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wi-fi charging for a HP-25 calculator

wi-fi charging for a HP-25 calculator

2023-04-30 12:49:33

Or, bringing a 45-year outdated calculator into the twenty first century.

The HP-25 was a revolutionary calculator for its time. Launched in 1975, it was the primary reasonably priced programmable engineering calculator. As years handed, extra superior fashions have been developed with extra performance. However the 25 struck a very good stability between options, measurement, and ease of use.

In the present day we reside in a really completely different world: all of us have a number of highly effective computing gadgets. No engineer would even take into account doing graphing on a calculator as we speak: we’ve got computer systems that may do that quicker and higher. Equally for algebraic manipulations or symbolic computing. What we nonetheless want, nevertheless, is an efficient engineering calculator — an simply accessible gadget that may carry out easy calculations rapidly and that has simply the suitable variety of capabilities.

This is the reason the HP-25 is changing into enticing once more. If you happen to work with electronics or mechanical design, you typically have to work with scientific or engineering notation and carry out calculations for which the straightforward calculator apps in your telephone are too restricted. The easy keystroke-based programmability means which you could rapidly automate duties: for instance, I typically have the parallel resistance components entered as a program, accessible utilizing a single keystroke. Enter two resistances, press R/S and get the outcome. The HP-25 was designed by engineers, for engineers, and you’re feeling it once you use it. Importantly, it suits nicely into your palm and can be utilized with a single hand, which is not true for a lot of in any other case wonderful later HP calculators (just like the Voyager sequence: HP-11C and HP-15C).

The HP-25 I’ve was purchased by my Dad, across the time I used to be born. I at all times loved utilizing it, however this entire sequence of calculators (dubbed “Woodstock”) was restricted by battery pack design. The unique battery pack contained two sealed NiCd cells, which clearly failed a few years in the past. Most individuals changed their NiCd cells with new ones, then with NiMh cells, and even alkaline AA batteries. This was at all times problematic: newer batteries have been barely bigger and by no means match nicely. Additionally, the facility consumption of a calculator with an LED show was important, so frequent battery replacements have been wanted.

The HP-25 was shipped with a “charger” (actually only a transformer), however the charging circuit was horrible: the charger with out load provided 10V AC: the next voltage than the calculator may deal with, with the idea that related NiCd battery cells would clamp the voltage. All the charging circuit consisted of a diode and a resistor! In case your cells weren’t making correct contact, or in case you related a charger with out the battery pack inserted, your calculator can be toast.

I made a decision I ought to do one thing to make my HP-25 usable day by day. So, I got down to design a chargeable battery pack, which replaces the unique one, however makes use of a contemporary Li-Po battery, and has Qi/WPC wi-fi charging with a USB fallback.

I now have the proper engineering calculator, that’s 45 years outdated and but sits fortunately on a Qi energy pad and recharges. If I haven’t got a pad accessible, I can take away the battery pack and use the micro-USB connector within the facet to cost it. However because it turned out, charging is not truly wanted that always — a 900mAh LiPo gives sufficient power for a lot of weeks with my utilization.

The design assumptions have been:

  • Ought to change the unique battery pack
  • No modifications to the calculator itself
  • WPC/Qi wi-fi charging
  • Micro-USB connector for wired charging
  • Li-Po battery lasting for at the very least a number of days of regular use
  • Low-battery indicator

I designed the battery pack enclosure in Fusion 360, taking measurements of the unique battery pack. That turned out to not be easy: the unique pack was designed for injection molding, and draft angles sophisticated the geometry. Since I do not plan to mass-produce these, I did not trouble with designing for injection molding and assumed 3D printing utilizing SLS (Selective Laser Sintering). This is the reason my battery pack opens in a different way from the unique one, offering higher entry to electronics and battery, however sacrificing the flexibility to be injection-molded.

No fasteners have been used: a easy spring clip is sufficient to maintain the quilt in place, and the pack is used within the calculator more often than not anyway, so there is no such thing as a threat of the quilt opening.

