Homemade Calculator Gets the CRT Display It Deserves

When I mention CRT (cathode-ray tube) displays, I usually say that they aren’t made anymore. But that isn’t quite true. While you won’t find a CRT TV at your local Best Buy, you can find some small CRTs for specialty purposes from online suppliers — though the inexpensive ones are probably built using NOS (New Old Stock). ALTco used one of those to make the coolest calculator ever.

This is the kind of CRT display you can find on sites like AliExpress for around $20. Unlike typical CRT TV designs, this is a “flat” unit similar to what was in the iconic Sony Watchman. It is monochrome and has a 4” screen, which is plenty for a calculator. ALTco just needed something for input, something to do the math, and something to output a video signal to the CRT.

The input is simple enough: it is just a keypad on a custom PCB connected to an Arduino Nano R3 development board. The Arduino scans the key matrix and looks for button presses, just like you’d see on any keypad or keyboard. The Arduino also does the math — again, easy enough to program (especially when the calculator just does basic arithmetic).

But the video signal output was a little more complicated, as the CRT requires an analog signal with precise timing and that isn’t something an Arduino normally likes to do. Fortunately, Avamander created the Arduino-TVout library several years ago and it takes care of all the fiddly bits for analog video signal generation. It can even handle both PAL and NTSC!

With that library, ALTco was able to put all of the pieces together and strap his keypad onto the CRT unit. The result is a handheld unit that can do numbers good, while also looking good with that sweet CRT glow.