Most of Nixie clock designs end up with at least four indicators that display time at once. More Nixie indicators require more space and power. But pinomelan decided that single digit IN-12 Nixie indicator is more than enough for building small portable digital clock.
He put small PCB with PIC16F84A which drives single Nixie with discrete transistor. Nixie tube needs high voltage to light up, so he used HV supply from disposable camera. Clock displays time by periodically flashing all digits in series. During display each digit can be set up by pressing button at the right moment. Nixie clock along with battery could fit in to 35mm cube enclosure if needed.