Fredrik Andersson wanted to hear a computer think, so he built one from enough relays to make the thinking audible.
Visit Zusie, the relay computer
Zusie is an 8-bit homebuilt computer inspired by the electromechanical machines of Konrad Zuse. Its data path is dominated by relays—the clicking switches used in early computers and telephone exchanges before vacuum tubes and transistors took over.
The specifications are serious: an 8-bit data bus, 16-bit address bus, three 8-bit accumulators, two 12-bit registers, a 12-bit program counter, a 12-bit stack pointer and an ALU capable of logical operations, addition, shifts, increment and decrement.
Memory and microprogram storage use solid-state components, but the machine’s visible logic remains gloriously electromechanical.
The project began with a museum visit and a junk box
Andersson traces the idea to a 2008 visit to Berlin’s technical museum, where he encountered Konrad Zuse’s early machines and became fascinated by the relay-based Z3.
Then the supply problem appeared.
A relay computer needs hundreds of relays, and new parts would make the project absurdly expensive. In 2009 Andersson found old telephone-exchange circuit boards being sold cheaply by a Swedish surplus dealer. He eventually made a 500-kilometer trip each way and bought roughly 100 boards containing relays.
After desoldering and sorting them, he ended up with about 1,500 usable switches.
The extraction process produced another piece of documentation that only a personal project page would preserve: heating old circuit boards with a hot-air gun inside an apartment kitchen worked extremely well and smelled, in his words, approximately like death mixed with poison.
Engineering lessons come in layers.
The finished machine uses a microcoded architecture. Andersson wrote a microcode assembler, a regular assembler and communications software for an old DOS laptop connected through a parallel port. The project page includes architecture notes, a build log, downloadable software and even an online simulator.
That depth matters because a homebrew computer is much more interesting when the design survives alongside the photographs.
The site also links Zusie into a wider Web ring of homebuilt CPUs and relay-computer projects. This was not one isolated eccentric machine; it belonged to a loose community of people rebuilding computation from first principles with whatever technology amused them most.
Modern processors switch billions of transistors silently at impossible speed.
Zusie is slower, larger and less efficient by every practical measure.
It also clicks.
Sometimes that is the feature.