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What is The Harwell Dekatron Clock? It's the culmination of two like-minded individuals who like to tinker with vintage display technologies and turn them into functional time-pieces. This clock is unique in many respects. The Harwell Dekatron Clock is a combination of old-skool technology married with a very modern micro-controller and 21st century case construction and aesthetics. It offers lots of customisation options, such as different "faces", LED colours (bling), chimes, GPS or WIFI disciplined timekeeping and is also open source in its design philosophy.

What is a dekatron tube?

The plethora of methods developed during the late 1940's to the 1970's for displaying/counting numerical information were numerous and varied. These included incandescent displays (e.g. edge lit displays such as those from Non-Linear Systems), neon gas based displays such as Nixie tubes and vacuum fluorescent displays (VFDs), E1T tubes, light-emitting diodes (LEDs) and liquid crystal displays (LCDs). Some of these possess a certain charm, such as nixie tubes and dekatrons with their warm orange glow and others not so much.The dekatron (or generically three-phase gas counting tube or glow-transfer counting tube or cold cathode tube) is a gas-filled decade counting tube. 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). 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. There are also 8 cathode and 12 cathode designs as well (also referred to as dekatrons).

A far more detailed explanation of how to electrically drive a dekatron can be found in the project documents. There are also some great resources on the web for history and data about dekatrons.

Although this webpage almost exclusively talks about using British dekatron tubes, there should be no reason why the design should not adapt to use Soviet or American tubes (it is our intention to make such changes).

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Describing the Hardware

The clock hardware is complex and we think comprehensive. We wanted to respect the rarity of these tubes by creating a first class clock to use them. The main elements of the hardware are:

The items marked * above are primarily intended to prolong the life of the dekatron tubes although they may have secondary benefits such as reducing power consumption.

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Suitable Dekatrons

The case and boards place constraints on the dekatrons that can be used:

Suitable dekatron arrangements are:

GC10B variants include GC10B/S, GC10B/L, GC10/4B, GC10/4B/L, CV1739, CV2271, CV2321, CV6044, CV6100, Z302C, Z303C, VX9194, VX9194/4B, M2465-401C.

There are variations to both the electronics kit and the case kit (espically changes to the engraving) depending on the arrangment to be used. So it is important that the choosen arrangement is stated when ordering either kit.

Dekatrons that are NOT suitable are:

If in doubt ask!

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Describing the Case and Cover Options

Build your own

The simplest case option is to craft your own design. The physical size and board layouts will limit what you can do. But the choice of materials is entirely your own.

Safety: If you have a metal case or exposed metal parts then you must electrically bond the metal to the clock's negative supply. There is a screw connector on the PSU board for this purpose.

Acrylic Light Guides and Case

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Describing the Software

For the SAM3X8C

This is written in C using Atmel Studio 7 and the GCC ARM compiler. It makes extensive use of the Atmel libraries.

The source code is open. You can download the complete source code from the project dropbox together with compiled binary file. The binary is burned into the SAM3X8C using the external programmer and BOSSA flash programming utility.

Instructions for burning the SAM3X8C using the external programmer and, should you wish, compiling the source code are in the project documentation.

For the MEGA324PA (on the dekatron board)

This is written in C using Atmel Studio 7 and the GCC AVR compiler. It is open source and uses the Atmel libraries.

The source code is open. You can download the complete source code from the project dropbox together with compiled binary file.

Instructions for burning the MEGA324PA and, should you wish, compiling the source code are in the project documentation. The MEGA324PA will normally be delivered burned with the latest firmware. The MEGA324PA contains both the operating software to drive the dekatrons but also contains a bootloader so the chip can be updated by the SAM3X8C using the TWI.

For the ESP32-BIN (if used)

The ESP32-BIN module uses compiled binaries from the ESPRESSIF website.

Instructions for burning the ESP32-BIN using the external programmer are in the project documentation.

For the MEGA16U2 (on the external programmer)

This is written in C using Atmel Studio 7 and the GCC AVR compiler. It is open source. It uses the Atmel and LUFA libraries.

The source code is open. You can download the complete source code from the project dropbox together with compiled binary file. The binary is burned into the SAM3X8C using the external programmer and BOSSA flash programming utility. The SAM module will normally be supplied fully assembled and tested with the latest firmware burned.

Instructions for burning the MEGA16U2 and, should you wish, compiling the source code are in the project documentation. The MEGA16U2 will normally be delivered burned with the latest firmware and the programmer partially built and partially tested.

About Nuggle

Ok, you are now thinking, what's a Nuggle?

What has a mythical creature from Shetland folklore got to do with The Harwell Dekatron Clock? I will explain some of the details…

Consider what happens when the clock is running:

Happenings or Events occur such as shown on the left … such as the clock ticks, the PIR senses motion and so on. The clock software must then decide what to do. So, for example, if the clock is currently showing the time in a hh:mm:ss format and the clock ticks one second then the dekatron display will need updating, the clock colons, if flashing, will need their state changing and if a tick sound is played then that will need to be started.

In other words, given a range of Events, the clock software must Decide on what Actions to take.

The starting events are fairly few in number as seemingly are the actions to take. But when you consider that there are twenty one events for the IR handset (one for each key), there are about a dozen sleep states and six types of tick (second, minute, hour, day, month and year) the starting event list grows in size. The actions you can take for the chime sounds is to play any audio file on the SD card (so the number of audio files is only limited by the SD card capacity), the number of beeps (frequency, duration, stereo channel) is similarly a large number, there are 32 bling LED modules, each module can (in theory) display over sixteen million colours. Against these numbers the dekatron tubes are much more defined with only 30 states to consider (36 for the seventh dekatron) giving a mere 216 options.

