work in progress, and it is headed for the shop wall once finished.
The signal chain is where the care shows. A divider taps the line side of a bridge rectifier, feeding an optocoupler for isolation, and a parallel SMPS supplies 5V for the logic and displays. A chain of op amps shapes the half sine coming off the opto's NPN side into a square wave, with rising edge detection to strip out noise. From there a decade counter's carry pin pushes 6Hz into a binary counter, and an AND gate resets it every six counts to yield the 1Hz tick. The seconds, minutes and hours are counted by cascaded 8-bit counters handing off down the line. My favourite detail is the roughly 5ms RC delay on the reset signals, which keeps the handoff between counters solid.
Time setting is handled mechanically: a DPDT slide switch pauses the seconds counters and enables the set circuitry, while two momentary buttons advance the minutes and hours, debounced by a dual op amp in a Schmitt trigger arrangement.
It is not all done yet. The hours display is still unwired pending more resistors, the housing is undecided, and true drift will only show up after a long run. Short tests so far suggest only a few milliseconds of drift per day.