Once I had finished the HV supply, I couldn’t just settle for a static number. Before I called it a night I had to make the numbers change.

Given that I have a tray of K155ID1 driver ICs switching the HV is pretty simple. The K155ID1 (SN74141) is a BCD to Decimal decoder and display driver designed for nixie tubes. The HV is supplied to the nixie anode via a 7.5kΩ current limiting resistor and each of the ten numeral shaped cathodes are connected to the ‘outputs’ of the K155ID1.

The K155ID1 is driven by a MC14510, a 4000 series pre-settable up/down BCD counter. The clock is provided by a small clock pulse generator my Dad built 20 years or so ago.

The reader must excuse the untidy breadboard prototype, in my defense it was late. The isolated HV supply can be seen in the background and the aforementioned clock generator to the right.

By the time I called it a night, or should that be ‘called it a day’ given that it was 1AM? I digress. By the time I called it a night I had a single nixie that counted from 0 – 9 and started again. A single counting nixie wasn’t enough though; I was already formulating a plan to have a larger counter. This four digit counter would prepare and test the prototype display driver that would be used when I finally build the prototype clock circuitry.