Weekend Hacking: Wireless Charging for a HP-25 Calculator
An Anonymous Coward writes:
https://partsbox.com/blog/wireless-charging-for-a-hp-25-calculator-05-2021.html
Or, bringing a 45-year old calculator into the 21st century.
The HP-25 was a revolutionary calculator for its time. Introduced in 1975, it was the first affordable programmable engineering calculator. As years passed, more advanced models were developed with more functionality. But the 25 struck a particularly good balance between features, size, and ease of use.
[...] The HP-25 I have was bought by my Dad, around the time I was born. I always enjoyed using it, but this whole series of calculators (dubbed "Woodstock") was limited by battery pack design. The original battery pack contained two sealed NiCd cells, which obviously failed many years ago. Most people replaced their NiCd cells with new ones, then with NiMh cells, or even alkaline AA batteries. This was always problematic: newer batteries were slightly larger and never fit well. Also, the power consumption of a calculator with an LED display was significant, so frequent battery replacements were needed.
The HP-25 was shipped with a "charger" (really just a transformer), but the charging circuit was terrible: the charger without load supplied 10V AC: a higher voltage than the calculator could handle, with the assumption that connected NiCd battery cells would clamp the voltage. The entire charging circuit consisted of a diode and a resistor! If your cells were not making proper contact, or if you connected a charger without the battery pack inserted, your calculator would be toast.
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.
The article includes pictures, schematics, and step-by-step descriptions of its design and implementation.
I had to make do with TI calculators in college but seeing this one with its LED display sure brings back many memories. Any Soylentils have an HP back in the day? What model?
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