AM/FM Nixie Clock Radio

Overview

These notes relate to my (multi-year) Nixie tube clock project. This project has gone through many revision stages, so much so that this clock is probably completely unrecognizable from my first version apart from the usage of the distinctively beautiful Nixie tubes.

What are Nixie Tubes?

Nixie tubes are soviet era display devices that can display numerous different symbols, but they’re most commonly used to display digits 0-9 (and sometimes a decimal place).

Nixie tubes come in all shapes and sizes but they all work on the same fascinating principle, cold-cathode gas discharge. It sounds very technical and physics heavy (and it is), but essentially Nixie tubes contain a metal grid which is used as an anode, and separated cut digits which are used as cathodes. The anode is powered to approximately +180V DC, and the cathodes are left floating to display nothing.

The tubes themselves are evacuated (contain a vacuum) from air, and contain a small amount of neon gas. Other noble gases can be used for other colors and different operating parameters such as striking voltage, but the soviets decided on neon due to their distinctive bright orange glow.

When the cathode digit is grounded, the gas around the cathode digit ignites, and begins to glow. Nixie tubes can also experience cathode poisoning, which is where digits are displayed for too long and they prevent other digits from displaying as brightly, as such they need to be cycled or multiplexed to prevent the digits from being on for prolonged periods of time.

Nixie tubes also come in a wide range of shapes and sizes, my personal favorite being the IN-12 Nixie tubes since they can be used for a much more compact clock package.

For more information about how Nixie tubes work, I’m exploring this in my Open Nixie Project, where I aim to document and replicate the Nixie tube manufacturing process.

Why I’m Making a Nixie Tube Clock (and why you should too)

Apart from the fact that they appear to be the main use of Nixie tubes in the modern era, there are many reasons why building a Nixie tube clock is a great project. Building a Nixie tube clock is by no means a beginner project (I should know, I started making my first clock as a beginner, not knowing how deep the rabbit hole went), but they’re a great way to learn about many different aspects of circuit building.

My clock includes quite a few features which “standard” Nixie tubes don’t include, mainly because I wanted to challenge myself. I decided to include an ESP32 for Bluetooth functionality, as well as a dedicated DAC and amplifier since the ESP32’s included DAC isn’t the highest quality. I also included a MicroSD card reader to store songs and user settings offline, and an AM/FM radio for an authentic retro radio experience.

To drive the Nixie tubes I could have used a modern IC such as the HV5222 shift register, but I decided against it for a few reasons. With 6 IN-12 Nixie tubes, 2 of the HV5222 shift registers are required, and they aren’t cheap. For that reason I decided to use the K155ID1, the very same IC used to drive Nixie tubes by the soviets. They however are also limited, most circuits use one of these ICs per Nixie tubes which is obviously a lot of routing complexity for 6 Nixie tubes, so instead I decided to take a more modern approach and multiplex the Nixie tubes, using the K155ID1 IC to display digits, and using the ESP32 to cycle through turning each digit on or off (by enabling or disabling the anode) very fast to make it look like they’re all on at the same time.

As you can tell from my decisions in this project, there’s many things to learn about circuit building apart from just making a clock that looks beautiful. It’s also an involved programming challenge, where you’ll have to manage binary coded decimal to decimal conversion, as well as I2C for something like an RTC clock module to keep track of the time accurately. In my clock in particular, I’ve had to learn about PCB routing standards, separating high voltage traces from other sensitive signals, PCB layout, careful component selection, and reading datasheets to implement complex functionality.

Project Funding

To fund my Nixie tube clock I will be crowdfunding, I’ve got all the designs ready to order, and the case ready to manufacture, I just need to fund it somehow. I’ve decided on crowdfunding to raise the initial capital, and I will be using the funds from the clocks I sell to fund the Open Nixie Project, where I aim to produce my own Nixie tubes to replace the decreasing supply of Nixie tubes, as well as producing new unique designs.

If you want to get involved, please reach out to me and let me know!

Research Notes