Nixie Tube Clock
I've wanted to build a nixie tube clock for quite a while. I ran across pictures of one on the internet several years ago and have been planning my own ever since. A few weeks ago I found a set of six Soviet-made IN-14 tubes that had been salvaged from old equipment and were being sold for only $34, after shipping from Ukraine. When I first started planning this project they were selling for up to $100 for a similar set, so I jumped at the chance.
Disclaimer: Any use of this information is at your own risk. High voltages are present and will cause injury or property damage if mishandled.
What is a Nixie tube?
A Nixie tube is a small tube filled with (mostly) neon gas with a mesh anode and several metal-cutout cathodes shaped as various symbols. When a voltage is applied to the anode, the neon gas inside breaks down and begins to pass current. The charged neon gathers around the cathode and passes off its' electrons. As the electrons are released the neon gas becomes excited and glows.
High voltage is required to push electrons through the gaseous neon inside the nixie tubes. Typically 150-180V at around 2.5 mA. During testing I am using a switch-mode power supply built from a kit made by Lumos specifically for driving nixie tubes.
The Lumos SMPS works very well for driving nixie tubes and is very easy to assemble, but I want to keep the costs down as I may produce and sell these clocks in the future, so I'll be switching to a 555-based SMPS for the actual build.
For now I am only driving these tubes with 1.8 mA at 180V. I don't have the correct resistor value to drive them at the rated 2.5 mA and I don't want to use one that would overdrive the tubes. As a result some of the numbers don't light up all the way. I think this will be less of an issue with a correct resistor, but it's also possible that these tubes have a large number of hours of runtime as they are salvaged. As nixie tubes age they tend to become more dim and eventually sections or entire digits fail to light.
I am basing the clock around a bare-chips version of the Arduino Uno built around an Atmel ATMega 328P-PU. To control the digits I am using SN74HC595 shift registers connected to the ATMega via SPI. The 595s in turn drive K155ID1s, a Soviet-made knockoff of the 74000 series BCD-to-Decimal converters designed to drive nixie tubes.
Now that I have a working driver and control circuit I need to start working on adding a real time clock into the mix as the Arduino is not accurate at keeping time on its own. I plan to use a DS3234, temperature-adjusted SPI real time clock (RTC). While these are fairly expensive (around $10 per chip), they are also very accurate, within 64 seconds per year.
The project got shelved at this point for about 8 years give or take. I started designing the PCB in FreePCB, then switched to KiCAD and didn't really have the motivation to re-do all that work, but I needed a more modern and widely-supported PCB CAD sofware. In the end it worked out well, after anxiously sending all the various gerber files off to PCBWay expecting a message that this or that was out of spec and I needed to fix a handful of things, to my surprise they gave my first ever PCB the stamp of approval and moved it to the checkout.
For reference, to export gerber files from KiCAD to use with PCBWay, these are the settings:
On the PCBWay website:
KiCAD drill file generator:
KiCAD Gerber file generator:
Remember the COVID chip shortage? Yeah that also came to bite me in the rear. I had planned on making a batch of these for Christmas presents but the ATMega328 and DS3234 chips were backordered for around 6 months. Ordering nixie tubes has been a practical exercise in geopolitics also after Russia invaded Ukraine. Tubes that were supposed to ship from a former satellite state turned out to have an originating address inside Russia, and while I'm still waiting on them to arrive at the moment, I am not holding much hope. At least eBay customer service is pretty helpful when this sort of thing happens and should be able to refund me if I turn out to be correct and these are not allowed through customs, but we'll see. US based sellers are more expensive but offer faster and perhaps more importantly, safer shipping.
I also found a new high-voltage power supply module, straight from the Big Rock Candy Mountain via eBay. Super cheap as well as smaller and less noisy (physically, probably not electrically :P ) than the previous ones.
Less than two weeks after placing the PCBWay orders the boards arrived at my house. At least the $50 in shipping got them here quickly. After dealing with the zoomed-in CAD software for so long they seem tiny in comparison. Right now the prototype sports a 3D printed case to keep stray fingies un-zapped. The next task will be to create the moulding to cast a cement case.