A common course would be to write code in C that handled a number of configurations where the actions taken for each event are programmed and burned into the SAM3X8C microcontroller. There is a disadvantage to this approach - editing or creating new clock configurations requires the source code to be edited, compiled, burned and then tested. It also makes the distribution of any new configuration more difficult. Therefore a different approach has been taken - to encompass the decision making process in a simple text script on the SD card. Here's an example:

program Time01 - Put hh:mm:ss on the dekatrons - Blank Bling ; customise the colour colour: Constant colon_colour 0x001000 Number Hr ; Called on start event_start: WriteLn "Time 01 - No bling" Default Stop ; Called every second event_tick: ; respect user's 12 or 24 hour time ; assume 24 hour clock to start- Hr := hour ; perhaps a 12 hour clock? If Low(fmt_time) == fmt_hr12 ; user wants 12 hr format? If Hr == 0 Hr := 12 Else If Hr > 12 Hr -= 12 IfEnd IfEnd IfEnd ; fill in the tubes Dekatron V1, Hr / 10 Dekatron V2, Hr % 10 Dekatron V3, minute10 Dekatron V4, minute1 Dekatron V5, second10 Dekatron V6, second1 ; light the time marker Dekatron V7, dot_time ; flash the colons in opposites If Odd(second1) Colon 0, 100 Colon 1, 0 Else Colon 0, 0 Colon 1, 100 IfEnd Default Stop End

The script "language" is a mixture of Pascal and C, and, for reasons that I'm going to leave obscure, I called it Nuggle. The text files on the SD card have a .NUG extension.

The example above does no more than display the current local time on the six dekatron tubes and to highlight the engraved "time" on the seventh. It does allow for the user time display preference (12 or 24 hour format). And it flashes the colons each second. But it makes no use of the other light guides, no use of the chimes and it certainly does not beep.

The Nuggle language manual is available from the the project dropbox. Nuggle has a Harwell Dekatron Clock supplement covering the Nuggle commands that are unique to this clock. The intention is that Nuggle will be extended to work with future Sgitheach clocks. You will find it used in the Fortress E1T Clock and it is likely to feature in future clocks.

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About Open Source

One intent of the Harwell Dekatron Clock project from the outset was that the hardware and software would be fully open. The project documentation is held on a dropbox where you will find:

If I have missed anything from the list that I should be providing then rest assured that I will.

The 3D printed parts and other case components were designed in Fusion 360. Whilst the F360 files are not immediately available on the dropbox, I will provide them if asked. The reasons I don't immediately provide them is they are a bit of a faff to export from F360, that I suspect the demand for them will be low and they would represent yet something else to keep updated.

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You should look in the "/Dekatron Projects/Harwell Dekatron Clock" and the "Common Stuff" folders.

Common Stuff includes:

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Built Clocks

Fully built, tested and configured clocks are available from Stock's Clocks together with your choice of British or American Dekatrons.

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Clock Kits

Harwell Dekatron Clock Kits have sold out!

Built and tested clocks are still available from Stock's Clocks.

N.B. None of these kits include the dekatron tubes - they are yours to provide or can be bought separately!

The prices of all these kits and dekatron tubes are here.

Minimum Electronics Kit

This kit includes (from the top clockwise):

If you take this kit then I assume you know what you are doing! There are a lot of parts to collect. You need to be confident to be able to solder parts as small as 0603 and a LQFP100 part with 0.5mm lead spacing. You will need access to a 3D printer and be able to cut acrylic tubing accurately.

The photograph of the minimum kit shows 2x3 panellised GPS and WIFI plug in boards. You will receive only one …

Of course the real minimum kit is nothing at all - you take the Gerbers and get your own boards made and get the custom transformer wound yourself. The minimum kit here represents what I think is a more likely kit where I supply the bare PCBs and the flyback transformer.

Standard Electronics Kit

This kit includes:

The list of the standard electronics kit includes both the GPS and WIFI plug in components. You will only receive those you buy …

The SAM3X8C programmer board uses a ATMEGA16U2 microcontroller that must be programmed using a suitable AVR programmer. If you wish I will burn the mega for you for no charge.

Case Kit

The Case kit includes:

What if the kit you want is not listed?

In principle, I will assemble a kit containing the parts you request.

But I am very reluctant to provide a kit containing all the SMD parts loose for several reasons. It takes a considerable time to bag and label all the SMD parts; a considerable amount of waste is generated by the kit purchaser when all the bags have been emptied. Unassembled kits I will normally fully refund (other than the postage) if returned. However, I cannot easily use short lengths of SMD tapes on my pick and place machine so I can only offer a partial refund.

Partly or Fully built?

Please contact me if you want a partially or fully assembled and tested clock.

Built and tested clocks are still available from Stock's Clocks.


The electronics kit is shipped from Scotland. Postage will be whatever I have to pay and no more. A complete kit will normally be shipped with tracking and insurance. Outside of the UK shipping will be by air. Inside the UK shipped Royal Mail "Special Delivery" is normally used. I will give you an estimate for shipping to your location based on Royal Mail prices and then the actual cost when posted.

You're welcome to come and collect the electronics kit … but I live near Drumnadrochit.

The case (with no electronics) is shipped from San Diego USA. USPS will normally be used.

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Please contact me with any questions or enquires about ordering or postage.

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Further Reading

Electronic Counting Circuits JB Dance 1967 Iffle Books Ltd.

Harwell Dekatron/WITCH computer

Stock's Clocks


Mike's Electric Stuff

Tube Tester

Three neuron's Pile o'Poo

Calculator Electronics

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