Relay Computer: You Can Hear It Think

Modern digital computers have complex instruction sets that runs on state-of-the-art ALUs which in turn are a consequence of miniaturized logic gates that are built with tiny transistors. These tiny transistors are essentially switches. You could imagine replacing with electromagnetic relays, and get what is called a relay computer. If you can imagine it, someone’s done it. In this case, [jhallenworld].

The Z3 was the first working programmable, fully automatic digital computer designed by Konrad Zuse. The board employs modern semiconductor devices such as memory and microcontrollers, however, the CPU is all relays. A hexadecimal keyboard allows for program entry and a segment display allows tracking the address and data. The program is piped into serial to the parallel decoder and fed to the CPU where the magic happens. Since the core is electromechanical it is possible to connect the output to peripherals such as a bell as demonstrated near the end of the video.

This project is a good balance of retro and modern to be useful to anyone interested in mechanical computers and should be a lot of fun for the geek kind. Hacking this computer to modify the instruction set should be equally rewarding and a good exercise for students of computing theory.

There is a SourceForge page dedicated to the project with the details on the project including the instruction set and architecture. Check out the video below and if you are inspired by the project, be sure to check out the [Clickity Clack]’a Videos on designing a relay computer bit by bit.

28 thoughts on “Relay Computer: You Can Hear It Think

  1. I used to work in a three story relay based computer. There was no CPU as such, as it would have been a single point of failure so it had distributed CPU resources that all worked in parallel.

    The relay clicks came across in waves and after working there for a decade you could here when an individual relay was falling in speed and needed adjusting.

    It’s amazing what was done with relays ‘in the day’. It was an expensive technology that took up lots of room and heaps of power. The 2 copper power rails had a cross-section of one inch by 14 inches and run right through the building.

    1. > I used to work in a three story relay based computer.

      I now picture you as a human relay. Shouldn’t be too hard, you just flick a switch when you feel the jumper cables tingling your feet.

  2. A year ago, I had no idea of ‘relay logic’, but devised a flip-flop using two relays and a diode (to show/hide a TV with a momentary button press).

    Thanks to the effect of Baader-Meinhof, I will never be allowed to forget that this is a ‘thing’.

    1. $$$
      Time. Developing the techniques necessary to reliably design small relays would take a long time.

      Most important: there were better alternatives already improving in price and quality while being much faster (orders of magnitude!). Relays were used as they were available and worked well enough.

    2. That has actually been done throughout the years.
      But as other comments have stated, other technologies have out competed relays in this application.
      Other applications where relays are still to this day used are however in need of smaller ones, and therefor smaller relays have been built.

      Smallest mechanical relay I have seen so far is around 1 cm long, 0.7 cm wide, and around 0.5 cm tall. So rather small.

  3. A friend of mine was restoring a 56 chevy pickup (I think it was a 56) and was looking at putting in power mirrors.
    He was looking at a kit that would let him control both the driver and passenger side window from one switch (which looked like an window hand crank).

    I can’t find the schematic now, but he showed it to me, and I looked befuddled at the large amount of relays… (maybe 20-30 of them?) took me a moment to see the first logic gate, then I quickly figured out what was being done… and told him that it was a series of logic gates, and explained how the whole thing worked. Then I told him I could do that on silicon for him and it would be about the size of one relay (not that it matters, plenty of space on those old trucks.)

    I guess some antique car shows don’t allow any silicon on their vehicles, so relay logic took over more complicated functionality (like pushing down twice to roll down the passenger side window, and once for the driver side).

    Very modern steampunk…

  4. I spent a few years of my life working for a welder repair company replacing relay-logic spot-welder controllers that fired huge ignitron tubes with silicon-powered SCR-based controllers. Some of those things were just works of art in all the things they sequenced.

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