Vintage Sinclair Calculator Restored with Arduino

The "Sinclair" name is most often associated with Sinclair Research Ltd., which is the British company responsible for ZX Spectrum 8-bit home computer. But before Sir Clive Sinclair turned his focus towards computers, his company Sinclair Radionics Ltd. developed a range of consumer electronics devices and scientific instruments. Like many other companies of the era, including Commodore, Sinclair Radionics produced calculators. Digital calculators were high-tech devices at the time that cost quite a lot of money. But Sinclair made affordable models — a feat they would later replicate in the computer world. Ynformatics purchased a non-functional Sinclair calculator on eBay and restored it to live with an Arduino.

This particular model, the Sinclair Cambridge Memory, hit the market in the mid-1970s. It was small enough to fit in a pocket and cheaper than most rivals. Similar models had some scientific functions, but the Cambridge Memory model was a more barebones model. It had an eight-digit seven-segment LED display and a keypad with 19 buttons. Ynformatics's calculator powered on, but had a couple of problems. Some of the buttons were not working correctly and the main C-595 processor chip was faulty. It is impossible to find new chips today, so Ynformatics turned to a modern microcontroller for help.

Ynformatics started by disassembling the calculator and giving the keypad contacts a thorough cleaning. As they are wont to do, the thin plastic fastener pegs broken off during disassembly. Ynformatics used ABS rods to replace those. They then reverse-engineered the keypad and display drivers. This calculator runs with a positive earth, which is somewhat unusual and means that the signal logic voltage is inverted. Ynformatics used an Arduino Nano during testing and was able to figure out how to drive the display and read the keypad matrix — a task complicated by the fact that the two share pins. But the Nano, even as small as it is, was still too large to fit inside the case. So Ynformatics designed his own small PCB to act as a breakout board for an ATmega328P microcontroller, which they flashed with the previously-developed code using an Arduino Uno. The code is simple and only replicates the standard functions, but now Ynformatics has a cool vintage Sinclair calculator that actually works.