The calculator was usually powered by two NiCd cells, which have a voltage of 1.2V every. I made a decision to supply 2.5V as an alternative of two.4V, assuming that the extra 0.1V would not harm something (in spite of everything, many individuals have been utilizing their calculators with alkaline cells, which have a voltage of 1.25V), and the elevated voltage would make the increase converter within the calculator barely extra environment friendly.

I made a decision to make use of one of many trendy low-power buck controllers from Texas Devices (TPS62740). Its low 360nA working quiescent present meant that I would not have to fret about having an on/off swap. There are controllers with decrease quiescent present (even right down to 60nA!), however I wished to have programmable voltage output — learn on to be taught why.

For WPC/Qi, I went with bq51050B (Texas Devices) and a coil from Wuerth Elektronik. This proved to be rather more difficult than I believed: designing gadgets with wi-fi charging is troublesome, requires a number of prototypes and measuring gear that I do not need. Ultimately, I went with design choices which could not be optimum, however on this case (with low energy necessities) present acceptable efficiency. In different phrases, I winged it. This was made considerably tougher by the truth that the bq51050B documentation is disappointing and probably not as much as TI’s requirements.

The board additionally has a Micro-USB port for wired charging, a charging controller IC (the favored MCP73832), a MOSFET pair performing as a swap for USB energy, a bunch of ESD safety for uncovered terminals, and a discrete thermistor to watch battery temperature, since many low-cost LiPos come and not using a thermistor or with one that does not match what the charging IC expects.

Designing a low-battery indicator was an attention-grabbing problem. Assuming that voltage is a proxy for battery cost (which is not totally true for LiPo cells), how do you measure the voltage with out drawing energy consistently and discharging the battery within the course of? Keep in mind that our energy funds for that is in nano-amps: the quiescent energy draw of your complete gadget needs to be beneath 1μA.

For the reason that coulomb-counting IC gadgets are manner too costly and often are available in unfriendly BGA packages, and in addition as a result of I did not need to over-complicate the design, I needed to discover one thing less complicated.

A microcontroller-based resolution might be designed, however for the primary time in a few years I wished to design a tool with out a microcontroller and software program.

Additionally, how do you show the outcome? You’ll be able to’t mild up an LED, as that might rapidly eat the remaining power, presumably with nobody even trying on the indicator. A “battery verify” button? Presumably, however these complicate mechanical design considerably.

It took some time, however I did give you an answer.

See Also

I spotted that the calculator truly does have a built-in low-battery indicator. When battery voltage falls beneath a threshold (which I measured to be 2.1V), the LED display lights up all of the decimal dots, besides the one that ought to usually be energetic. With decimal dots “inverted” this manner, you possibly can nonetheless use the calculator, however you clearly see that batteries want changing.

So, I used a voltage (reset) supervisor with a 250nA provide present to watch the battery voltage. If it falls beneath 3V, the output goes low. That output is related to one of many voltage selector pins on the TPS62740 buck regulator (this is the reason I wanted a programmable voltage output half) and causes it to start out producing 2.1V as an alternative of two.5V. In different phrases, battery low, decimal dot LEDs mild up, mission completed!

This turned out to work fairly nicely in observe, and the two.1V threshold labored for all of the HP-25 models that I had. The one limitation is that with most LiPo batteries, their safety circuitry will reduce the output off slightly below 3V, so you do not have a lot time after your dots mild up.

My exams confirmed that the 900mAh LiPo is sufficient for about 10 hours of steady use, earlier than the low-battery indicator dots mild up, and for 10-20 minutes afterwards. Adequate for me!

All in all, I am very pleased with the results of this weekend hacking mission: my HP-25 calculators (sure, I’ve a couple of) are helpful once more and I can use them day by day with out worrying about batteries. I simply place them on charging pads once in a while. The battery life is so good, that doing this as soon as a month or so is sufficient.

If you consider it, it is fairly wonderful {that a} 45-year outdated calculator obtained a brand new life because of twenty first century expertise!

Jan Rychter (PartsBox founder)

(If you happen to loved studying this and you’re employed with electronics, please take a look at PartsBox — it is an indispensable device for corporations, and it is free for Hobbyists/Makers)

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