26 November 2022

The 4-weeks challenge: deliver a Nixie clock from scratch

Last weekend of October a curious teenager guest noticed the Nixie clock in our living room. The most eye-catching element was the short depoisoning routine that runs once a minute. B-5750 Nixies have their numbers 0-to-9 back-to-front, which is an even more eye-catching feature.

So I quickly explained him and his family some basics of these lovely tubes, showed them my lab/workshop/messy room and of course I told them I had designed and built that clock.

The next day I came up with a self-challenge: design and build a fully functional Nixie clock that I could give the teen at our next (and probably last for a very long time) meeting four weeks ahead.

With a small personal "library" of hardware design and AVR firmware I was quite sure I could get over all the small challenges inside this project:

  • design a working circuit and receive the PCB from the fabhouse
    • sub-task: fix the KiCAD footprint for the chosen ZM1020 Nixie
  • design a suitable case and get it 3D printed (this then turned to laser cut)
  • build it without easy access to my lab for the same period of time
  • write a user's manual

The PCB was designed with KiCAD and etched by JLCPCB. To save on board space and stay into the chosen round design I used a bare AVR ATmega uC with its internal 8 MHz clock; it also drives the voltage booster. On the board I included an ICSP header which is very useful if you're debugging or adjusting the firmware code. I used a ZM1020 round top view tube

While waiting for PCBs to arrive I learned how to use OpenSCAD to design the case, which is plain geometry applied to programming. I also risked to have no AVR microcontroller for the clock as the ATmega48 doesn't have enough program memory (or I would have needed to do heavy optimizations). I managed to unbrick a couple of ATmega168 that are enough for my firmware (wrong fuse settings).

The circuit diagram was correct. The PCB was suitable, meaning I kept HV and LV separated enough. The laser cut case (two plates) was perfect and I picked it at 3.5 weeks into the challenge. In the last week I did some firmware corrections and upgrades and today the clock has been delivered to a happy teenager.

Here it is!

Single Digit Nixie Clock with ZM1020
Single Digit Nixie Clock with ZM1020