Shown below are the two boards that make up the entire clock. The top board is the back/foil side of the nixie tube driver board. Each Nixie digit is directly driven via an MPSA92 high voltage NPN transistor. I used surface mount parts so that the entire driver circuitry would fit under the Nixie tubes.
The second board is the timebase and microcontroller board. Each Nixie digit is connected directly to a port pin of the ATMega165p. The approach I took was to make the PCB construction as simple as possible. That meant single sided boards and routing traces from port pins to the driver input pins along the edge of the board without any jumpers. The result is that the digit to port pin assignments are all over the place. However, the AVR microcontroller has instructions to address pins directly, so the firmware isn't particularly complex. The two boards connect together, one on top of the other and the result is a very compact design.
The Nixie tubes operate at +200V at about 2ma per digit. To provide this voltage from the low voltage source that drives the logic, I developed a switch mode power supply based on the ubiquitous 555 timer chip from ideas found on the web. My SMPS design differs from other 555 based versions in that I use an LMV431 shunt regulator instead of a NPN transistor, for tighter voltage regulation.
Not shown is the board that holds the mode and forward/back set buttons that are accessible from the rear of the main clock enclosure. That board also contains the +5V regulator needed to power the logic circutry.
