The clock uses the 50Hz mains signal to keep time. Therefore, we need some kind of circuitry to count the pulses, and convert them into minutes and hours.
For this I am using a series of CMOS logic 4017 Decade Counters. These chips basically count from 0 to 9, incrementing at each pulse on the Clock pin, and output a pulse once per cycle. However, they can be forced to have a shorter cycle, e.g. 0 to 5.
Therefore, to convert a 50Hz pulse into a 1Hz pulse, I pass it first through a decade counter from 0 to 9, producing a 5Hz pulse, and then pass that through a decade counter from 0 to 5, producing a 1Hz pulse.
This process can be repeated until I have a pulses with period 1 minute, 10 minutes, 1 hour and 10 hours. The counters generating each of these pulses can then drive the Nixie tubes.
This could all be done on a micro-controller very easily, and this would reduce the chip count from about 9 chips to 2 chips, however that is a bit boring and this way I learn more about digital logic. The result of this is some pretty complex wiring.
My original plan was to use my CNC to produce a single PCB for the logic board, however I found that the schematic was so complex that the board would either have to be double sided, or the traces be very thin. Both of these options are not feasible on my CNC, so the possibility of having a PCB was out the window.
Making the circuit on stripboard is much more fiddly and slow, with every connection having to be wired individually, however it actually worked out relatively compact and I’m happy with the results. Of course, if I wanted to produce multiple boards, it would make much more sense to design a PCB and get it made professionally.