relay computer

13 Articles

Capacitor Memory Makes Homebrew Relay Computer Historically Plausible

March 5, 2026 by 21 Comments

It’s one thing to create your own relay-based computer; that’s already impressive enough, but what really makes [DiPDoT]’s design special– at least after this latest video— is swapping the SRAM he had been using for historically-plausible capacitor-based memory.

A relay-based computer is really a 1940s type of design. There are various memory types that would have been available in those days, but suitable CRTs for Williams Tues are hard to come by these days, mercury delay lines have the obvious toxicity issue, and core rope memory requires granny-level threading skills. That leaves mechanical or electromechanical memory like [Konrad Zuse] used in the 30s, or capacitors. he chose to make his memory with capacitors.

It’s pretty obvious when you think about it that you can use a capacitor as memory: charged/discharged lets each capacitor store one bit. Charge is 1, discharged is 0. Of course to read the capacitor it must be discharged (if charged) but most early memory has that same read-means-erase pattern. More annoying is that you can’t overwrite a 1 with a 0– a separate ‘clear’ circuit is needed to empty the capacitor. Since his relay computer was using SRAM, it wasn’t set up to do this clear operation.

He demonstrates an auto-clearing memory circuit on breadboard, using 3 relays and a capacitor, so the existing relay computer architecture doesn’t need to change. Addressing is a bit of a cheat, in terms of 1940s tech, as he’s using modern diodes– though of course, tube diodes or point-contact diodes could conceivably pressed into service if one was playing purist. He’s also using LEDs to avoid the voltage draw and power requirements of incandescent indicator lamps. Call it a hack.

He demonstrates his circuit on breadboard– first with a 4-bit word, and then scaled up to 16-bit, before going all way to a massive 8-bytes hooked into the backplane of his Altair-esque relay computer. If you watch nothing else, jump fifteen minutes in to have the rare pleasure of watching a program being input via front panel with a complete explanation. If you have a few extra seconds, stay for the satisfyingly clicky run of the loop. The bonus 8-byte program [DiPDoT] runs at the end of the video is pure AMSR, too.

Yeah, it’s not going to solve the rampocalypse, any more than the initial build of this computer helped with GPU prices. That’s not the point. The point is clack clack clack clack clack, and if that doesn’t appeal, we don’t know what to tell you.

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Most complex blinking light

2025 One Hertz Challenge: 16-Bit Tower Blinks At One Hertz

July 23, 2025 by 3 Comments

We’ve seen our share of blinking light projects around here; most are fairly straightforward small projects, but this entry to the 2025 One Hertz Challenge is the polar opposite of that approach. [Peter] sent in this awesome tower of 16bit relay CPU power blinking a light every second.

There’s a lot to take in on this project, so be sure to go look at the ongoing logs of the underlying 16-bit relay CPU project where [Peter] has been showing his progress in creating this clicking and clacking masterpiece. The relay CPU consists of a stack of 5 main levels: the top board is the main control board, the next level down figures out the address calculations for commands, under that is the arithmetic logic unit level, under the ALU is the output register where you’ll see a 220 V lamp blinking at 1 Hz, and finally at the base are a couple of microcontrollers used for a clock signal and memory. [Peter] included oscilloscope readings showing how even with the hundreds of moving parts going on, the light is blinking within 1% of its 1 Hz goal.

It’s worth noting that while [Peter] has the relay CPU blinking a light in this setup, the CPU has 19 commands to program it, enabling much more complex tasks. Thanks for the amazing-sounding entry from [Peter] for our One Hertz Challenge. Be sure to check out some of the other relay computers we’ve featured over the years for more clicking goodness.

2025 Hackaday One Hertz Challenge

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This Relay Computer Has Magnetic Tape Storage

June 11, 2025 by 9 Comments

Magnetic tape storage is something many of us will associate with 8-bit microcomputers or 1960s mainframe computers, but it still has a place in the modern data center for long-term backups. It’s likely not to be the first storage tech that would spring to mind when considering a relay computer, but that’s just what [DiPDoT] has done by giving his machine tape storage.

We like this hack, in particular because it’s synchronous. Where the cassette storage of old just had the data stream, this one uses both channels of a stereo cassette deck, one for clock and the other data. It’s encoded as a sequence of tones, which are amplified at playback (by a tube amp, of course) to drive a rectifier which fires the relay.

On the record side the tones are made by an Arduino, something which we fully understand but at the same time can’t help wondering whether something electromechanical could be used instead. Either way, it works well enough to fill a relay shift register with each byte, which can then be transferred to the memory. It’s detailed in a series of videos, the first of which we’ve paced below the break.

