A Reverse Polish Calculator For Your Keychain

As the smartphone has eaten ever more of the gatgets with which we once surrounded ourselves, it’s with some sadness that we note the calculator becoming a less common sight. It’s with pleasure then that we bring you [Nekopla]’s keychain calculator, not least because it’s a little more than a conventional model. This is a calculator which uses Reverse Polish Notation, or RPN.

A full write-up in Japanese (Google Translate link) carries an impressive level of detail about the project, but in short, it takes an Arduino Pro Micro, an array of keys, and an OLED display, and packages them on a couple of fiberglass prototyping boards in a sandwich between laser-cut Perspex front and rear panels.

The RPN notation is what makes it especially interesting,a system in which where you might be used to writing 2+2=  to get 4, in RPN you would write 2 2 + . It allows the use of much simpler code with a stack-based architecture than that used in a conventional calculator. It’s a system that’s usually the preserve of some pretty exclusive machines, so it’s great to see on something with more of the toy about it.

If RPN interests you, then you might like to read our look at the subject, and even feast your eyes on the teardown of a vintage 1975 Sinclair RPN calculator.

23 thoughts on “A Reverse Polish Calculator For Your Keychain

    1. My HP-48sx has a SPC key in addition to the enter key. Thus you could hammer out a long string of numbers, hit enter and they’d all be put neatly on the stack. “2 2+” is a valid sequence. Dunno about earlier RPN calculators.

  1. RPN took me a few days to adapt but once i learned it, i didn’t want to go back to “normal” calculators anymore. I learned it around 2001 on my (now defunct) HP 49G, which 48G fans hated because of its rubber keyboard.
    nowadays i have no use for a calculator anymore, since i no longer do any calculus. whatever non trivial math I do, a spreadsheet is the tool of choice.

    1. Right you are. I learned RPN in college, because I bought an HP-25. Reduced keystrokes and a more logical way of evaluating an expression. Still use an emulator on my PC, but you’re right about spreadsheets…if only I’d had them when I was in college…

      1. Learned RPN in high school (1972) on a 5 function calculator larger than a typewriter (yeah, we had those too). Have finally given up on HP, as assuming they hate their customers is the workable model for explaining their calculator decisions (after doing thousands of calculations on an HP48, I could never adjust to the jiggered key placement in the HP50). But can’t give up on RPN so here’s a plug for the RealCalc phone app and Excalibur desktop app.

        1. I grew up on HP calculators in the 70s and 80s as my dad worked for HP at the time. Today, I use the 48sx app on my Android phone. I still have my real 48sx, but it stopped powering on and I never could get into it to figure out why.

        2. I don’t have any experience with HP calculators but I bought an HP printer and in the foggy blur at the tail end of a marathon installation of bloated, glitch-riddled, horrifically bad, practically malware, omg-what-do-I-have-to-do-to-print-a-test-page, software, I got suckered into signing up for Instant Ink. Accordingly, I can say with authority and certainty that HP absolutely despises their customers and is likely planning to mail anthrax to all of us in the near future.
          * After weeks of nagging notifications that I needed to restore the printer’s internet connection, I informed HP that the printer had (thankfully) met it’s end some time ago. I didn’t mention this occurred when I threw it into the wall and then kicked it to small pieces. Undeterred, they continue gleefully billing me for the ink, which they no longer mail to me since they can’t connect to the printer. I did the math, on my Casio FX116, and it seems unlikely that I will live long enough to justify hiring a lawyer to make them stop stealing $3.99 / month from me, so I satisfy myself by relaying my experience to everyone I encounter in any circumstances. When the class action inevitably comes up, I’ll probably jump in there.
          On other notes, that little calculator is adorable and RPN looks intriguing. I look forward to investigating both in the coming days.

      1. Agreed. RPN is a joy. In engineering school in the early 80’s a HP41 was required and we had numerical methods classes where we actually were taught to program those calculators. After learning the system, using a standard calculator just isn’t an option for me. My brain simply won’t let me successfully do it. The young engineers at work suspect I’m an addled old fart because I can’t make a simple calculator work. Then, I pull out my Android phone running droid48 and spend 20 minutes evangelizing the clear superiority of RPN. Their suspicions are then confirmed.

    2. I’ve still got an HP50g, which reverted to the nice plastic keys; I break it out when I’m doing any calculations that need more than a four-banger. This little guy in the article looks like a lot of fun, though.

  2. When I got to college my first was the hp-11c, then 15C, and finally 16C. Still have the 15C and 16C on my desk at home. Find I use the Free42 emulator more often than not if sitting at a desktop.

    Neat little key-chain calculator there.

    1. Still have my 16C as well, and use it practically every other day. It’s been dropped, chewed on by our dog, got spilled on, dropped something heavy on it more than once. Replaced batteries 3 times, since about 1990. Seriously the best ever calculator I have bought. It was expensive to buy, but looking at the worth I got out of it, it was actually really cheap!

      To be honest, I don’t quite understand their business model. They made a calculator that can take heavy abuse without even winking for over 30 years, and sold it for (well, relatively expensive…) peanuts.

      I also ported the ‘nonpareil’ emulator to iOS so that I can have an emulator of the 16C running on my phone, for the cases where I don’t have my actual calculator with me. It’s in my github. You’ll have to build it from source yourself though, and use your own Apple developer account to install it on a real iDevice. It’s for personal use only.


      1. I am on my second HP-16C. The first one got ‘lost’ when I was working at a job site out of state. Luckily I later found and bought a like new one to replace it years later. So it goes :) .

  3. > it’s with some sadness that we note the calculator becoming a less common sight.

    There is a scientific calculator on my desk and most of my (power system engineer) colleagues. It is just the quickest way of going those supporting calculations that you do frequently. It doesn’t do RPN though. But what do you expect for £9.95

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