compareeHub

YOU CAN BUILD A 16-BIT COMPUTER OUT OF THE SWITCHES THAT RAN TELEPHONE EXCHANGES

A working 16-bit computer built entirely out of clicking electromechanical relays, the kind that ran telephone exchanges before transistors.

by Peter (HPRelCo)

FULL CAD BOM FIRMWARE DOCS

Open-hardwareScience

Built withArduino

difficulty
●●●●●
time
months of evening work
license
unspecified
repo
repo ACTIVE0 stars

WHAT YOU’LL NEED

1
Jump to section

COMPAREE VERDICT

This is a multi-month engineering project for someone who already understands CPU architecture and has done FPGA work. The published circuit diagram is complete and the FPGA coprocessor code is there, but you are on your own for sourcing the relays — no part numbers, no supplier links, no cost estimate. Peter says the project will never really be finished, which is the honest read: this is not a paint-by-numbers build, it is a framework for your own relay computer. The single hardest part will be acquiring the right relays in quantity and debugging timing issues when your specific relay model does not match the builder's. If you want to hear a computer think and you have the skill to reverse-engineer a schematic into a parts list, this is extraordinary. If you need a BOM and step-by-step instructions, it will waste your year.

GOOD TO KNOW

  • —Complete system circuit diagram is published in the project downloads
  • —The VHDL code for the FPGA maths coprocessor is provided in the project files.
  • —Relay timing tests and signal analysis are documented
  • —No explicit licence is stated anywhere on the project
  • —No bill of materials or relay part numbers
  • —Memory is implemented with an Arduino, not relays — the relays do CPU work only
  • —Commercial use restrictions unknown due to absent licence

Parts to buy

7 items

From our check of the build. Exact quantities and part numbers are in the creator’s BOM.

  • Hundreds of electromechanical relays (no part numbers given)Find
  • FPGA dev boardFind
  • Arduino for memoryFind
  • 32x32 LED matrixFind
  • Power suppliesFind
  • WireFind
  • Whatever mounting hardware you chooseFind

BUILDS OF THE WEEK

Five open-source builds worth your weekend, every week.

Checked like this one: what’s really in the repo, what it costs, how hard it is. One email, unsubscribe anytime.

Can I build this?

Printnothing required
Buyhundreds of electromechanical relays (no part numbers given), an FPGA dev board, an Arduino for memory, a 32x32 LED matrix, power supplies, wire, and whatever mounting hardware you choose
ToolsFPGA toolchain (for the VHDL coprocessor), oscilloscope or logic analyser for timing work, soldering iron, wire strippers, multimeter, and patience for debugging relay timing
SkillsCPU architecture understanding, FPGA programming, circuit reading fluency, and relay timing analysis — this is not a learn-as-you-go project
Timemonths of evening work, possibly years if you are learning relay timing from scratch
Cost$$$, dominated by the relays themselves — pricing depends entirely on which models you source and where
SafetyThe relay system runs on 24 V DC, so ordinary electronics care applies; if you add a contactor switching a mains lamp as Peter did for a demo, treat that part as mains wiring.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

More builds like this

All projects
You can build the machine that grows a laser crystal on your benchScienceYou can build the 4,750-dollar machine that makes rapid test strips, for 200 dollarsScienceYou can build the machine that spins thread from scratchScience

Gallery

hackaday.io
hackaday.io
hackaday.io
hackaday.io
hackaday.io
hackaday.io

Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Read the complete system circuit diagram in the project downloads (This is the foundation — everything else depends on understanding this schematic)
  2. 2.Study the FPGA coprocessor code to understand what the relays are NOT doing(Memory and maths coprocessor are separate systems)
  3. 3.Reverse-engineer a parts list from the schematic(No BOM is provided — you will be sourcing relays based on the circuit diagram alone)
  4. 4.Review the relay timing tests before buying anything(Your relay models must match the timing assumptions or you will be debugging for months)

KNOWN ISSUES

  • No explicit licence — the legal status for commercial or derivative work is unknown
  • No bill of materials, no relay part numbers, no supplier links — you are reverse-engineering the schematic into a shopping list
  • Memory is implemented with an Arduino and the maths coprocessor is an FPGA — the project page is honest about this, but do not start thinking the relays do everything
  • Relay timing is critical and model-specific — the timing tests in the docs are for the builder's specific relays, yours may behave differently
  • The project is explicitly never finished — expect to iterate and debug, not follow a fixed plan
  • Cost is unknowable without a parts list — relay prices vary wildly by model and supplier

Do the relays actually do all the computing?

No. The relays implement the CPU — instruction decode, the accumulator, the ALU and branching logic. Memory and the clock generator are Arduinos, and an FPGA acts as a maths coprocessor. The builder is open about this in the project description.

How long does a single operation take?

Slowly: calculating pi to two decimal places took 1,377 instructions and a little over eight minutes, roughly a third of a second per instruction. The clock is not a single signal but four separate pulses, because the self-retaining relay registers must be cleared before they can be written.

What instruction set does it use?

Nineteen instructions: loads with direct, indirect and double-indirect addressing, arithmetic through the accumulator, branches on carry/zero/sign, and CALL/RET giving one level of subroutine via an SR register.

Can I use different relays?

Possibly, but you will need to redo the timing analysis. The four-phase clock system depends on specific relay switching times.

Is there a PCB or is it all point-to-point?

The photos show a constructed system but no PCB files are provided. Implementation method is up to you.

Community builds

No community builds yet — be the first, we feature the best ones.

Discussion1

FROM THE COMPAREE TEAM

It calculated pi to two decimal places in a little over eight minutes, with an instruction set of just 19 commands. If you built one, would you go for speed or more instructions first?

CompareeTEAM24d agoedited

Practical notes from our verification: the project files on Hackaday.io include the complete circuit diagram, the VHDL code for the FPGA maths coprocessor and the measured relay timing tests, but there is no bill of materials and no relay part numbers, so sourcing parts means working from the schematic. The project page is clear that the relays do the CPU work while the memory and the clock generator are each built with a separate Arduino; the FPGA board is a later add-on that emulates the relay CPU and acts as a maths coprocessor. No licence is stated on the page. This is a framework for building your own relay computer rather than a follow-these-steps guide, and Peter says himself that the project will probably never really be finished. Correction (4 October 2026): we re-checked this page line by line against the project's own repository, documentation and videos, and fixed errors in earlier versions.

Peter (HPRelCo)

Peter has been building this relay computer since 2023 — first simulated in the game Logic World, then in real 24 V relays — and documents it in more than 140 Hackaday.io logs, on relaiscomputer.at and on YouTube. He describes the project as never really finished: every improvement reveals another idea to try.

Web

Star the project on GitHub

DISCLAIMER

  • Comparee is not the author of the projects featured here. All rights to each project belong to its creator — every page links to the original source, and we never host creators’ files.
  • Information is provided without warranty and may become outdated as projects evolve. Prices are indicative bands only — always check the creator’s parts list for current costs.
  • Building and operating any project is at your own responsibility. Protective equipment, safe workshop practice and compliance with local regulations are the builder’s responsibility.
GitHub repo