Nixie watch part 1

Published 2018-11-09

Nixie tubes are fascinating things. Once very important for the electronics industry, they have become something of a historical curiosity.
I’ve stumbled across several articles and videos for a while now, and it was finally a video on the EEVBlog that made me purchase a set of six IN-17 Nixie tubes. These are smaller than the more popular IN-12 model.

When I received the small tubes, I decided to make a Nixie wristwatch.

After a while in development, I’ve now finished it!

How it was done

This is the first part in a series of posts detailing some of the steps that went into creating this wristwatch, mistakes and all.

Battery management

I wanted this wristwatch to be rechargeable with an integrated battery. I settled on an ultra-thin LiPo battery.

The battery needs a battery management circuit. There are several solutions for LiPo batteries, and I settled for the FAN5402, a relatively new battery management IC that fits well with my requirements. The downside with this component is that it’s in a tiny BGA package. The upside is that is has a very small footprint and requires few external components.

High voltage

Nixie tubes need to be driven at a relatively high voltage, about 170V. The LiPo battery voltage is around 4V, which means that we must multiply the voltage by a factor of more than 40.

Nixie tubes are very popular with electronics hobbyists, and people have created all kinds of voltage converters to achieve the required voltages. I found my solution in the “Typical Applications” section of the LT3580 datasheet, featuring an input voltage range of 3.3V to 5V and an output voltage of 350V (adjustable down to 170V by adjusting a resistor value).

The solution proposed in the LT3580 datasheet uses a discontinued miniature 1:10 transformer. I could not find any good replacement of the same form factor, so I decided to wind my own transformer using a toroid core and enameled copper wire.

To control the Nixie tube digits’ cathodes, I used the HV509 16-channel high voltage driver with push-pull outputs.


I wanted to leave open the ability for the watch to be able to communicate over USB to a host computer, so I chose a microprocessor with built-in USB support: the EFM8UB1 from Silicon Labs.

I thought about using the venerable ATMega328, but it’s relatively expensive and does not have hardware USB support. The EFM8UB1 also has a small form factor and features a built-in 3.3V regulator that proved to be useful.

Other chips

For the real-time clock chip I chose the MCP7940M for its low price and small form factor.

I wanted the wristwatch to be water-proof, so I avoided having any buttons on the case. Instead, I wanted to be able to control the watch by tapping on the case. I chose to include an accelerometer to detect movement and taps; the LIS3DE from STMicroelectronics.

Next steps

In the next few posts I will be describing the design, from 3D model using Blender to KiCad schematic and PCB layout and finally creating the housing using FreeCad. Soon..!


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