Wireless charging for a HP-25 calculator

[jwr]

Thanks for all the kind words! I'll try to reply to most things in a single post:

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. Don't try to power the calculator directly from the LiPo, it will damage the calculator. That's why this thing wasn't so obvious to design — although it isn't really a complicated design, either.

Yes, this took much more than a single weekend :-) There is no way to design a perfectly fitting pack the first time. I think I went through three iterations of the case, two iterations of the lid, and two iterations of the PCB with electronics.

For people talking about purchasing the base — this speaks the standard Qi/WPC protocols, so it can be used with pretty much *any* Qi charging pad that can charge a modern phone. The charging coil is fairly small, so you have to position the calculator fairly precisely on the pad, though.

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.

I don't have any specific plans for what to do next. This really was a weekend hacking project, with blatant advertising for my PartsBox (software for electronic parts inventory) business.

The first problem with selling this is that selling and shipping LiPo batteries is a major pain. I think the only thing I could potentially sell would be a kit, without the LiPo. And to connect the LiPo you would have to use a crimping tool to crimp a connector onto the cables. Hmm, actually I think LiPos from Sparkfun or Adafruit should work, I always tried to use the popular connector and polarity.

The second problem with selling is that it makes little sense financially: it would need to be quite expensive (low-volume production) and even then it isn't really a business.

Making it open-source is not something I'm considering right now. I did publish projects as open-source in the past, both software and hardware, but then I end up having to support them and respond to questions. I just don't have the time right now. Also, some of my projects ended up being sold on Etsy, using my (pirated) photos, which I find immensely annoying.

In other words, I don't know what to do next. Perhaps I'll find someone who has a business already and wants to add this to stuff they sell. I don't know.

Oh, also, open-sourcing does not mean that this thing is easy to build. You need to be comfortable handling 0402 components, QFN chips, reflow/hot-air soldering, and the case probably wouldn't print well on a home FDM printer. It's designed for SLS, the walls are 1mm thick.

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.