A 1961 Relay Computer Running in the Browser
Postedabout 2 months agoActiveabout 2 months ago
minivac.greg.technologyTechstory
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Relay ComputerEmulationVintage Computing
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Relay Computer
Emulation
Vintage Computing
A user has successfully emulated a 1961 relay computer in the browser, showcasing a remarkable feat of technical achievement.
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Nov 16, 2025 at 9:36 PM EST
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Jump to the original sources
Read the primary article or dive into the live Hacker News thread when you're ready.
https://www.computerhistory.org/timeline/computers/
It looks like transistorized computers were dominant at the point when integrated circuits were introduced.
When the HP 35 came out, it was cheaper than the annual maintenance contract for the Friden. They bought one, and passed it around to try out for a week, then all of the Fridens went into the dumpster. Of course he brought one home, and we got to play with it.
(Ands they had to either have an extra set of batteries handy or access to an outlet to plug in the cord since the possibility of the batteries dying during a test was a real likelihood.)
https://www.calling315.com/relay-logic
It turns out both are true [1]. The "integrated circuits" were sort of "flat-packs" of RTL circuits. I had forgotten that early IC's were not quite what we envision today. Regardless I suppose ICs were RTL before they were TTL (before they were CMOS, etc.).
[1] https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Logic...
https://computerhistory.org/exhibits/ibm1401/
One of the crucial things I didn't get to yet is making that wheel controllable - some circuits in the manuals do require you to turn it. That's high in the todo list (with mobile support).
I also really want to share all of the videos I've been collecting of similar "wheels" - in (modern-ish) elevators [0] and light shows [1]... And also pinball machines! [2]
Re: boards, I actually did grow up with the "300 in 1" kit, so yes... it's full circle for me. :-)
[0] https://www.instagram.com/reel/C3dk5GkJni6/ and https://www.instagram.com/reel/CzuHXKRpuN-/
[1] https://www.instagram.com/reel/C6qqCdhJbM2/
[2] https://www.youtube.com/watch?v=ue-1JoJQaEgm
If anything though, I want to go build a physical relay computer mechanism myself now (if even just a 4-bit adder or something).
[0] https://minivac.greg.technology/manuals/1961-minivac601-book...
The biggest unlock was finding Willy McAllister's excellent Circuit Sandbox [1], which provides the Minivac Simulator's underlying electrical math. I tried so many approaches to simulate electricity (a doomed DIY approach, Falstad, Spice...) but Circuit Sandbox's DC analysis did the job perfectly.
Ping me for questions and would love your feedback!
[0] https://news.ycombinator.com/item?id=45945762
[1] https://spinningnumbers.org/a/circuit-sandbox.html
Feature request: I was wanting for a bit more sound… I mean, when I see "relay computer", I'm heading there for lots of soft clicks.
I totally feel you - the one reservation is that I’m using (gasp) not the original Minivac Relay sound. I need to go to a Computer Museum that will let me record one to offer a more authentic simulated experience.
So tldr - let me get a clean real sound and then we can come back to this question :)
The relay click sound you're hearing should only happen when a relay turns on, and when it turns off. There is no sound when a button is pressed - you can check this by loading an empty simulator and pressing any of the buttons at the bottom (the simulator won't make any sound):
https://minivac.greg.technology/simulator/
((Once/if I get to see a Minivac in real life, I may learn that buttons do make sounds..))
When a relay turns on and off, a sound is played. In the following circuit, check this by pressing and releasing the button 1:
https://minivac.greg.technology/simulator/#wires=1%2B%2F1C%2...
An alternative might be going to a regional VCF show where one will be (possibly by pre-arrangement with a collector). There are five or six shows held around the country every year. Unlike a museum almost all the devices there are available for hands-on interaction.
Steam engine concepts were already there before Jame Watt, logic by electrical circuits was already there before Shannon. People provide incremental guidance for the change, like river banks do to the flow of the river. No single part of the river brought the river upto that point.
For that incremental change, the name of its inventor is appropriate.
However, you are right that too frequently people fail to distinguish what was new and what was old in an invention, and they misleadingly attach the name of the inventor/discoverer to the entire big system or theory, not to the small features that are truly new.
The inventors or discoverers are frequently guilty of this themselves, by failing to properly acknowledge their sources and by making exaggerated patent claims, which nowadays are too frequently accepted by patent offices that do not perform an adequate search of prior art.
https://youtu.be/YDU47MxfHnQ
1) Are any of them for sale? :)
2) Would you be open to recording a clean sound of one of its relays turning on and off? And helping me figure out some specifics about the motor wheel (its speed and whether it experiences any speed up)
Thank you! My email is:
hi at greg.technology
https://youtu.be/_j544ELauus?t=373
The Enigma-cracking Bombe (on display at the extraordinary Bletchley Museum of Computing) also used relays
https://www.tnmoc.org/bh-10-bombe-description
And the Z3 as well!
https://en.wikipedia.org/wiki/Z3_(computer)
It was a hybrid electronic-relay computer, containing more than twenty thousand electromechanical relays and more than ten thousand vacuum tubes.
This is likely the biggest and most complex relay computer ever made, though its main arithmetic part and its fast registers were made with vacuum tubes, for increased speed in comparison with a pure relay computer.
https://en.wikipedia.org/wiki/IBM_SSEC
I'm fixated on speed. I connected some reed relays in a 3 stage ring oscillator and it ran at 1.8 kHz. That has me thinking that with a pipeline 100 instructions a second might be attainable. Fast enough to drive a terminal interactively. Reed relays are fast enough for a UART at 50 baud.
I'm forever tinkering with the control design such that it'll probably never be done. Karnaugh maps are my take on Sudoku.
They are relays in the form of tiny metal filaments, sealed in a vacuum or argon-filled glass bulb. They are very reliable, switching potentially billions of times. And they are very fast for purely mechanical components; I built a three-stage ring oscillator out of them once and it settled around 1.8 kHz.
They were used in the old phone system for logic and memory. The registers for receiving dialed digits needed to be fast and reliable.
Even after control moved to transistorized computers the switching fabric itself continued to be made out of relays so that a direct electrical connection was set up between the two terminals. (As late as the 1990s if you called another line on the same 1ESS exchange, there was direct physical continuity - a literal metal circuit - between your telephone's line and the other party's telephone line.)
[0] https://en.wikipedia.org/wiki/Reed_relay