Nixie-clock using neon lamps as logic elements

[photo of my neon clock]
The above shows my home-built digital clock. It uses Nixie-tubes for readout. In contrast to most other nixie-clocks being built these days, my clock does not use any transistor or IC for driving the tubes. Instead, the driving logic is built from neon lamps, together with resistors, capacitors and silicon diodes.

The project started in 2002, when our university library was selling old outdated or otherwise superfluous books, and I very cheaply bought the book "Electronic Counting Circuits" by J.B. Dance, published in 1967, and apparently only ever lent three times by our library, all in 1973. It described how neon lamps can be used as logic elements in a ring counter, exploiting the fact that they need a higher voltage to ignite (the striking voltage) than to stay lit (the maintaining voltage):
[schematic from Dance's book]
Unfortunately, if one substitutes the neon bulbs that are available in electronics shops nowadays, the circuit doesn't work. Dance used lamps that were specifically manufactured for this type of application, with a large difference between their striking and maintaining voltages. Nowadays, such lamps are (presumably) no longer manufactured; the neon bulbs that are still available in shops are meant as indicator lamps, and have a much smaller difference between their striking and maintaining voltages. This required changing the circuit's resistor values, and makes its operation more critical; furthermore, the lamps need to be selected for matching characteristics.

This is one of the ring counters in my clock:
[photo of one ringcounter]
Four of these are used, to divide the 50 Hz from the mains power (see here for stability measurements) first by 10 (yielding 5 Hz), then by 5 (yielding 1 Hz, i.e., one pulse per second), then further by 10 and 6 to yield one pulse per minute. Note the paper labels still dangling at the cathode wires of the lamps: these are needed to look up the measured properties of each lamp.

Four more ring counters are used dividing by 10, 6, 10 and 3, to count the minutes, tens-of-minutes, hours and tens-of-hours and drive the Nixie tubes:
[photo of one ringcounter with LDRs]
The nixie tubes are driven through Light Dependent Resistors (LDRs): under the influence of the light from the neon lamp, their resistance lowers, connecting one nixie cathode to the negative power supply. In order for the LDR not to be influenced too much by ambient light, while still allowing the neon bulb to be visible, an optical attenuator and filter is used between them, consisting of a black cardboard disk with a small hole in it, and two layers of red foil, held together by glue and shrink tube:
[photo of one ringcounter with LDRs]

The ring counters are rather sensitive to ambient light: in complete darkness, they tend not to work. Even though there are always a few bulbs active (if only in the power supply, which is not shown in the photographs), my clock still needs a bit of external ambient light. I'm experimenting with blue LEDs for providing this extra ambient light. This seems to be quite effective: illuminated by just two blue leds, the clock ran perfectly one night long in otherwise complete darkness:
[photo of clock with 2 blue leds]
Note though that the blue in this photo is more intense than it looks like in reality: apparently the camera is more sensitive to this shade of blue than the human eye.

Some other things that I ran into while designing this clock:

The clock is now electrically functional, but still some work remains to be done. The power supply needs to be built tidily, the aligator clip test leads eliminated, and the whole thing put into a (transparent) enclosure for safety.

Movie and circuit diagram

A short movie (AVI format, 10 MB) of the clock in operation is available here.

Furthermore, the circuit diagram is available in a PDF file. This schematic diagram contains some extra explanation of how specific parts work. This diagram is meant to document and explain the details of my clock, and there will probably be some minor changes made in the future. The diagram is not meant as a complete basis for building another such clock; for example, while some of the resistor values are quite uncritical and determined by what I happened to have at hand, many depend critically on the characteristics of the neon lamps used. (Hopefully needless to say, any prospective builders should take proper safety precautions for working with the high voltages involved.)


Epilogue (written in 2017)

I wrote the above text in January 2007, after getting the clock working on the workbench. I proceeded by building an entirely transparent enclosure for it, hanging it on my livingroom wall, and enjoying using it to tell the time:

[photo of the clock hanging on the wall]

Unfortunately, it wasn't quite stable. Occasionally, the amplifier/buffer stages needed to be readjusted. The power supply neon bulbs started to have trouble starting reliably, so I replaced them by a circuit using a 150B2 stabilizer tube (which works on the same principle as regular neon bulbs, but is made for this purpose), eliminating the silly difference between the +151 and +157 V supply rails in the circuit.

But as time passed, it became harder and harder to keep the circuit working properly; even replacing neon bulbs which others with same striking and maintaining voltages, as once measured, didn't help anymore. Apparently, these bulbs continued aging, and more parameters than just those two voltages play a role. After one or two years or so, I had to admit defeat: the clock simply wasn't usable anymore...

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Copyright © 2007.
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