The Original Nixie Clock

I decided to use 4017 CMOS counters driving high voltage switching transistors, with the reset pins and carry pins configured appropriately. The resets are also connected together by diodes, with an AND function to reset the whole thing when "24" is shown in the hours.

The timebase uses an Arizona Microchip PIC16F84 and a 4MHZ crystal, as a kind of programmable divider, clocking the first 4017 once every minute.

The provision of the power supply is interesting, eventually I decided to buy two "Tesco Value" clock radios at £2.49 each (I don't know how they do it) in order to get 2 small mains transformers. Each transformer turned out to have a 12v centre-tapped winding, so a bridge rectifier, smoothing cap and 7805 provides the 5V rail, and the other transformer has its secondary winding connected to the first one - giving a safely isolated HV supply. A high voltage bridge rectifier and smoothing capacitor provides a supply of about 140V when loaded with the nixies (over 300V at no load).

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