I wanted to build nixie clock, but the problem was - I had no nixie tubes! I stayed at the idea with neon, but I solved it completely different way.
As a displaying device, I used a giant 4-digit 7-segment display consisting of 55 neon glow lamps.
Digit height is almost 9cm (3.5''). Clock runs in 24-hour rhythm, each segment is formed by
2 neon glow lamps. The control circuit I got from an old useless alarm radio clock.
The heart of the clock is LM8560 circuit, clock frequency is controlled by the mains. The display is controlled by the weird multiplex - the duplex, over 13 high voltage transistors. Duplex is controlled by mains half periods - in one half cycle half of segments lights and in the other half cycle the other half of segments. In the same way it was made in a clock radio, from which I used the IC. Circuit initially enabled and an alarm clock, time off radio, radio alarm clock and backup battery, but I did not use these functions. 7.5V transformer I got from the same alarm clock, 200V transformer I wound myself. I salvaged the IC with a piece of PCB, in which I left only one diode, resistor and capacitor. Setting the clock is simple - buttons for hours and minutes. Clock is quite accurate, because nowadays the mains frequency has very small deviation (all Europe runs synchronously) plus, positive deviations compensate with the negative. Note: In the schematic the 50/60Hz selector is set to 50Hz. Disconnect pin 26 to set 60Hz operation.