A Nixie clock does not light up. It ionises. Inside every tube sits a stack of wire cathodes, each one bent into the shape of a numeral and suspended in a low pressure neon and argon Penning mixture behind a fine mesh anode. Apply roughly 170 volts across the gap and the field strips electrons from the gas. The discharge collapses onto whichever cathode is pulled to ground, wrapping that single digit in a sheath of glowing plasma sitting a fraction of a millimetre off the metal. The colour is not a coating or a filter. It is neon relaxing back towards its ground state, radiating on characteristic lines between roughly 585 and 640 nanometres. No pixel grid. No backlight. Just excited gas doing what excited gas does. Building one is a genuine high voltage exercise: - Boost stage: a switching converter lifts 5 or 12 V DC up to 170 to 200 V, classically an MC34063 or a 555 driving an inductor and a fast diode - Anode resistor: a series resistor on each tube limits current to a few mill...