Wireless charging for a HP-25 calculator

[jwr]

Hi fellow HP enthusiasts,

I thought you might enjoy my weekend hacking project. I decided I should do something to make my HP-25 usable every day. So, I set out to design a rechargeable battery pack, which replaces the original one, but uses a modern Li-Po battery, and has Qi/WPC wireless charging with a USB fallback. I now have the perfect engineering calculator, that is 45 years old and yet sits happily on a Qi power pad and recharges. If I don't have a pad available, I can remove the battery pack and use the micro-USB connector in the side to charge it.

For more details: https://partsbox.com/blog/wireless-charg...-2021.html

[charger73]

fantastic and very cool.

I like one for my HP41, HP71 and HP75

Thank you :-)

[Maximilian Hohmann]

Hello!

What a phantastic project. But that must have been a very long weekend. Designing and 3D-printing the box alone would occupy me for much more than a couple of days...
Do you have any intention of getting a small batch of these boards made? If so, I would be very interested!

Thanks
Max

Addition: I just ordered a couple of ready-made wireless chargers from Aliexpress (shortened link: shorturl.at/puPQ7 - if that link doesn't work after some time just search for "wireless charger receiver" there and sort the list lowest price -> highest price) at a whopping 2 Euros a piece. It is small enough to fit inside an empty - or 3D printed - Woodstock battery box. Accoring to the listing this can be directly connected to a LiPo battery. In two weeks or so I will know more.

[PANAMATIK]

Fantastic project! And very nicely designed!

It makes my HP-25 Low Power circuit useless, although the charger alone doesn't give you more memory. It would be perfect to place my "Woodstock" calculators to the wireless charger from time to time, the HP-25 Low Power only twice per year.

Definitely I would order one base station and several batteries, if available.

Bernhard

[Maximilian Hohmann]

Hello!

(05-12-2021 02:07 PM)PANAMATIK Wrote:  Definitely I would order one base station and several batteries, if available.

I just ordered 4 fitting batteries from AliExpress (https://de.aliexpress.com/item/100500225...4c4dO8SMav). I took the 800mAh version because they are a little bit smaller than the 900mAh ones.
And I will definitely install this wireless pack into my wonderful Panamatik Low Power Woodstock because, for some reason I could never find out (my guess is the voltage converter for the GPS receiver), it keeps draining it's batteries.

Regards
Max

NB: These days I try to order as much small stuff as I can from China because in July the import duty excemption for shipments below 22 Euros will expire. After that, even small orders will become quite expensive, not because of the 19% duty but because the post office will charge an extra fee of 6 Euros for each package. A 2-Euro-Battery will then cost 8.50 Euros.

[jjohnson873]

(05-12-2021 09:09 AM)jwr Wrote:  Hi fellow HP enthusiasts,

Hi Jan...is this an open source project? If so, I'd be interested in where to find the BOM and stl files. A very nice project!
~ Jim J. ~

[rprosperi]

Like Bernhard, I would like to purchase 2 of the battery packs and a base/charger, if you will be selling those too.

My 29C and WLP-25E deserve wireless charging, they've earned it by many years of faithful service!

[PANAMATIK]

You found also a very clever solution, and the only true one, for indicating "low battery". The original HP-25 manual states that there is only one minute left after the decimal dots appear. Compared to that. your "10-20 minutes afterwards" are very comfortable.

Bernhard

[Dreato]

I'm interested! Will these be produced / sold?

Regards,

Dave

[jwr]

(05-12-2021 02:07 PM)PANAMATIK Wrote:  It makes my HP-25 Low Power circuit useless,

It most definitely does not — I've seen your work and I highly respect it. In fact, I've been thinking about buying your kit. Your solution is better in many ways except one: it requires serious modifications to the internals of the calculator.

The biggest drawback of this rechargeable battery pack is that if you don't have a 25C, you will lose memory contents when you switch the calculator off. And even if you have a 25C, your display mode won't be remembered, for example, you only keep the registers and program memory.

Very happy to read what you wrote about the low-battery indicator: I was worried and a bit disappointed about the short time (it was actually closer to 10 minutes), and never knew that it was originally designed to be even shorter. Good news!

[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.

[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

[jwr]

(05-13-2021 08:35 PM)Maximilian Hohmann Wrote:  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.

That's not likely to work well — LiPo voltage varies a lot, the usable range is roughly 3.0-4.2V. With two diodes, you'd be getting 3.0V when the battery is fully charged (or charging) and 1.8V when it's nearly depleted.

(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.

(05-13-2021 08:35 PM)Maximilian Hohmann Wrote:  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...

You're right, I actually forgot about that — it's not a huge problem (one would need a buck-boost instead of a buck converter), but it does require a redesign.

(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.

(05-13-2021 08:35 PM)Maximilian Hohmann Wrote:  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 :-)

bq51013 is a wireless receiver, with no charger. What it does is produce a constant voltage on the other end. The Adafruit device is connected to a LiPo charger in their demo. I use the bq51050b, which has an integrated LiPo charger (though I placed another charger in there as well, to be able to also easily charge from USB if needed).

