Dekatron

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An octal-base dekatron.

In electronics, a Dekatron (or Decatron, or generically three-phase gas counting tube or glow-transfer counting tube or cold cathode tube) is a gas-filled decade counting tube. At the end of the Second World War a 24 year-old scientist called John Reginald Acton was working at the AC Cossor valve factory at Loudwater, Buckinghamshire. John proved that by sending a negative voltage spike pulse to the unlit cathode he could move the glow from one cathode to the other, and it would stay in the new position when the pulse ended. John thought about this problem and in 1947 conceived the idea of moving the cathode glow down a line of ten cathodes. He saw how this could be done, using intermediate “guide cathodes”, but by now his job was designing a special cathode ray tube, so once again the idea remained just that – an idea. In 1948 John was headhunted by Mr R.C. Bacon, the head of the new Ericsson Telephones valve design laboratory at Beeston, Nottingham and he was appointed chief valve designer. When he came to Beeston both John and Ray Bacon supposed the future lay in developing new hot cathode valves, and had no idea that cold cathode valves were the key to the future. However John was soon introduced to John Pollard, head of Ericsson’s electronic laboratory, who showed him the standard mechanical 50 way uniselector device used to route telephone calls. John immediately realised that his glow moving valve could be developed to create a multi-way electronic selector inside a gas filled valve. So early in November, just a month after he had come to the labs, and watched by Bill and two or three others in the lab, John was able to demonstrate the world’s first multi-state valve. As John pressed the button to send a pulse, the pale blue glow moved round from cathode to cathode. The idea worked! The news spread round the labs and then into the whole factory and people crowded in to see this strange new device. From that moment on John was always known in the factory as “John the counting valve man”. Ericsson made protecting their rights an urgent priority, and a provisional patent (archived by Espacenet as GB19490001324) in the name of John Reginald Acton was granted on the 18th January, 1949 for “Improvements in or relating to electronic counting arrangements”, the first of several patents concerning the new kind of valve. Dekatrons were used in computers, calculators and other counting-related products during the 1950s and 1960s. "Dekatron," now a generic trademark, was the brand name used by Ericsson Telephones Limited (ETL), of Beeston, Nottingham (not to be confused with the Swedish TelefonAB Ericsson of Stockholm).

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A dekatron in operation.

The dekatron was useful for computing, calculating and frequency-dividing purposes because one complete revolution of the neon dot in a dekatron means 10 pulses on the guide electrode(s), and a signal can be derived from one of the ten cathodes in a dekatron to send a pulse, possibly for another counting stage. Dekatrons usually have a maximum input frequency in the high kilohertz (kHz) range – 100 kHz is fast, 1 MHz is around the maximum possible. These frequencies are obtained in hydrogen-filled fast dekatrons. Dekatrons filled with inert gas are inherently more stable and have a longer life, but their counting frequency is limited to 10 kHz (1–2 kHz is more common).

Internal designs vary by the model and manufacturer, but generally a dekatron has ten cathodes and one or two guide electrodes plus a common anode. The cathodes are arranged in a circle with a guide electrode (or two) between each cathode. When the guide electrode(s) is pulsed properly, the neon gas will activate near the guide pins then "jump" to the next cathode. Pulsing the guide electrodes (negative going pulses) repeatedly will cause the neon dot to move from cathode to cathode.

Detail of the top of a dekatron — central anode disk surrounded by 30 internal cathode pins.
Sending sequenced pulses to guide electrodes will determine the direction of movement.

Hydrogen dekatrons require high voltages ranging from 400 to 600 volts on the anode for proper operation; dekatrons with inert gas usually require ~350 volts. When a dekatron is first powered up, a glowing dot appears at a random cathode; the tube must then be reset to zero state, by driving a negative pulse into the designated starting cathode. The color of the dot depends on the type of gas that is in the tube. Neon-filled tubes display a red-orange dot; argon-filled tubes display a purple dot (and are much dimmer than neon).

Counter (common-cathode) dekatrons have only one carry/borrow cathode wired to its own socket pin for multistage cascading and the remaining nine cathodes tied together to another pin; therefore they don't need bases with more than 9 pins.

Counter/Selector (separate-cathode) dekatrons have each cathode wired to its own pin; therefore their bases have at least 13 pins. Selectors allow for monitoring the status of each cathode or to divide-by-n with the proper reset circuitry. This kind of versatility made such dekatrons useful for numerical division in early calculators.

Dekatrons come in various physical sizes, ranging from smaller than a 7-pin miniature vacuum tube to as large as an octal base tube. While most dekatrons are decimal counters, models were also made to count in base-5 and base-12 for specific applications.

The dekatron fell out of practical use when transistor-based counters became reliable and affordable. Today, dekatrons are used by electronic hobbyists in simple "spinners" that run off the mains frequency (50 Hz or 60 Hz) or as a numeric indicator for homemade clocks.

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