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posted by hubie on Monday February 03, @12:40PM   Printer-friendly

http://www.nablaman.com/relay/about.php

I amuse myself by constructing a computer almost entirely out of relays. Relays were used to construct computers well before vacuum tubes, transistors or integrated circuits were feasible for the task. The main inspiration is the machines by Konrad Zuse of the late 30s and early 40s.

Why relays? In addition to constituting an important historical link between the mechanical and electronic computers, relays are especially fun to work with since they

  • are big and slow, with huge propagation delays and a tendency to oscillate if you hook them up wrong.
  • are noisy, especially when lots of relays switch at the same time.
  • consume lots of power to do even the simplest of calculations.
  • subscribe to Lenz' law, i.e. generate lots of EMF and flyback current that make for all sorts of interesting interference in places you couldn't even guess.

So all in all, relays require you to think in very new ways compared to normal solid-state devices.


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  • (Score: 5, Interesting) by VLM on Monday February 03, @02:08PM (4 children)

    by VLM (445) on Monday February 03, @02:08PM (#1391368)

    The fan-in fan-out ratio varies from miserable to ridiculously good. So if you have a coil that draws half an amp and contacts rated at one amp, your relay can only "fan out" to two more relays. On the other hand, if you have a tenth amp coil and ten amp contacts, thats a 100 to 1 fan out ratio.

    Another thing the linked article didn't discuss is there's no reason to connect the coil of a relay to its own contacts. Depending on your logic levels etc, you can do things as simple as make an XOR gate by simply throwing a coil across the two inputs; DC coils usually don't care which end is positive. Generally coil to contact isolation is pretty good on relays.

    Another thing the linked article didn't mention is lots of poles means regular and inverted outputs are "free" whereas with transistors if you want a NAND instead of an AND you need an inverter, with relays you usually solder to a different pin.

    Something not mentioned is there's a lot of logic level "standards" such as 1 = +12 0 = very well grounded, vs 0 = disconnected (to make wired-or simpler, essentially the relay equivalent of open collector)

    Another thing not mentioned is the joy and excitement of latching relays. A latch in TTL might be a pile of NANDs to make a D flip flop or MS flip flip. A latch using a latching relay is a latching relay plus maybe something gating its input (again depending on wiring).

    You can interface relay logic very well with TTL digital logic using optocouplers and 555 timers (to debounce).

    Power supply spikes are a little brutal on relay logic. My unimplemented plan was float charge a motorcycle battery and run it all on 12V. The battery is pretty happy with 10+ amp current spikes as long as they're rather short.

    Relays, in bulk quantity, generate a surprising amount of heat over time. Not as bad as vacuum tubes, but they'll get warm if you have a lot of them.

    Finally there are some differences between level logic and edge triggered logic I don't remember with relay vs TTL.

    Its a fun thought experiment but I never got far designing and building my own.

    • (Score: 2) by epitaxial on Monday February 03, @03:10PM (2 children)

      by epitaxial (3165) on Monday February 03, @03:10PM (#1391374)

      Are you using car horn relays or something? I can get relays that switch 10 amps with a coil current of 30mA at 12 volts.

      • (Score: 0) by Anonymous Coward on Tuesday February 04, @05:09AM

        by Anonymous Coward on Tuesday February 04, @05:09AM (#1391494)

        Now I want to see a Zusie made of large industrial contactors. 6 pole, auxiliary contacts, great fan-out, amazing noise.
         

      • (Score: 2) by VLM on Tuesday February 04, @02:07PM

        by VLM (445) on Tuesday February 04, @02:07PM (#1391524)

        I was using easy numbers. Its possible to purchase some really low performance relays, and usually pretty easy to avoid.

    • (Score: 3, Interesting) by Mojibake Tengu on Monday February 03, @04:06PM

      by Mojibake Tengu (8598) on Monday February 03, @04:06PM (#1391388) Journal

      Two state flip-flop relays use impulses, not continous current. So the total heat produced is not as bad as for coil powered all the time.

      Minimal impulse width must be sufficient enough only to overcome double triangle mechanical aretation with spring.

      I used one such 12V-rated about 50 years ago, it's precisely a 1-bit mechanical memory, just like JK flip-flop. It used two coils, for left and right impulse to bring it to relevant state.

