Nixie-clock using neon lamps as logic elements (2007) | Hacker News Reader

Nixie-clock using neon lamps as logic elements (2007)

pa3fwm.nl

119 pointsIvoah16 comments

The "integrated" form of that is the Dekatron.[1] That's essentially 10 neon bulbs in one tube, with the electrodes arranged so they act as a counter.

Digital counting took too many parts before transistors. The Dekatron was one attempt to get the parts count down.

[1] https://www.youtube.com/watch?v=o3HLZOJKEQU

I see this clock on Copenhagen Main Station fairly often [1], made with giant neon tubes.

It looks fairly old. Does anyone know how it originally worked, and if it still works that way?

(My searching skills aren't good enough in Danish, but there are plenty of Danish speakers on HN.)

[1] https://1684654000.rsc.cdn77.org/sites/default/files/public/... from https://www.magasinetkbh.dk/indhold/neon-bog

There's surprisingly little information about it. All I can find is that it was originally build in 1950 and was renovated in 2008, and it used to use relays, which were broken for some time.

http://www.aok.dk/byliv/vintertid-koebenhavn-i-en-klokkestre...

http://www.jernbaneklub.dk/jernbanen/2008/jernbanen_nr4-augu...

This is cool as hell. Shame to hear it's no longer working.

Woz is really well known for his interest in nixie clocks: https://www.youtube.com/watch?v=m4R3hODnTGo

Could you have used the individual Nixie digit tubes themselves as the counting state tubes?

More good work by radio amateurs. PA3FWM is a ham.

Even given how unreliable this ended up being over time the very fact that it was built in the first place is remarkably impressive.

So, looking at Wikipedia and comparing with his circuit PDF and what he said, it sounds like there wasn't thought given to current stability. Apparently these bulbs are very sensitive to too much current, so it sounds as if he was literally breaking the bulbs to get it to work.

I wonder then if there might have been some way to control for this properly?

You have to match each bulb with a resistor as each bulb has slightly different requirements.

Edit: The properties change over the lifetime of the bulb in a non-linear way to make things even harder.

Why not use a constant current supply instead? Inline resistors are a very primitive way of limiting current.

You would need one for each bulb as the problem is that the bulbs characteristics are not tightly controlled like they are for vacuum tubes and vary widely even in bulbs from the same lot.

Circuit for multiple output current mirror [0] is very simple but then you start using transistors to run the neons and I think not using transistors was point of the project

[0]: https://electronics.stackexchange.com/questions/256530/curre...

Neon bulbs require a certain voltage to strike (for small ones, several 10's of volts). Until they are struck they are very high resistance, once alight they are low resistance until the voltage drops well below the striking voltage.

This leads to an easy way to make a neon oscillator; put a capacitor across the bulb, and a high value resistor in series to a 90v supply. The capacitor charges to the strike voltage, then discharges until the neon extinguishes; the oscillation rate and ratio characterises the particular bulb.

Don't know if it's still true, but they used to include radioactive tracers in the neon to reduce the striking voltage (also I believe the striking voltage varies with incident illumination).

> I believe the striking voltage varies with incident illumination

That is the principle behind this entry for the flashing light prize:

https://www.youtube.com/watch?v=h8VJft5Xq5g

Thanks for that! Great demo, and using strong UV shows the effect in spades (I'd guess in this case you wouldn't want the radioactivity, so the illumination effect from the photons is more pronounced).

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