You want Retro? We did, when we started our retrocomputing challenge. [Peter] decided that transistors weren’t retro enough, and sent us this lovely homebrew relay computer, complete with 16- bit CPU, which is rather more bits than one normally associates with clicky clacky contacts.

The architecture is very simple– it just uses an accumulator register, ACCU, and goes from there. All mathematics and save/load operations go through ACCU. There whole instruction set is only 19 commands, and he’s used that set to program such lovely things as calculating 3 digits of Pi– which only took 8 minutes of glorious clicking. There’s a demo video of that embedded below. [Peter] has even implemented a display by hooking his computer to a 32×32 LED matrix, but don’t expect it to relay updates really quickly.

If this computer looks familiar, it’s because its earlier incarnation was one of the more “extra” entries in last year’s one-hertz challenge, where it was used to blink an indicator lamp. Yes, even relay computers apparently get started with the “blinky” sketch.

If you want in on the fun, our retrocomputer challenge runs until October 27th, so there’s lots of time left to turn back the clock.

What is it about retrocomputing? For some people, it’s nostalgia. For others, it’s the appeal of simplicity. For still others, it’s the chance to save old machines from the graveyard. Whatever your motivation, there’s no denying that we’ve seen a metric ton of retrocomputer projects here at Hackaday. And we’d like to see more!

We’re happy to announce the 2026 Hackaday Retrocomputing Challenge!

Now’s the chance to put your retrocomputer project up for all to see. Open up a Hackaday.io project that features your retrocomputer project, and we’ll pick our three favorites for a $150 gift certificate courtesy of this contest’s sponsor, DigiKey. You have until Tuesday, October 27th. So get hacking!

Honorable Mention Categories

• • Old Iron: Are you saving an old C64 from the scrapyard? Restringing the core memory of a PDP-8? Or working on even more esoteric ancient devices? If you’re restoring or running the old hardware itself, it’s an “Old Iron” project.
  • Modern Retro: Computing power is plentiful, but the nice interfaces of past computer made them a joy, or a terror, to use. This honorable mention category is for emulations, work-alikes, and all the other retrocomputers that are TRS-80 on the outside, but modern electronics on the inside.
  • Retro-Modern: You love your classic machine, but you also love reading Hackaday on the Internet Of The Future, so you built a web-modem to bring the old beast into the modern era. This category is for keeping old hardware relevant.
  • Coding Like It’s 1999: Retrocomputers don’t run without software, and it’s a continual source of amazement to us how programmers today can get more out of old machines than we could back in the day. Think of the demoscene, for instance. But it doesn’t have to be showy either. If you’re writing new code for old boxes, it fits in here.