IN-12A Nixie Tube Clock

Sep 7, 2020 · 3 min read · embedded projects featured ·

Nixie clock – front side

Back of the board (with all components soldered on)

Summary

I managed to complete my first major embedded development project: a nixie tube clock. A nixie tube is a display device invented in the 1950s that is typically used to show numerals. Nixie tubes illuminate the current digit using a bright glow discharge.

In this post I will go over the entire process I went through to create the clock, from initial hardware experimentation through programing the microcontroller.

Initial Hardware Experimentation

Lighting Up A Nixie Tube

Initial testing of a nixie tube. I was learning how to power on the IN-12A nixie tube, which requires a high voltage (170V) to function!

The first task to work through was figuring out how to light up one of the digits of a nixie tube. Unfortunately, a nixie tube isn’t as easy to work with as an LED; you can’t just connect a nixie tube to a 5V power supply with a resistor and expect it to work. Nixie tubes require high voltage to turn on. According to the datasheet of the IN-12A – the nixie tube i decided to use – the nixie tube requires 170 volts to turn on! For reference, a wall outlet in a North American home supplies 120 volts.

Here I am testing one of the dot indicators () that separate the hours from the minutes and the minutes from the seconds on the clock. You could see on the multimeter that the dot indicator is running at 166.8 volts. For comparison, wall outlets in North America run at 120 volts.

To supply the nixie tube with the required voltage, I bought a high voltage DC power supply designed for nixie tubes. The power supply can boost a 12V input up to 235V. When I was trying to first turn on a nixie tube, I did not have a 12V input for the high voltage power supply. I ended up using the 5V supply from a USB cable as an input to a boost converter outputting 12V. I then used this 12V output as the input to the high voltage power supply. I was able to to power on the nixie tube with 166.8V, slightly less than what the data sheet recommends. I decided to keep this voltage output to the nixie tubes for the final clock design. With resistance kept equal, a lower voltage means lower current running through the nixie tube. A lower current will extend the lifetime of the nixie tubes in the clock.

Initially I planned to use an Arduino Uno as the microcontroller for my clock. However, the relatively small amount of memory and lack of wifi capabilities led me to rule out using the Arduino Uno. I opted to use the ESP32 instead.

Extra Photos

I wanted to see what a nixie tube looked like when all the digits were turned on. From the front, all the digits blur together into a jumbled mess. In the front view picture below, I can barely make out the "3" and the "6" digits. From the side, the physical difference in location of each digit is very evident. The digits appear as a column of light with small gaps in between.