Calculators are so ubiquitous and so familiar that they are easy to take for granted in many different ways. [lcamtuf] points out one that has probably never occurred to many of us: the user interface for a calculator is an unexpectedly complex thing.

Resolving something like 1 + 2 = is pretty straightforward but complexity compounds rapidly after that, with numerous special cases. Let’s imagine one decides to program a simple calculator UI as a weekend project. The development process might look a little like this:

1. User types in 1 + 2 = and the calculator displays 3. What happens if the user immediately presses -?
2. No problem, just consider the result of the previous operation as an already-there input. So we’ll have 3 - for this next operation, and wait for more.
3. Unless we should have treated that - as a negative sign for whatever number is coming next, making it a negative number? No, ignore that. Just treat whatever results from pressing equals as a pre-typed input.
4. Unless the user hits a number. Because if they hit 2 (for example) then we’ll have a 32 and not a 2 which they probably, definitely don’t expect. So that’s a special case and we should insert a clear if that happens.
5. Oh, better clear if the user enters a decimal, too.
6. I’m going to need a coffee…

And that’s just the tip of the iceberg. Imagine trying to figure all this out for the very first time, without the benefits of habit and history to fall back on.

The fact is that supporting the apparently trivial behavior of a simple calculator requires an underlying complex state machine that deals with all kinds of special cases in order to make the UI feel intuitive. And that’s just for a basic four-function calculator; we haven’t even touched on how special keys like % should behave.

We know [lcamtuf] speaks from experience, not just because of their deep knowledge of calculator history but because they rolled their own calculator that uses voltmeters as digit displays and there’s nothing like actually implementing something to make one appreciate it.

[lcamtuf] is not just a calculator superfan, but also a skilled builder. That much is evident in the fabulous  design of Calcumator 2000, an electromechanical calculator that uses voltmeter readouts as digits (plus one at the bottom to represent decimal place). There are plenty of high-quality build images, so give it a look!

Calcumator 2000 is a bit of a love letter to a time when display technology hadn’t quite yet produced anything suitable for calculator use. This resulted in calculator designs that are generally unrecognizable compared to the 7-segment display based devices we see today. The Calcumator 2000, in all its electromechanical glory, would have fit right in that era.

The Calcumator 2000 has all the usual buttons one would expect from a simple calculator and drives a total of seven readouts, one of which acts as the decimal point. The idea of using voltmeters as digit displays came from [lcamtuf]’s voltmeter clock, an earlier work with a similar attention to detail in its design and assembly.

We want to take a moment to admire how clean the blue panel is. [lcamtuf] made it by painting one side of an acrylic panel, cutting the letters and design out on a CNC mill, then filling with white paint. The depth of the cuts gives the white elements a nifty multi-layer effect that really complements the design.

Want to see it work? Oh yes, you do. Check out the video, embedded just below.