In order to have the counter work as a clock, it needs to count once every second - except:

  • At 59 seconds it needs to advance not one count, but 41 counts to get to the next minute
  • At 59 minutes and 59 seconds it needs to advance 4041 counts to get to the next hour
  • When the clock needs to either for to 00:00:00 (24 hour) or 1:00:00 (12 hour) the counter needs to reset to 00:00:00
  • If it is 12 hour mode, at 1:00 it needs to reset and then count to 10000 times

The software to do this is simple:

  1. Keep track of where the counter is
  2. Get the time
  3. Convert the time to an integer (2:45:02 => 24502)
  4. Find the difference of the current count and the current integer
  5. If the difference is positive, advance the counter that far
  6. If the difference is negative, reset the counter and then advance to where it needs to go

Advancing the counter may take more than a second, so it may need to advance again after getting to the current integer. But that make the display look cool! :)

The counter I have, General Radio 1192-B from 1972 (based on date codes on chips inside) has a beautiful Nixie-tube display, and the one I found has 2 of the optional extra digits, so has the perfect 6 needed for a clock.

One requirement for the clock is the reset. I guess all frequency counters have a reset button on the front. I have another counter, a Global Specialties "5001 Universal Counter Timer", but it lacks a remote count reset. Luckily the General Radio counter has Start and Stop inputs on the back. Only the Start input is needed to reset the count.

The Start input requires a 5v level to reset, so I used a simple level shifter using a 2n3904 transistor and 2 10k resistors.