Dekatron Clock Tells The Time, Sans Semiconductors

A dekatron-based clock with a GPS receiver and a plastic dinosaur on top

Over the years, there have been several memory and display technologies that served a particular niche for a while, only to be replaced and forgotten when a more suitable technology came along. One of those was the dekatron: a combination memory and display tube that saw some use in the 1950s and ’60s but became obsolete soon after. Their retro design and combined memory/display functionality make them excellent components for today’s clock hackers however, as [grobinson6000] demonstrates in his Dekaclock project.

A dekatron tube is basically a neon tube with ten cathodes arranged in a circle. Only one of them is illuminated at any time, and you can make the tube jump to the next cathode by applying pulses to its pins. The Dekaclock uses the 50 Hz mains frequency to generate 20 ms pulses in one tube; when it reaches 100 ms, it triggers the next tube that counts hundreds of ms, which triggers another one that counts seconds, and so on with minutes and hours.

The Dekaclock uses no semiconductors at all: the entire system is built from glass tubes and passive components. However, [grobinson6000] also built an auxiliary system, full of semiconductors, that makes the clock a bit easier to use. It sits on top of the Dekaclock and automatically sets the correct time using a GPS receiver. It also keeps track of the time displayed by the dekatrons, and tells you how far they have drifted from their initial setting.

Both systems are housed in sleek wooden cases that perfectly fit the tubes’ retro aesthetic. [grobinson6000] was inspired to make the Dekaclock after watching another dekatron clock we featured earlier, and designed the GPS receiver to work alongside it. Dekatrons are surprisingly versatile devices: you can use them to make anything from internet speed gauges to kitchen timers.

19 thoughts on “Dekatron Clock Tells The Time, Sans Semiconductors

  1. it sounds pretty neat but, just as a matter of semantics, isn’t a dekatron tube a semiconductor? it conducts or doesn’t conduct in response to a control signal, which i thought was the definition

      1. aren’t they? i’m surprised that this isn’t the common usage. i always figured relays, tubes, and transistors existed on a continuum of different kinds of semiconductors. but wikipedia, at least, says semiconductor only refers to “solid-state” semiconductors. i guess because the underlying motivation for the term was the discovery of “semiconductor substances”….before that, people didn’t refer to the tubes as “semiconduct devices”. i guess?

        anyways even so, this may qualify? if i understand, the glowing gas is a substance that is conducting or not conducting? isn’t it a semiconductor substance? the innovation of a dekatron is that it’s not a vacuum tube…unlike vacuum tubes, the semiconductive aether it uses has substance.

        don’t tell me a gas isn’t a substance :)

        1. You can ask, what property of vacuum would stop an electron from traveling through it? That property would be its resistivity, and the reciprocal to resistivity is conductivity.

          Therefore, you can understand a vacuum tube as having “perfect” conductivity, not “semi”-conductivity, or that the concept of conductivity doesn’t apply at all because there’s nothing in there – at least ideally speaking – and the transfer of current through the device is not controlled by the change in conductivity as such.

        2. >the glowing gas is a substance that is conducting or not conducting?

          There shouldn’t be glowing gas inside a vacuum tube… because it’s a vacuum tube. The heater filament is glowing.

          There are types of gas-filled tubes like thyratrons, but these are highly non-linear devices that work on different principles of breaking down an insulating gas with high voltage and then continuing the arc through the ionized gas. The material there is an insulator – not a conductor or “semi” conductor.

    1. Semiconductors are a material like silicon, germanium, gallium-arsenide, and another bunch of materials that change their electrical conductance when a force is applied. The force could probably be anything, but most commonly are electricity, light, magnetism and pressure.

      So, as a tube doesn’t contain any material that changes its electrical conductance in response to a force, a tube is not a semiconductor.

      A tube IS a kind of analog switch, like a transistor. What you are basically describing is a switch. :)

      1. a vacuum tube doesn’t contain any material but dekatron tubes are not vacuum tubes. not all tubes are vacuum tubes.

        these tubes do contain material (an excitable gas) and that material does change its conductance in response to … i guess in response to arc flowing through it?

        1. Yes. A high voltage applied breaks down and ionizes the gas, flipping it from an insulator to a conductor. It is never “semi” conducting, but flipping from one state to another.

  2. Isn’t this why we have 60hz grids? It just makes this kind of clock (well not exactly this clock, but ones that use mains freq for timing) a lot simpler to build? I think I remember reading that somewhere

    1. Not at all. It’s just a coincidence that our (United States) power uses 60Hz which is convenient for clock circuits. That frequency was originally chosen (by Westinghouse) as an optimization for powering lights (arc lights at the time) as well as induction motors from the same generators. Much of the world (all of Europe, China, etc.) currently use 50Hz.

      1. Early electrical systems used to have much lower frequencies all the way to 12-25 Hz for running motors.

        The reason was the high impedance of early large AC motors, and getting motors to turn more slowly so you wouldn’t need as much reduction gearing. The old induction motors would have to start from simply throwing a big switch, and the pull-up torque would be greatly reduced at higher frequencies.

        Arc lights actually needed much higher frequencies around 125 – 140 Hz to not turn into stroboscopes, because the arc would blink off twice per cycle. Westinghouse actually considered 33 Hz sufficient for filament lighting.

        1. I can agree that 60 is a number nicer than 50. However I still cannot imagine the advantage of starting from 60 cycles per second over 50 cycles per second. We need to multiply here, not to divide.

    2. Power frequency in the US is somewhat arbitrary. Early railroad traction engines tended to operate on 25 Hz power. I have a a pre General Electric Warren Telechron power station master clock from about 1920 which was originally sold with a 25 Hz 1500 RPM rotor in its synchronous Telechron motor frame. The clock’s previous owner swapped in a 60 3600 RPM rotor. Telechron rotors are pretty standardized and easily swapped, even between different line frequencies. Rotors usually have a 1 RPM output gear. Master clocks use a differential’s output to drive a large pointer to compare the 1 RPM output of an accurate pendulum regulator to a Telechron motor 1 RPM output. The motor is driven by a power plant alternator. A machine operator watched the pointer to adjust the throttle of the plant’s alternator’s power source. This made selling synchronous motor electric clocks feasible, which was the business Warren wanted to establish. Warren Telechron was later bought up by GE.

      Areas of the US which were powered by Westinghouse power plant equipment tended to be 60 Hz. 60 Hz is believed to a compromise to balance line losses, size of transformer magnetics and machine operating speed.

      Areas of the US in the early years of GE tended to be 50 Hz, influenced by GE’s European partner AEG who favored 50 Hz. Notably southern California and Los Angeles were GE territory and 50 Hz. Only a year after committing LA to 50 Hz, GE standardized on 60 Hz. Almost immediately, there was pressure to update to 60 Hz because the new Boulder Dam power station was going to operate at 60 Hz. Los Angeles stayed with 50 Hz power up to 1948 when a three-year trade-in program was established for clocks, appliances, etc.

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