You always hear that people talk about the weather. But it seems to us we see more clocks than we do weather stations. A case in point is [frank_scholl’s] clock made from an old hard drive. We found it interesting that the clock has no microcontroller at all. The custom PCB is all digital and uses the line frequency to drive counters which, in turn, drive the motors.
The one catch is that you have to have a hard drive that uses a very particular motor scheme for this to work. The platter rotation shows the hour and the head’s track position counts off the minutes from 0 to 59. Two buttons can speed up either rotation for the purpose of setting the clock. You can see it all in the video below.
The schematic is a little hard to follow since it doesn’t use conventional logic symbols, but you can probably figure it out. Obviously, it would need a little tweaking for 60 Hz and your motor connections are unlikely to be the same.
Even if you can’t find a hard drive that fits the bill, it would be easy enough to refit a modern drive with some motors. As cool as the logic circuit is, we would have been tempted to punt and do it with a microprocessor, too.