Even if you’re obsessed with owning the latest and greatest tech, there’s probably still an older gadget you still have a soft spot for. Maybe it’s the original Game Boy, or a classic PDA? For Jan Rychter, it’s a Hewlett-Packard HP-25 calculator, which they recently upgraded so it was compatible with at least one modern convenience: wireless charging.
In the 46 years since Hewlett-Packard first introduced the model 25, the capabilities of calculators have evolved by leaps and bounds, with touchscreens and graphing capabilities and more buttons than you’ll find in Nasa’s mission control. But Rychter is convinced that several decades after its debut, the HP-25 still represents “a particularly good balance between features, size, and ease of use.” It was designed by engineers to feature the specific functions and basic programmability that engineers would actually need, and today remains a streamlined tool for quickly performing calculations when reaching for a computer is overkill.
The only drawback to still using an HP-25 today is its ancient power source. Its power-hungry LED display originally relied on rechargeable nickel-cadmium batteries which decades ago would have lost their ability to hold a charge. That coupled with a power adapter that would potentially fry the calculator when plugged in without batteries inserted meant that a classic piece of tech was in desperate need of an upgrade.
The solution was to first recreate a perfect match of the HP-25's original battery pack enclosure by modeling it in 3D software, updating the design to accommodate modern electronics and features, and then using a selective laser sintering 3D printer to churn out the part in high detail, including a working spring clip to keep it securely in place when installed.
Instead of ancient nickel-cadmium batteries, the upgraded power pack now features a rechargeable lithium-polymer battery that can be topped off using a USB cable connected to a microUSB port that’s accessible when the pack is removed from the calculator. But even charging with a cable feels old-fashioned at this point, so Rychter also included a Qi-compatible compact wireless charging coil which allows the battery to be topped off by simply plopping the calculator down on a charging pad like a smartphone.
Thanks to the rest of the calculator still using original inefficient power-hungry components, battery life is still limited to about 10 hours of continuous use, which translates to about a couple of weeks’ worth of calculations with regular use. But one other clever trick Rychter managed to get working was making the HP-25's original low-battery indicator—which would invert the illuminated decimal points on-screen when it was close to dying—work with the modern battery upgrade. This eliminated the need to introduce battery status LEDs which potentially wouldn’t last long enough to actually be noticed with what little charge was left in the battery, or a battery check button which would add additional and unnecessary complications to the replacement power pack’s design.