Ok, so I know making a nixie clock isn't necessarily the most original of clock ideas, but I wanted something elegant to tell the time in my living room. When I made this I lived in a rented house where I couldn't mount a clock on the wall, so I needed it to stand on my (CRT!) telly. I'd been playing with nixies (see the name badge - I didn't take any pictures of it once it was assembled, but it had a 555 to switch between "Hi I'm" and "Simon") and I'd recently bought a 50s telly which inspired the styling: all curves and fillets. Add knowing someone who makes gorgeous guitars and has off cuts of some very nice sustainable woods and it almost designed itself.
I've been meaning to make an instructable on this for about 3 years. I didn't take quite enough photos on making it so sorry about that. Hopefully this should give you enough to inspire something similar though.
Step 1: Introducing Nixies
What are they?
Nixies are what was around to make numeric displays in scientific equipment back before LCD and LED displays were made. Technically Nixie was a brand name, but it has become the generic term for this type of indicator lamp. They are individual glass tubes with a number of shaped electrodes for each of the digits.
How do they work?
The numbers are illuminated by cold cathode emission, just like the neon indicator you get to say your 4-way mains extension is powered.The electrodes sit one in front of another and are pretty thin, so you can see the illuminated one from the front.
To achieve the cold cathode emission they run on over 150 volts. Once the gas inside starts to ionise its conductivity increases so a ballast resistor (similar to using LEDs) is used to limit the current and keep the glow rather than allowing an arc. PWMing them also works, although everything has to be happy with voltages higher than usual 5V electronics so high voltage transistors are required.
My Nixies are from Russia. The Soviet Union continued to produce Nixies long after they were obsolete elsewhere. Mine were made between 1975 and 1984, but others were made into the early nineties. The model is ИН-14 (IN-14 in Latin alphabet). These are one of the more affordable nixies, and even when they were made they were low cost... they cost reduced them by using an upside down 2 as a 5. I quite like this quirky look, although it clearly isn't to everyone's taste so these tubes are cheaper than others.
Step 2: The Base Design and Build
I was on a mission to make sure that for once things didn’t look shoddy, design was the starting point.
As I said, I was inspired by a 50s Bakelite TV I had recently bought, I used my then limited CAD skills to draw up the body form in Solidworks based on the dimensions of an off cut of beautiful wood (Padauk) offered to me by a friend who makes beautiful guitars (small plug for Organic Guitars…).
The body was lofted from three rounded rectangles (for each pair) to six circles, one for each tube. A cuboid extension was added to the back to house the electronics, which was filleted to an appropriate roundness. A cavity was added for the circuits to go in, with a cover to screw on the back. Holes were then put in for each nixie's wires, with a filleted counterbore to support the base of the tube. Two more holes were put in for the colon tubes, and the CAD was about done.
My friend then CNC milled the base for me so I can't really describe any hard graft here...
While he wasn't supposed to do more than that he also spent much of a weekend sanding and oiling it for me. Not only was I extremely grateful for him taking the time to do this for me, I was even more grateful when I discovered I'm allergic to the wood and doing this myself would have left my hands very red and uncomfortable!
Step 3: Electronics
My basic design principle is similar to the way people often drive small LED displays. I connect all the tubes' cathodes with all the same number cathodes from the other tubes. I then individually switch each tube's anode. By doing this I only need to control 10 cathodes and 6 anodes. By connecting one anode and the correct number cathode I can make any digit glow. Because once recently lit the tubes relight instantaneously, I can pulse through the digits quickly and brightly and persistence of vision makes it look like they are all lit at once.
The control of the entire clock is achieved using an Arduino. I initially breadboarded with an UNO, but I went for a Nano when I needed to fit it in the box. Everything was designed to fit the piece of stripboard I cut to fit in the box. I also soldered dusk till dawn to get it ready just before a Maker Faire...
The bit that is harder than LEDs is that any switching needs to be happy at around 170V, and that isn't Arduino territory! Each output needs high voltage transistors to do the switching.
For the cathodes this is easy as our Soviet friends also made (until 1992) a handy chip to drive their old school indicator lamps, the К155ИД (K155ID). This chip is a high voltage sink to pull down the selected cathode based on a low voltage binary coded decimal (BCD) input. You give the 4-bit number you want displayed and it pulls down the appropriate cathode.
For the anodes I used some 300V bipolar transistors I had lying around. To switch the anode a PNP is required, but to switch the PNP a high voltage sink is required, so I used an NPN to pull each of six PNPs down to control the anodes to select the active lamp. Current limiting resistors are required on the base pin of each transistor as well as on the anodes to control current through the nixies. I didn't write the values on my diagram (I probably worked them out as I went), but if you are less colour blind than me you can hopefully work it out from the 4th photo where there is a close up of the anode transistor array.
I used DIP sockets to connect one ribbon to the cathodes and one to the anodes.
Now there is control, all the electronics needed to illuminate the nixies is a high voltage supply. To make the project a little safer I used a low power 12V DC supply and a step up PSU to power the nixies. The 12V comes in a barrel connector in the back plate with a flying lead to the board. I have used a few different step up PSUs for nixies and I have not found any as good as the Taylor Electronics one. This works reliably and is very well designed. In practice I think this involved an extra resistor not drawn in the diagram to set the voltage, but it comes with good documentation to say how to do this. I set it to about 180V.
Having had one clock project ruined by the DS1307 RTC failing to keep time, I splashed out on the Sparkfun Dead On RTC. This is easy to implement with the RTClib library over SPI and keeps time exceptionally. I have not had to correct the clock in over 3 years! I added some code to the Arduino to automatically adjust for British Summer Time.
The colons on the clock are made from sub miniature mains neons connected directly to the power supply. Each neon has its own current limiting resistor in series. Resistor values need to be carefully chosen to avoid outshining the nixies. The legs are bent to structurally position the neon lamps. Each colon assembly is then carefully positioned in a mini test tube to fit in with the nixie styling. Clear silicon sealant applied at the opening stops them sliding out.
Step 4: Wiring in the Lamps
Another feature of the cheaper ИН-14s is the fact they have flying leads rather than sockets. This makes them difficult to mount neatly and very tiresome to replace. That said mine have been running for 3 years 24/7 without issue. The counterbore I added to the base covers any untidiness, but nothing can prevent the need to do 66 fiddly insulated solder joints to mount these nixies.
The anode is connected to one ribbon that is split into 6 strands, making it nice and easy to feed in through the hole in the wood. The cathode ribbon is much harder, since there are actually two ribbons, one from the previous tube and one to the next tube. These have to be folded to get through. Each of the solder joints needs to be made in turn, splicing ribbon two wires onto each cathode wire, while trying not to forget to put the little bit of heat shrink on first!
Feed the wires through, solder, shrink, solder, shrink, solder, swear, unsolder, put the forgotten bit of heat shrink on, solder, shrink... etc.
When I was done I used clear silicone sealant to mount the tubes, using a block of wood with parallel holes drilled to hold them straight while it cured.
Step 5: Finished
Fitting the electronics into the back, screwing on the back cover, and giving the wood a final polish are the last steps to complete a beautiful glowy nixie clock.
If you have enjoyed this instructable and like the clock please vote for me in the Time and Make It Glow contests