Introduction

Even if you don't know the name, Nixie Tubes are an iconic piece of display technology that most anyone can recognize. I've always found them quite fascinating and having just completed my Microcontroller Applications course, I wanted to challenge myself and try to make something that uses an MCU, without using the entire dev board.  

Design Requirements

I know already that I'm looking for the final design to meet the following criteria:

1. I want to understand everything going on in the circuit. 

This sounds kind of strange, but there's some really complicated power supply circuitry out there that is beyond my current understanding. I want something that I can point to each component and with confidence say This does this, and this does this, so a basic linear power supply it is.

2. Be as small as possible, and looking proportional to the size of the tubes.

This is completely subjective. Basically I want it to look good.

3. The PCB must be entirely designed by me, with no MCU dev boards.

Again, this is so I can learn how to design a PCB that uses an MCU without using the whole dev board. I want to figure out what exactly is needed to get the MCU to run. Some challenges this could cause are providing stable power, programming/debugging, and missing jumpers/connections that are needed for the MCU to behave as I expect. This will require digging through a lot of documentation.

4. Gotta count seconds

More numbers = more pretty

Initial Design

After digging through some Nixie tube forums online, I found that the HV5122PG-G would be appropriate for controlling the tubes with an MCU. This IC is just a 32-bit shift register with the outputs connected to the gates of high-voltage MOSFETs. This way, I can just serialize the time from the RTC module of an MCU. For the MCU itself I opted for the STM32G031K8T6. I've gotten very comfortable with STM32 microcontrollers as it's what we used in class and the dev board is less than $10 so I can afford to blow a few up if (when) I make mistakes.

I decided for a user interface a nice single-button design would be the best. It's easy to use and easy to design around, though this will be slightly more challenging to program. 

For the tubes themselves I decided to go with the IN-8-2 which would later become the IN-8 for reasons explained later. 

With all that set in place I started designing the schematic in KiCAD 

Schematic

Above is the initial electrical design. Let's break it down.

In addition to driving the tubes, the MCU also needs ~3 volts so I selected a component that can convert mains to that level. Yes this does sort of violate design requirement 1 but hey, I don't have all year to make this work, just gotta make compromises sometimes.

Testing the setup so far, I am able to drive and shift through each of the digits on a single tube as seen below.

If you look closely, you'll notice the tube color is slightly dark, and some of the segments are partially illuminated (2 and 9), this is because this test was done without a linear regulator. These tubes are sensitive to voltage drops and without a regulator in place, the voltage drops to about 140, which is enough to keep some of the smaller segments fully illuminated, but causes issues on larger segments with area that's further away from the center. After including the linear regulator, this issue is resolved.

Moving forward

This is mostly where the project currently stands, as of now. Because of tariffs on foreign imports, PCB prototyping and specialized components have become very expensive and are making it difficult to work on this project economically. Once I can afford a second round of PCBs, more components and new tubes, I will post an update.