Retrofitting a 46-Year-Old Calculator with Wireless Charging

Jan Rychter added a new battery and wireless charging to a retro HP-25 calculator.

Cameron Coward
a month ago3D Printing / RetroTech

We live in a world full of devices designed with planned obsolescence. Apple wants you to buy the new iPhone when it comes out, so they purposefully design their smartphones to become useless within a few years. Many, if not most, modern electronic devices and appliances adhere to that philosophy. But that wasn't always the case. Manufacturers used to design products to last, because the durability and longevity of their products were selling points. For that reason and many others, some people prefer to use products that are now several decades old. But those products often lack features, which is why Jan Rychter retrofitted a 46-year-old calculator with a new battery and wireless charging.

The calculator in question is a Hewlett-Packard HP-25, a model that was in production from 1975 to 1978. It was a programmable scientific calculator designed for engineering applications. It used Reverse Polish Notation (RPN), which was popular at the time but has largely fallen out of favor. It had a ten-digit LED display that looks great, but which consumes quite a lot of power. That power originally came from rechargeable NiCad batteries. Those batteries gave up the ghost a long time ago and were inconvenient to recharge even when they were new. So Rychter killed two birds with one stone by retrofitting wireless charging and a modern battery.

The replacement battery is a 900mAh lithium-polymer cell, which holds enough juice for Rychter to use the calculator for weeks between charges. A replacement battery case holds that LiPo cell. Rychter designed that case in Autodesk Fusion 360 CAD software and fabricated it on a SLS (Selective Laser Sintering) 3D printer. It fits right into the same compartment as the original, so no modifications to the calculator itself were required. The LiPo battery is chargeable through a micro USB cable, but Rychter also added a Qi wireless charging coil. That means that they can recharge this vintage calculator by placing it on any modern wireless charger. The calculator's original low-battery indicator still functions, so no additional LEDs or indicators were needed.

Modern calculators may have more features, but the HP-25 offers exactly what Rychter needs and now they can use their calculator without any hassle.

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