      --
      Rust programming language offends both my Intelligence and my Spirit.
  • (Score: 2, Interesting) by Anonymous Coward on Monday February 03, @02:34PM (3 children)

    by Anonymous Coward on Monday February 03, @02:34PM (#1391371)
    • (Score: 2) by Tork on Monday February 03, @04:33PM (2 children)

      by Tork (3914) Subscriber Badge on Monday February 03, @04:33PM (#1391397)

      https://www.youtube.com/watch?v=-ReqdyCxZ9I [youtube.com]

      Thank you for sharing this! I have a question- Is this sort of computer system why the Enterprise computer in the original Star Trek series does a ker-chunking kinda sound sometimes when it's working on something?

      --
      🏳️‍🌈 Proud Ally 🏳️‍🌈
      • (Score: 0) by Anonymous Coward on Monday February 03, @06:59PM

        by Anonymous Coward on Monday February 03, @06:59PM (#1391422)

        You're very welcome. Yes, I enjoyed sci-fi, including "Lost In Space". By that time serious computers had no relays, at least not in logic paths, but maybe in power supply control. But relays were still in other types of controllers like elevators (lifts) and machine controls like factory processes. They're mostly replaced by PLC (Programmable Logic Controllers) which of course are solid-state electronics.

        All that said, movies / entertainment in general, sci-fi especially love as much sight and sound as possible, so all that relay rattling enhances the experience. I always loved the big reel-to-reel tape drives and lots of blinking lights which really were data, address, and status lights for the CPUs and peripherals. Fun stuff!

        But yes, for decades many computers had many relays. Many (most?) computers were more special purpose. Think preprogrammed routines and you were limited to programming the computer to run one of the hard-wired routines, rather than the very general-purpose CPUs we're all used to these days.

        https://en.wikipedia.org/wiki/Plugboard [wikipedia.org]

        Relays are pretty robust. I'm sure there are still many building elevators running on big relay logic controllers. You've probably heard about early vacuum tube (valve) computers needing much maintenance replacing failed tubes. Tubes were much much faster of course, but factor in the maintenance downtime and relays didn't look so bad for many operations.

      • (Score: 4, Interesting) by DannyB on Monday February 03, @07:15PM

        by DannyB (5839) Subscriber Badge on Monday February 03, @07:15PM (#1391425) Journal

        I remember both the 1960s Star Trek and Lost In Space making clickety-clackety sounds.

        I assumed some of that might be like the "logic" mechanical parts of an adding machine, of that era. At least the sound effects may have been intended to evoke that mental image.

        I did get to use an ancient IBM 1130 in college for one semester. There were lots of real sounds. The primitive disk drives made seeking and of course spinning sounds. It had a selectric typewriter ball onto pin-feed paper right on the computer "desk" in addition to the separate high speed line printer. Both the selectric and line printer were noisy. And then there is the punched card reader / puncher. Decks of cards were stacked in the input hopper. The machine would read and run one deck, print the output, then read from the next deck, print more output, etc. All of this made noise.

        In the adjacent room were about eight 029 keypunch machines. Those also made a lot of clickety-clackety noises.

        Now I was using this on its trailing edge, 1979. But in the 1960s when it was current equipment, people might have associated all those sounds as just part of how computer rooms sounded.

        --
        Satin worshipers are obsessed with high thread counts because they have so many daemons.
  • (Score: 2) by kolie on Monday February 03, @08:48PM (1 child)

    by kolie (2622) Subscriber Badge on Monday February 03, @08:48PM (#1391442) Journal

    As a fellow homebrew computer enthusiast and experimenter - I think it's really cool to see a well known relay based project in the homebrew community be featured here.

    • (Score: 0) by Anonymous Coward on Tuesday February 04, @05:11AM

      by Anonymous Coward on Tuesday February 04, @05:11AM (#1391495)

      I like it too, and bonus: no politics.

  • (Score: 4, Funny) by corey on Monday February 03, @09:40PM (1 child)

    by corey (2202) on Monday February 03, @09:40PM (#1391449)

    …I think the guy mustn’t have kids or a wife.

    • (Score: 0) by Anonymous Coward on Tuesday February 04, @05:03AM

      by Anonymous Coward on Tuesday February 04, @05:03AM (#1391493)

      Or a great supportive family.

  • (Score: 2) by driverless on Tuesday February 04, @03:49AM

    by driverless (4770) on Tuesday February 04, @03:49AM (#1391484)

    The progress log [nablaman.com] stops in June 2011, fourteen years ago. If you want to follow one that's still actively worked on, try DiPDoT's one [youtube.com].

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