A 16-Bit Relay Computer Calculates Pi and Drives a 32×32 LED Display
A homebuilt 16-bit relay computer entered Hackaday’s 2026 Retrocomputing Challenge with a 19-command instruction set, an eight-minute calculation of three digits of pi, and a 32×32 LED-matrix display.
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A homebuilt 16-bit relay computer has entered Hackaday’s 2026 Retrocomputing Challenge with the sort of specification that makes modern computing feel almost excessively fast: a 19-command instruction set, an accumulator-based architecture, and enough working hardware to calculate three digits of pi in about eight minutes. The result is displayed on a 32×32 LED matrix, turning a deliberately slow computation into something that can be watched as well as measured. Hackaday’s report describes the computer and its pi calculation.
The project is retrocomputing in the most literal sense. Rather than recreating an old machine in software, it builds a functioning computer from relays, gives it a compact instruction set, and lets the hardware execute a program through an accumulator-based design. There is no need to pretend that the machine is competitive with contemporary processors; the point is that the underlying operations are visible in the physical system, with the display providing an immediate record of what the computer is doing.
The pi calculation is a particularly good demonstration because it makes the machine’s limitations part of the result. Three digits in roughly eight minutes would be an absurdly small workload for modern hardware, but here the elapsed time is evidence that the relays, instruction set, memory model, and program are all working together as a computer rather than merely serving as a static exhibit. The LED matrix gives that process an output with the same physical character as the computer itself.
The project is one of the submissions to Hackaday’s 2026 Retrocomputing Contest, which runs from August 18 through October 27, 2026. The official contest page describes it as a competition for retrocomputer projects and listed 98 submissions when fetched, placing this relay machine inside a broader collection of attempts to rebuild, reinterpret, or rethink older computing systems.
There is something satisfying about the scale of the achievement. A 19-command computer that takes about eight minutes to produce three digits of pi is not useful because it is fast; it is useful because every part of the computation has been brought back into view. The relay logic, the instruction set, the accumulator, and the LED display form one complete system, with the slowness no longer hidden behind layers of abstraction but made into the project’s most legible feature.