To be clear, the circuit I built is not rocket science, but power management is tricky, especially with LiPo batteries. I built this because it turned out that I couldn't easily assemble it from available off-the-shelf components.

[Maximilian Hohmann]

Hello!

(05-14-2021 07:53 AM)jwr Wrote:  That's not likely to work well — LiPo voltage varies a lot, the usable range is roughly 3.0-4.2V. With two diodes, you'd be getting 3.0V when the battery is fully charged (or charging) and 1.8V when it's nearly depleted.

Unfortunately... So a voltage converter is a must.

(05-14-2021 07:53 AM)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.

So the diode and maybe a resistor will have to go between the wireless receiver and the LiPo battery instead. In one of my shoeboxes I found a bag with battery protection boards (BBP), so while I wait for the wireless receivers and LiPos from China I tried out this (the original (dead) Woodstock battery pack is not connected to anything, it is just there to show the size):



A single cell LiPo battery is connected via a BBP to a small buck converter which I have set at 2,5V. The BBP should protect the battery from both overcharging/overvoltage and deep discharging and the calculator will never see more than 2,5V, no matter what the output of the wireless receiver may be. It should all be small enough to fit inside an original battery case.
I tried it with my Panamatik LP25 and it worked well. The only problem is that my cheap buck converter draws 5mA even when the calculator is off, which means that it will drain the battery within three or four days. Not good. On the other hand it will have to stay on all the time when one of the "C" Woodstocks is going to be powered.

(05-14-2021 07:53 AM)jwr Wrote:  To be clear, the circuit I built is not rocket science, but power management is tricky, especially with LiPo batteries. I built this because it turned out that I couldn't easily assemble it from available off-the-shelf components.

Well, rocket science is more my background, but I am afraid that I can't design a circuit like yours nor do I have the tools and skills to assemble it :-)

Regards
Max

[Maximilian Hohmann]

Hello!

An update on my "poor man's wireless Woodstock charger made from readily available components".

I have received more parts from China, unfortunately not yet the small LiPo battries that fit inside the battery holders, so that at least I can test my setup outside the calculator.

1. I have thorougly tested the small battery protection board. It reliably turns off the charging current when the battery reaches 4,2V. I left it connected in a safe place for three days in a row. As can be seen in the picture I power my wireless charging pad through a "smart" USB cable with an integrated Volt- and Ampere-meter. The current drops to 0,1A once the battery is fully charged. Those 0,1A seem to be what is required for the communiation between the transmitting and receiving coils. It is the same when I place a fully charged phone on the transmitter.

2. I got a better voltage converter for the calculator now (Pololu 2.5V, 500mA Step-Down Voltage Regulator D24V5F2). When the calculator is turned off it only draws 1 microAmpere, which means no battery drain is to be expected. The maximum current is 170mA with the calculator on and displaying all "8"s. Alternatively I have bought some AMS1117 2.5V low drop voltage regulator chips. I have not tried them out yet.

3. As can be seen in the pictures the circuit works with and without the coil receiving engery just as it should. Other than with an "normal" Woodstock it should also be possible to use this contraption without a battery safely, because the voltage regulator will protect the calculator form overvoltage.

The only thing left to do once I have the batteries is to "make" proper battery connector springs that also hold the battery pack in place.

Regards
Max

[jwr]

> I have thorougly tested the small battery protection board. It reliably turns off the charging current when the battery reaches 4,2V.

Max — I really strongly suggest using a LiPo charger circuit. Yes, the battery protection board is supposed to intervene in extreme conditions (such as under- and over-voltage), but it does not control charging. With small LiPos you might be able to get away with things, but I would not rely on it, because mistakes with LiPo batteries cause serious fires. I wouldn't risk it.

Lithium polymer batteries should only be charged by a specialized IC. You can easily get little modules which will do the job, such as this one from AdaFruit: https://www.adafruit.com/product/1904

Your D24V5F2 synchronous buck is a much better choice than the 1117 linear regulator, because a linear regulator wastes lots of power.

[rprosperi]

(05-12-2021 02:33 PM)Maximilian Hohmann Wrote:  And I will definitely install this wireless pack into my wonderful Panamatik Low Power Woodstock because, for some reason I could never find out (my guess is the voltage converter for the GPS receiver), it keeps draining it's batteries.

Max, thanks for this comment!! I too was having an issue with my WLP-25E draining batteries in only a few weeks, even if not used at all. I even replaced the batteries with new Tenergy AA flat-top cells, and the problem remained. So after seeing you comment, I hunted in the manual to check if there is a master power-on/off setting for the GPS and indeed there is, and indeed I found mine had been left on, for many, many months, thought I clearly forgot about it.

To turn GPS OFF, press [EEX] while in GPS mode (activated by [g] [g] [STO] on the WLP25). It will then tell you "GPS OFF". This is an on/off toggle so make sure you are leaving it turned off.

Thanks again for mentioning that!