If you want more cassette tape goodness, while this may be the slowest, someone else is making a much faster cassette interface. Continue reading “This Relay Computer Has Magnetic Tape Storage” →

Homebrew Relay Computer Features Motorized Clock

August 3, 2024 by 6 Comments

Before today, we probably would have said that scratch-built relay computers were the sole domain of only the most wizardly of graybeards. But this impressive build sent in by [Will Dana] shows that not only are there young hardware hackers out there that are still bold enough to leave the transistor behind, but that they can help communicate how core computing concepts can be implemented with a bundle of wires and switches.

Created for his YouTube channel WillsBuilds, every component of this computer was built by [Will] himself. Each of the nine relay-packed protoboards inside the machine took hours to solder, and when that was done, he went out to the garage to start cutting the wood that would become the cabinet they all get mounted in.

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Relay Computer Consumes Six Years And 4.5 Suitcases

October 23, 2020 by 38 Comments

If you thought your home-brew project was taking a long time, [Jeroen Brinkman]’s MERCIA Relay Computer project probably has you beat. He began working on this impressive computer back in 2014, and has been at it ever since.  In fact, the ongoing nature of the project is embedded into the name itself — the English translation of the acronym MERCIA is “My Simple Relay Computer Under Construction”.  Being interested in old analog and relay computers from an early age, [Jeroen] took on this project to educate students about how computers work.  The entire computer is build only using relays, diodes, and capacitors, not to mention color-coded wire based on signal functions. Using relays as the primary switching elements is at the core of his educational goal — anyone can understand how a relay works.

Understandably, this thing is big.  But he has cleverly packaged it to visually show the major building blocks of a computer.  While the exact size isn’t stated, we can estimate based on the photo of [Jeroen] standing next to the modules that these panels are about 1.5 m tall and perhaps 60 cm wide.  The whole computer is nine panels wide, making it about 5 meters long.  Except for the ROM assembly, pairs of panels are hinged together and they fold like a book and carried like a suitcases when being moved.  If you enjoy the clickety-clack sound of relays, be sure to watch the relay longevity test in the video below and check out our article on the 1958 FACOM from last year.

This is a fascinating project, but unless you have a couple thousand relays laying around and a decade of free time, it’s probably better to just enjoy [Jeroen]’s work rather than build your own.  We hope he releases schematics and other documentation once the project is finished.  You can follow his Facebook build log if you want to keep track of the progress. Thanks to [David Gustafik] for the tip.

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Visiting The FACOM 128B 1958 Relay Computer

December 6, 2019 by 9 Comments

If you study the history of computing you might have heard of the FACOM 128B, a Japanese relay computer from 1958. It holds the distinction of being a contender for the oldest computer that still works in its original form, as it resides in a Fujitsu building in Numazu Japan. [CuriousMarc] visited the old computer and created a video about it as well as painting a picture of other contemporary machines. You can see the video below.

[Marc] explains how a relay machine was already behind the times in 1958, and also shows how the 5,000 relay machine is laid out. The machine on display came from a Tokyo university and did the kind of computations you might use a computer for today to do engineering design.

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Electromagnetic Field: TIM, A Relay Computer

September 8, 2018 by 12 Comments

We are probably all familiar with computing history to the extent that we know the earliest computers were surprisingly simple devices. While early electronic machines such as Colossus or ENIAC were hugely complex racks of tubes, once expressed as a schematic or as a network of logic gates they would be relatively straightforward for today’s electronic engineer to understand their operation. Those who have made an in-depth study of computing history may have heard of the work of Konrad Zuse in the mid-20th century, his relay-based machines predate their fully electronic cousins by several years.

A relay-based computer can be simple enough to be built by a home constructor, and at the recent Electromagnetic |Field hacker camp [Rory Mangles] outlined his TIM relay computer built while he was at school. It’s an engaging story starting from first principles and describing a series of TIM devices from a simple binary adder to the final fully Turing-complete computer. He describes the design process for his ALU, eventually going with a 1-bit serial design to economise on relays.

The machine has a Harvard architecture, with the program pathway consisting of a paper tape from which the code is run directly. The instruction set is called BLT, which of course means Basic Language of Tim, and there is a T++ assembly language. Loops and if statements are handled in a nod to the classical Turing machine by looping the paper tape. The original TIM is a few years old, but he reveals that he’s recently brought it out of storage and added a parallel port. Thus the finale of the talk is a demonstration, printing a “Hello World”.

We’ve placed the full video below the break, meanwhile we were lucky enough that [Rory] brought TIM along to the EMF Hackaday Readers village for our bring-a-hack, so the header image is from when we had a chance to examine it. If you’re curious to know more, he has a web site with some more TIM details.

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