January 19, 2022


The fine art of fashion

Vintage HP-25 Calculator Receives Wi-fi Charging

[Jan Rychter] actually likes his many HP-25C calculators, but the unique battery pack style and design is crude and out-of-date. No issue — he whips up a substitution making use of Fusion 360 to style an enclosure, prints a number of on his SLS 3D printer, and packs them with LiPo batteries and Qi/WPC wireless charging circuits.

In his website put up, he points out the objectives and various style and design choices and compromises that he manufactured together the way. We like [Jan]’s frank honesty as he remarks on anything we have all been responsible of at a single time or a different:

In the stop, I went with design and style selections which may well not be ideal, but in this scenario (with small electrical power needs) present satisfactory effectiveness. In other text, I winged it.

1 trouble which proved complicated to resolve was how to deliver a low battery indicator. Considering that small voltage on a LiPo is various from the original HP-25’s NiCad cells, it wasn’t clear-cut, specifically since [Jan] challenged himself to make this devoid of using a microcontroller. He found that the HP-25’s inner minimal battery circuit was activated by a voltage of 2.1 volts or lessen.

In a definitely clever hack, [Jan] came up with the thought of making use of an MCU reset supervisor chip with a small voltage threshold of 3. volts, which corresponds with the minimal voltage threshold of the LiPo battery he is using. The reset signal from the supervisor chip then drives a single of the pins of the TPS62740 programmable buck converter, switching its output from 2.5 volts to 2.1 volts.

This venture is exciting on various levels — extending the daily life of a helpful but finish-of-life calculator, enhancing the authentic battery structure and introducing new charging methods not out there in the early 1970s, and it is a thing that a hobbyist can afford to pay for to do in a home electronics lab. We do speculate, could these types of a modification could change an HP-25 into an HP-25C?

We’ve penned about battery pack substitution challenge in advance of, like one particular for the Sony Discman and a further for an electrical drill. Let us know if you have any battery pack replacement accomplishment (or failure) stories in the responses below.