A Nixie Clock Project

Bringing Past Technology Back to Life

By Steve Dryja

What are Nixie Tubes?

Take a look at any digital display, say on a clock, stove, or media player. Most likely they'll have red or bluish-green digits. If they're red, LEDs are probably being used and if they're bluish-green, it's vacuum fluorescent display (VFD) that are behind the scenes. No matter the color, both of these display sources use "segments" in separate bars to make their displays so prominent.

Before LEDs and VFDs overtook displays in consumer products, there were Cold Cathode Numerical Display Tubes, or Nixie tubes. They were originally used primarily in measuring and scientific instruments like frequency counters, calculators, voltmeters and multimeters.

Nixie tubes are essentially glass tubes containing a wire-mesh anodes and shaped wire cathodes, filled with a low pressure gas. Although they resemble vacuum tubes, they do not operate like them. Instead collisions between atoms, electrons and ions are responsible for releasing a photon which produces an attractive neon orange glow.

The name Nixie is actually a brand name, it comes from "NIX I", an abbreviation for, "Numeric Indicator eXperimental No. 1", named by Burroughs Corporation, who were the first to introduce Nixie tubes back in 1955. "Nixie" came about accidentally, after "NIX I" was misinterpreted, but the name stuck.

My Nixie Clock Project

I fell in love with Nixie tube displays. ANYTHING with a Nixie tube gets my attention! I love that the digits appear to move when changing, giving a 3 dimensional look to the display. One day, I saw two clock-like displays up for auction on E-Bay. I didn't pay much attention to the details, the fact they were Nixie was all I needed to place my bet.

Once I got them home and took them apart, it was clear that they were just displays, not really a clock, since there was no logic inside to keep time. There were two large Cannon (military) connectors on the back, wired in parallel, which led me to believe that originally there was a very large cable with 46 wires. This cable would have carried the 5VDC "time" signals to the boards with transistors that are inside.

Nixie tubes run off of 170-300VDC and you need a transistor to sink the 18-22mA of current at that voltage to turn on the digits, plus you need a transistor for every digit. 5VDC is used to drive the transistor to its ON state in order to light a single digit. I confirmed this by applying AC to the displays and applied 5VDC to each digit to make sure all the Nixie tubes were good.
Nixie TubesNixie Tube

I am an Amateur Radio Operator (Ham) and we use Greenwich Mean Time (GMT) in our logs. We usually have two clocks, one displays GMT and the other local time in 24 hour military format. I thought, why not make the "clock" logic to drive the digits and make each of these into a standalone clock, one for GMT and one for local time?

Designing and Building My Nixie Project

Working with Multisim Blue, many parts, datasheets, and Eagle layout software, I came up with a design.

My project requirements were:
  • Standalone, no external timing
  • Use the 60Hz AC for clock timing
  • Circuit board had to fit inside the display case
  • 24 hour display only
  • Able to be set by hand
  • Cheat on 5 VDC by using a wallwort instead of designing a 5 VDC power supply
SchematicNixie Clock Project Circuit Diagram (click to enlarge)

I created a breadboard design using CD4017 decade counter chips since these came in DIP and SOIC packages. These chips were perfect since only one output is on with each clock cycle, and it had 10 outputs (one for each digit). Once I got the basic clock circuit working, I wired it to the Cannon connector, and it worked perfectly!

60Hz Pick Off Circuitry

I used Multisim Blue to proof out a design and then put it on a breadboard. The idea was to create a voltage divider and feed that into a CD4017. In the process, I blew up my Heathkit digital trainer board because the 120VAC wire popped out of the breadboard and touched the 5VDC supply wire, letting the magic smoke out of not only the trainer but every CD4017 on the breadboard! After re-ordering more CD4017s and replacing them, I was able to take on the task of the 60 Hz division.

I placed a CD4017 right after the 120VAC voltage divider and wired the now 2.5VAC (rms) to the clock end of the chip. Viewing the waveform revealed the output had some jitter and wasn't quite square. I did three things, added a diode after the voltage divider to create a DC voltage, changed the voltage divider to give a little more voltage and added a CD4018 Quad AND gate chip. I needed the AND gate for the 24 hour reset signal anyway and I decided to use one of the left over gates as a nice 5VDC buffer. I ran the now 3VDC half wave signal to both inputs of the gate. It worked! A nice clean 5VDC square wave came out of the CD4017. I tied the "6" digit output to the reset line which gave me a Divide By 6 signal and fed that to another CD4017. The Carry Out (CO) signal produces a Divide By 10, thus a perfect 1Hz signal. I would argue that the 60Hz signal is a good time base. There is an informative white paper on the accuracy of the 60Hz.

Setting the Circuit

You might think setting the circuit is pretty straightforward, but it's not since buttons have "bounce" which can cause extra transitions that are seen by the chips. I added multiple capacitors and resistors based on research regarding switch debouncing. I also added capacitors on each chip (5VDC) as close as I could get them to the chip, as recommended by many electrical engineers.

Getting a Custom Circuit Board

With every change in the design, I always update the schematic in Eagle and layout the board again. I always use Eagle's "Auto Route" feature to route the circuit board traces instead of doing it by hand. There were a couple wires I had to move by hand after the router was finished. Once I finally got the layout I wanted with the right size, I started looking for a board house to produce them for me. The first board house wanted $800 for 10 boards! Well, that was not going to fly with my wife, so I kept searching. My next attempt got me closer at $400, but was still too much. After searching and finding nothing really cheaper than $300, I put my project on the shelf.

I told everyone I knew about my clock project, and one day I found out about Seeed Studios. They allowed me to reduce the size and quantity of the boards, and the price reduced accordingly. I ordered 5 boards for $16; two boards were for the clocks, one was for "show and tell" and two in case I screwed up the design and need to hack up the boards with red wires. The boards are excellent quality and the best part, the design was PERFECT, no red wires were needed.

ClockNixie Tube Clock

Circuit BoardCustom PCB for Nixie Clock

The ClockNixie Clock Project Parts

Lessons Learned

This was my very first circuit board design and the only help I had was my research. I recommend that you do the design and put it on a breadboard to make sure things work the way you thought they would. It's also good to lay it down for a short while; sometimes you will get a really good idea by backing off the project for some time. I also learned that things don't always work the way they are designed, as was evident with the 60Hz to 1Hz circuitry.

When it comes to soldering, tin one pad of the SOIC chips and solder, and then do one other leg on the other side. This will allow a nice solder joint, and the chip and its legs will sit flat. Tinning all the pads first causes the chip caps and resistors to be up on one end and SOIC chips to not sit flat either.

I replaced the Military AC input connector to a typical "D" type computer AC cord, since they are so plentiful. Also, I added a switch to turn off the 250 volts to the Nixies so that if I am sleeping or not home, the Nixies don't need to be on, but the timing circuitry still runs.

I had great fun and enjoyed every minute of bringing these old units back to life. I learned a lot and hope this article inspires you to do some project that gives you great joy. If you lack the skills or knowledge of design, (I'm self-taught), try it anyway! Letting magic smoke out of things teaches you a lot fast!

How have you used electronics to liven up your holiday decorations? Share your stories at [email protected].
Steve Dryja is from Milwaukee, Wi. and is a consultant assigned to an Industrial Automation manufacturer testing their energy related firmware and web applications using LabVIEW and eggPlant. His hobbies include meteorology, electronic gadget design, and ham radio. He has been a Jameco customer for too many years to remember.