The nixie tube clock is comprised of two circuit boards. The main board is populated with four IN-12B nixie tubes, an Atmel ATtiny26 microcontroller and associated circuitry, and some digital logic ICs to control output to the tubes. The secondary board is a simple high voltage power supply which provides power for the nixie tubes.
Main Circuit Design
The main circuit is comprised of an ATtiny26 microcontroller which is connected to a 32768 Hz quartz crystal for timekeeping. The microcontroller contains the necessary circuitry to drive the crystal and derive a stable input from it, which drives an internal timer. There are also two momentary push buttons, which, combined with the internal pullup resistors and software debounce, provides a way for the user to set the time.
Every time the display needs to be updated, the microcontroller feeds the new values to the nixie tubes via two 74HC164 serial-to-parallel shift registers. The output of the shift registers forms the input to the four K155ID1 nixie tube driver ICs, which are special purpose HV-tolerant BCD to decimal converters for nixie tube use.
The board also contains a 78L05 5 volt regulator in a TO-92 package, current limiting resistors for the nixie tube anodes, and decoupling capacitors for the ICs. Note also that the ATtiny's RESET pin is held to 5 volts through a resistor, allowing for in-circuit programming of the micrcontroller.
I bought IN-12B nixie tubes from an Eastern European supplier on eBay. They were sold in packs of six, giving me two extra tubes to keep around as replacements. These are "end-view" tubes, so the final configuration of the clock is best suited for hanging on the wall.
High Voltage Power Supply
WARNING: Do not build this at home. High voltage power supplies can be extremely dangerous and you risk death if you do this without training.
Here's the schematic I used for the high voltage power supply for the nixie tubes:
The high voltage power supply is a switching DC boost converter which converts the 12 volt DC input to the system into a much higher voltage for the nixie tubes. The design uses a 555 timer as a controller and was selected because I had the parts for it on-hand already.
Note that I can't recommend this circuit for any new projects. The 555 isn't really an ideal fit for this application, and switching controllers such as the Maxim MAX1771, TI TPS40210, or MC34063 are now available to hobbyists affordably from suppliers like DigiKey and Mouser.
Circuit Board Layout
Although the IN-12B tube footprint I used ended up being too small, and the tubes barely fit next to K155ID1s, I managed to fit everything in the end. I'll measure and create my own new footprint next time I design a board around these tubes.
Here's a photograph of the assembled nixie tube clock, displaying the time. The "5" digit in the IN-12B is, indeed, an upside down version of the "2" digit.