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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Relay Computer Knows The Sequence

October 25, 2025 by 11 Comments

When we first saw [DiPDoT’s] homebrew computer, we thought it was an Altair 8800. But, no. While it has a very familiar front panel, the working parts are all based on relays. While it isn’t finished, the machine can already do some simple calculations as you can see in the video below.

Turns out, the Altair front panel isn’t a coincidence. He wants to put the device in an Altair-style case. This limits him to two backplane cards, but he’s running out of space, so part of what he does in the video is redesign the backplanes.

We need to watch some more of these videos to figure out how he’s making his logic gates. A common approach is to wire AND gates as series relays and OR gates as parallel relays. However, there are some advantages to using relays as two-to-one multiplexers, which can create any logic gate you want.

If you just want to see the computer run, you can watch it generate a Fibonacci sequence around the 14:30 mark. Glorious sound from a beautiful bunch of relays. Not exactly a speed demon, mind you, but not half bad for a bunch of electromechanical relays.

There was a time when computers like this were state-of-the-art. In a way, we miss those days. But then again, in some ways, we don’t.

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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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Homebrew Relay Computer Looks Like It Could Be A Commercial Product

January 19, 2021 by 21 Comments

You may not have noticed, but we here at Hackaday really love our clicky stuff. Clicky mechanical keyboards, unnecessarily noisy flip-dot displays, and pretty much anything made with a lot of relays — they all grab our attention, in more ways than one. So it’s with no small surprise that we appear to have entirely missed perhaps the clickiest build of all: a fully operational 8-bit computer using nothing but relays.

What’s even more amazing about our failure to find and feature [Paul Law]’s excellent work is that he has been at it for the better part of a decade now. The first post on his very detailed and very well-crafted blog describing the build dates from 2013, when he was just testing LEDs in the arithmetic-logic unit (ALU). Since then, [Paul] has made incredible progress, building module after module, each containing a small portion of the computer’s functionality. The modules plug into card cages with backplanes to connect them, and the whole thing lives in an enclosure made from aluminum extrusion and glossy black panels for a truly sleek look. The computer is incredibly compact for something that uses 400+ DPDT relays to do its thinking.

In addition to the blog, [Paul] has a criminally undersubscribed YouTube channel with a quite recent series going over the computer in depth. We included the overall tour below, but you should really check out the rest of the videos to appreciate how much work went into this build. We’ve seen relay computers ranging in size from single-board to just plain ludicrous, but this one really takes the prize for fit and finish as well as functionality.

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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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