Steampunk Nixie tube clock: The electronics are up and running

September 09, 2016

Max The Magnificent-September 09, 2016

Good grief -- I can't believe how much time has passed since I last wrote about this project (see Home-made Nixie tube clock: Time is ticking). The problem is that there are so many fun things to do and so little time to do them all in.

Shortly after penning the aforementioned column, I got sucked more deeply into our Caveman Diorama development (see Caveman past meets post-apocalyptic future (and then things get complicated)). I'm happy to report that this little scamp is coming along in leaps and bounds; in fact, we've entered it into a modelling competition, which is set to take place on Saturday September 24, so you can bet that I'll be posting an update with pictures in the not-so-distant future, but we digress...

As you may recall from my previous writings on this topic, I was lured into the idea of building my own Nixie tube clock when I saw some of Paul Parry's amazing creations at Bad Dog Designs. At first I toyed with the idea of whipping up the electronics myself ("How hard could it be?" I thought, with the naiveté of youth). Fortunately, I ended up investing in a Spectrum Kit created by Pete Virica at PV Electronics and targeted at the uber-large Z568M Nixie tubes. The reason I say "fortunately" is that it turns out that there's a lot more to driving Nixie tubes than first meets the eye (we'll return to this topic later in this column).

Andy and Mandy Blackett of Engraving Studios have already fabricated three brass panels to my specifications. Meanwhile, my master carpenter chum Bob is working on the cabinet, which is going to look like a fine Steampunk-Victorian timepiece. The cream on the cake, of course, is my set of custom copper and bronze R|Z568M Nixie tubes, which were hand-crafted for me by Dalibor Farny.

Before we look at a video of the current state of my Nixie tube chronograph, I'd first like to show you a few of the many photographs Dalibor shared with me during the process of creating my tubes.

Cutting and polishing the copper anodes
(Click Here to see a larger image. Source: Dalibor Farny)

Shaping the copper anodes and adding the cathodes
(Click Here to see a larger image. Source: Dalibor Farny)

Adding the gasses and sealing the tubes
(Click Here to see a larger image. Source: Dalibor Farny)

Sealed tubes awaiting their bases
(Click Here to see a larger image. Source: Dalibor Farny)

Close-up look at a pre-base tube
(Click Here to see a larger image. Source: Dalibor Farny)

Preparing and polishing the bronze bases
(Click Here to see a larger image. Source: Dalibor Farny)

Adding the pins
(Click Here to see a larger image. Source: Dalibor Farny)

Ready to ship
(Click Here to see a larger image. Source: Dalibor Farny)

I'm ashamed to say that the tubes arrived from Dalibor a couple of months ago, but it was only this past holiday weekend that I finally got some time to sit down and solder everything together. The 40-page step-by-step assembly guide provided by Pete to accompany his Spectrum Kit is great, with failsafe checkpoints along the way, such as verifying and tuning the low-voltage and high-voltage (170V) portions of the circuit as they were implemented before proceeding to the next stage.

The image below shows the board sitting on my kitchen table performing the first tube test, in which all of the tubes cycle around displaying the digits 0 to 9. I have to say that it was a great feeling when I flicked the power switch and everything sprang into life.

Hurray, it works!
(Source: Max Maxfield)

As you can see in this video, there's a lot more to the clock than meets the eye, not the least that Pete has added all sorts of effects to his code. For example, when a digit has counted up to 9 and is poised to rollover back to 0, instead of simply transitioning directly to 0, it quickly flickers down through all of the other digits (8, 7, ... 2, 1, 0).

At the 50 second mark of each minute, the current time (hours, minutes, and seconds) is replaced by the date (month, day, and year). Instead of simply swapping one for the other, the new value scrolls in from the side. Also, once every 10 minutes at 10 seconds into the new minute, all of the digits go into a wild random "slot machine" mode.

I wasn't quite sure what this effect was doing, initially, but it turns out that Nixie tubes can suffer from something called cathode poisoning. The idea is that when one of the cathodes is on, microscopic particles of material "splutter" off the active cathode. These particles can land on and stick to the inactive cathodes, thereby degrading their ability to glow over time.

Consider the two right-hand ("seconds") tubes, for example. These typically count from 00 to 59, which means the left-most of these tubes only activates its 0, 1, 2, 3, 4, and 5 cathodes, leaving 6, 7, 8 and 9 unused. Similarly, in the case of the two left-hand ("hours") tubes, in 12-hour mode with no leading zero, the left-most tube will only ever have its 1 cathode activated for hours 10, 11, and 12. If we turn on leading zeros while remaining in 12-hour mode, then the left-most tube will only have its 0 and 1 cathodes activated. In the case of a leading zero and 24-hour mode, the left-most tube will only have its 0, 1, and 2 cathodes activated.

Thus, the trick of swapping between time and date and back again by scrolling in from the side, coupled with the occasional slot machine display, are all geared toward addressing and rectifying any cathode poisoning problems.

Last but certainly not least, I just discovered that -- until now -- I have completely misunderstood how Nixie tubes work. I never really gave this much thought before. I think that, at the back of my mind, I'd vaguely assumed that they contained a vacuum and that the cathodes acted like filaments in regular incandescent light bulbs.

I couldn’t have been more wrong if I'd tried. It turns out that the tubes are filled with a mixture of ~99% neon and ~1% argon with a smidgen of mercury thrown in for luck, all of this at approximately 1/100th atmospheric pressure (the exact mix and pressure is the result of a great deal of experimentation). When a cathode is activated, the electrons streaming off the negatively-charged cathode toward the positively-charged anode excite the gas molecules close to that cathode and turn them into a plasma, which explains how the active digits end up being a pleasing combination of "sharp" and "fuzzy" at the same time. And don’t even get me started on the fact that Nixie tubes exhibit negative resistance. As usual, I'm amazed by the number of things I don’t know.

So, that's where we are at the moment. When people come into our bay and see the Nixie tubes quietly glowing away, they invariably say that this looks amazingly cool. Even folks who have seen Nixie tubes in the past are impressed because most such tubes used to be quite small (typically less than 1" in diameter and less than 2" tall), so my monster R|Z568M Nixie tubes cause quite a stir. The next step will be to build the cabinet. I will, of course, report any ongoing developments. In the meantime, I look forward to seeing your questions and comments.

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