Michal Zalewski and Son Build a Relay-Driven Calculator in a Nod to the Early Days of Computing
With a fistful of Omron relays — and, admittedly, an eight-bit microcontroller — this calculator clicks its way through your math.
Developer Michal Zalewski has decided to go back to basics with the build of a homebrew calculating machine created, in keeping with classics from history, using electromechanical relays: the Calc-U-Later.
"Some time ago, my eldest son decided to build a single-bit computer using discrete transistors," Zalewski offers by way of background to the project. "As dictated by the iron laws of sibling rivalry, his younger brother soon approached me and asked if I could help him build a computing device that’s even more obsolete. After some back-and-forth, we settled on a relay-based calculator. I was given further instructions: the PCBs must be old-school and off-yellow, and all relays must be see-through."
Relay-based computing is nothing new — in fact, quite the opposite. The earliest computers, primarily built as fire-control calculators for the military, used electromechanical relays as binary switches for logic elements either exclusively or in conjunction with vacuum tubes: the Harvard Mark I, Harwell Dekatron, Zuse Z2, and perhaps most famously of all the Turing-Welchman Bombe, inspired by Marian Rejewski's Bomba Kryptologiczna and used to break the German Enigma cipher at Bletchley Park in World War II.
Zalewski and his offspring, though, set their sites sensibly lower for their project. The Calc-U-Later is a compact computer which uses Omron G6C relays with, as requested, transparent housings to form the Boolean logic gates required for a basic calculator. The finished machine isn't entirely relay based, however: there are 12 relays in total, forming an adder, which are supported by a Microchip ATmega644V eight-bit microcontroller.
"The microcontroller is there to take care of a variety of auxiliary tasks that would require hundreds of additional relays," Zalewski explains. "This includes keypad scanning, digit decoding, operation sequencing, error handling, and memory. That said, the adder has a clearly-delineated role and is there to perform actual math."
The calculator itself is paired with a display board, which drives six seven-segment LED displays entirely with electromechanical relays — requiring 42 in total, wrangled using six 74HC259 eight-bit latches to reduce the number of data lines required. "The idea was to make the entering digits as satisfying as watching the ALU at work," Zalewski says.
Zalewski's full write-up is available in his Substack newsletter, including a mesmerizing video of the display in action.