Wireless charging for a HP-25 calculator

[Maximilian Hohmann]

Hello!

(05-13-2021 07:31 PM)jwr Wrote:  If you plan to rig something like this on your own, you will need to add a voltage regulator to get the LiPo voltage down from 3-4.2V to 2.4-2.5V which the calculator expects.

I was thinking of two silicon diodes in series between the battery and the calculator to achieve that. I don't really have much use for my calculators, I mainly collect them - if this will cost an hour of operation on each battery charge then I can live with that.

(05-13-2021 07:31 PM)jwr Wrote:  This kind of pack could definitely be designed for other HP series, but only the schematic would be the same — every pack would need a different PCB and obviously a different case.

And don't forget that most classic HP calculators need three batteries, so even a fully charged 3,7V LiPo may just not be sufficient. So one would either need a 2-cell LiPo or a voltage booster. More complications...

(05-13-2021 07:31 PM)jwr Wrote:  I... with blatant advertising for my PartsBox (software for electronic parts inventory) business.

Although I have accumulated quite a few boxes of parts over the years I think it's still not enough for an inventory software. My calculator collection, on the other hand, might benefit from one :-)

(05-13-2021 07:31 PM)jwr Wrote:  Also, some of my projects ended up being sold on Etsy, using my (pirated) photos, which I find immensely annoying.

Are you familiar with Tindie (https://www.tindie.com/)? Maybe that could be a platform for your projects.

(05-13-2021 07:31 PM)jwr Wrote:  Perhaps I'll find someone who has a business already and wants to add this to stuff they sell.

There are two or three familiar names on this forum who could be candidates for that...

(05-13-2021 07:31 PM)jwr Wrote:  Please be careful with those "wireless charger receivers" from AliExpress — the ones I saw have USB connectors, so they produce 5V. Definitely not what your calculator wants to see on its battery terminals, and this will not charge a LiPo, you need a charge controller for that.

I ordered some which are based on the TI BQ51013 chip and seem to be clones (or vice-versa?) of this one here from Adafruit: https://www.adafruit.com/product/1901
The Aliexpress sellers claim that the receiver can charge LiPo cells directly (there are pictures of a LiPo connected to the module) whereas Lady Ada writes that a LiPo charger module should be inserted in the circuit. I will carefully try it out - outside the calculator and outside the house, at work they keep showing us what LiPo fires look like - and see how it goes. I am 95% certain that somewhere in one of my univentorised boxes with stuff there must be some LiPo charging modules :-)

Anyway, should you ever consider to make a batch of your devices I am certainly in for one or two!

Regards
Max