Russian Nixie Tube Based Clock(s)

There has been a recent surge of interest in Nixie tubes, which were the popular numeric indicators on test equipment and computers until LED's and LCD's became practical in the 1970's. They used stacked flat number-shaped electrodes to light the neon gas inside. You've seen them in the movie "The Matrix" although for some reason they were colored green, not the soft orange of a real Nixie tube. And real Nixie tubes don't have fully-formed alphabetic characters except for a few special-purpose models. Many hobbyists are building clocks with Nixie tubes, many very technologically advanced and artistic. I think the last Nixies were made in the US maybe 15 years ago or so, and are now fairly hard to find new. They still make tube equipment in Russia (although I don't think that even they make Nixies) and I obtained some IN-18 Nixies, which, besides being new, are pretty large and easy to read at a distance.

Figure 1. IN-18 Nixie clock.

Figure 2. Interior View. Note ribbon cable and wire-wrap construction.

Construction

The wire-wrapped PCB has a power and a microcontroller section. The microcontroller section isn't very groundbreaking or even economical, using a Ubicom SX-28 clocked at a crystal controlled 32.768 Khz. Three 74LS373 latches control six 74141 BCD to decimal decoders. In an ironic twist, my Nixie and crystal oven based Hewlett-Packard 5326B, 30 years old and still working fine, was used to trim the crystal via a trimmer capacitor.

The unit is powered by a 9V wall wart. The power supply's fairly sophisticated high-voltage section uses a MAX 1771 step-up switching regulator, coil, cap, and a power FET to produce a regulated 180 volts DC for the tubes. The low voltage is regulated with an LM323K (which I prefer although it's more expensive than a 7805, because it's beefy, even more so than a 7805 in a TO-3). There is a fairly unusual three volt vanadium pentoxide coin type rechargeable cell for battery backup, which an LM317 recharges. Time/date setting switches are not exposed because (hopefully) there will be little need to set them. The clock will only work properly until the year 2059 (leap years are not handled). I hope this will be a problem!

Even harder to get than Nixies are Nixie sockets, particularly for Russian tubes! The case is made out of stained oak; I measured the sockets as best I could and laid out a template for myself using Visual Basic and Windows Paint. I printed it and laid this on top of the box and marked the pin holes with a punch, and then drilled them with a 3/32" drill. RS-232 socket pins that I acquired (at criminal prices) from Digi-Key were hammered into the holes, and fit so well that only a dab of "thick" cyanoacrylic glue was needed on each terminal. I regret not using a drill press for the holes, even though I had one available. I damaged and ruined three tubes stupidly trying to force the pins into the (unopened) socket pins.

I used magnetic reed switches for adjustment of the clock, so the case has no holes other than for power; a piezo element adds a "tick" to the clock each second.

Unlike LED's, Nixie tubes have a finite life; the tubes were rated at 5000+ hours, but the Russians are known to spec very conservatively. I run them with 23.5K resistors, which is also very conservative. I hope that they will last for many years in moderate use.

Recently, I built a second unit, similar to the first. The case is slightly smaller .

Figure 3. New, Smaller Clock.