Weekend hacking: wireless charging for a HP-25 calculator | Hacker News Reader

Weekend hacking: wireless charging for a HP-25 calculator

partsbox.com

73 pointszdw18 comments

Pretty cool hack, these things are just beautiful and math is math, so no need to retire these things.

Two things I stumbled over:

It says alkalines are 1.25V, could've sworn they're 1.5V. don't have any around to check...

Also, first they state with normal AAs you'd need to replace the batteries frequently, but with the 900mAh lipo pack it's good for several weeks of usage. Aren't AAs at over 1000mAh? Again I don't actually have any around to check...

You are right about alkalines, they are definitely marked as 1.5V nominal. It is the reasoning behind the warnings on Gameboy manuals to not use rechargeable batteries (1.2V), but they worked just fine nonetheless.

Anyone know what they were worried about happening?

I still feel guilty...

IIRC it was mainly a problem with cart based accessories like the camera.

The low battery indicator doesn't work particularly well with recharchables.

With a standard non rechargeable alkaline the voltage drops gradually as the battery discharges, so the battery light coming on at a certain voltage gives the user enough time to save their game and replace the batteries.

Rechargeable AA batteries have a much flatter voltage curve - they basically give a stable 1.2V until they run out of juice and then drop rapidly.

Edit: not lithium, Ni-MH

> Lithium ion batteries have a much flatter voltage curve - they basically give a stable 1.2V until they run out of juice and then drop rapidly.

Did you mean Ni-MH (and possibly Ni-Cd earlier)?

Lithium based rechargeables are usually not in AA format and the voltage is closer to 3.6V.

Oops, yes. For some reason I thought rechargeables like enerloop were lithium but I was remembering wrong.

perform calculations for which the simple calculator apps on your phone are too limited

C'mon now. I know this guy's too smart to think the only calculator apps are the ones that come preinstalled. There exists every conceivable calculator program, including half a dozen HP-25 emulators. You're allowed to like using an old calculator just because it feels good and you enjoy it.

One thing your phone can't emulate is clicky buttons

time for a calculator -> bluetooth keyboard project

In the native ios calculator, rotate your phone to landscape for more advanced options!

I use an early '00s TI calculator daily. It's great for quick calculations that are a bit beyond my mental math capabilities.

I've considered these types of mods before. The biggest reason I haven't is the ROI feels impossibly low. A set of 4 disposal batteries last >6 months. A lithium ion battery lasts ~4 years before needing to be replaced. Doing a lithium ion mod would save me less than 32 disposable batteries before I would need to replace the rechargeable battery. It seems way way faster to just keep some disposable batteries in my desk, and not have to worry about charging or replacing a lithium pack. This is cool, but feels impractical.

Your TI calculator use 4xAAA batteries, yes? Why not use NiMH rechargeables instead of disposables? NiMH are so cheap these days that they pay for themselves after only a handful of recharge cycles. The other advantage of NiMH is that they rarely leak (I've never seen one leak). In contrast to disposable batteries which tend to leak and destroy electronics.

Not that your overall point is wrong but lithium ion batteries aren't that bad. I've have quite a few over a decade old that work just fine.

As a person with no soldering skills and no 3D printing knowledge/equipment, this makes me very happy and only slightly less jealous. Sometimes I do get ideas about building things to address design issues in everyday items but I'm not confident about 3D design (I think I can afford to buy a 3D printer, if I was confident).

Can anyone who has gone from zero to actually designing/producing 3D printed items share how difficult the learning process will be?

I’ve made small things for repairs using a ToyBox 3D printer and OpenSCAD. OpenSCAD is just like programming.

Examples:

https://blog.jgc.org/2022/12/the-rogers-watch-retro-display-...

https://blog.jgc.org/2022/11/restoring-my-commodore-p50-calc...

https://blog.jgc.org/2022/11/restoring-my-rockwell-8r-calcul...

All small things but satisfying and you can see the OpenSCAD code in the repos.

These look very impressive. For all the software I have written, I still feel my skills being incomplete because I have no electronics skills.

Thanks for pointing me towards OpenSCAD.

lol, works with same phone charger I have.

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