Geek hacks an old-school HP calculator for wireless charging

Shane McGlaun - May 19, 2021, 7:54am CDT
Geek hacks an old-school HP calculator for wireless charging

We take wireless charging for granted with many of the gadgets we use today, such as smartphones, among others. Four decades ago, electronics like calculators were relatively new inventions, and most people were just surprised to have them and never even considered the ability to charge the devices wirelessly. When new, old electronics typically relied on old-school batteries for power.

One such old-school electronic is the HP-25 calculator, initially introduced in 1975. It was a breakthrough as the first affordable programmable engineering calculator with a good balance of features, size, and ease of use. The HP-25 is a notable device designed by engineers for engineers, and while it’s old, it still has use today.

One electronics hacker has an HP 25 his dad purchased around the time he was born, and one of the drawbacks was its limited battery pack design. That battery used a pair of NiCd cells that stopped working many years ago. To make the calculator more usable today, the owner set out to design a rechargeable battery pack to replace the original one using a modern lithium polymer battery and wireless charging capability along with USB charging.

The result is an old-school calculator more than four decades old that can be placed on a Qi power pad for charging. The battery pack can also be removed and connected using a micro-USB cable to charge when wireless isn’t an option. The modern battery pack also packs in 900mAh of juice, providing lots of power.

The new battery pack was designed to replace the original with no modifications to the calculator required. It also features a low-battery indicator. The battery pack was designed using Fusion 360 and was based on the measurements of the original battery pack. That was a challenge because the original battery pack was designed for injection molding and had complicated geometry. Much of the hardware used in the design came from Texas Instruments, and full details can be seen here.


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