Panel meter calculator with floating point
lcamtuf’s thing
SubscribeSign in
Panel meter calculator with floating point<br>Yes, there's a video at the end of the article.<br>Jul 12, 2026
Share
Unusual clocks are a common idea for electronics DIY projects. The difficulty is never the timekeeping part; it’s making the clock look presentable. For my own take on the theme, you can check out the article I posted in May:
A nicer voltmeter clock<br>lcamtuf<br>May 16
Read full story
Some readers might recall that I’m a bit of a calculator nerd. For a while, calculators lagged behind general-purpose computing because of the lack of a suitable display technology. Some of the early designs used ticker tape, cathode ray tubes, or incandescent lightbulb panels to display the result:
Casio 14-A with a lightbulb-based display (1957).<br>A bit surprisingly, though, there were scarcely any calculators with electromechanical displays, so I decided to address this glitch.<br>I started with a 3 mm sheet of acrylic. I spray-painted the back side blue:
Display panel, rattle-can fun.<br>I then selectively removed some paint with and cut a number of openings on a CNC mills:
Display panel, machining.<br>The areas with lettering were then painted again to give these areas contrast and a cool three-dimensional look.<br>I constructed the display itself out of six generic “SO-45” panel voltmeters from Amazon, plus one vintage edgewise voltmeter scored on eBay. The meters are fitted with custom faces printed on adhesive paper (template file):
Display panel, meters with original faces.<br>Here’s the assembled panel, also embellished with two Dialight 656 series panel indicators (catalog page) that signal negative results and overflows:
Panel close-up.<br>And yep, the edgewise meter in the middle is obviously used for floating point.<br>The panel was the easy part; next, I had to come up with an enclosure. Because the panel is fairly massive, I decided to use a non-standard keyboard layout: ten digits and a decimal point in a two rows on the left, and then five operator keys in a cluster to the right.
A rough 3D sketch of the enclosure.<br>Electrically, the keypad is still a 4×4 grid with four driven rows and four column sense lines.<br>I made the enclosure from thin maple lumber stock resawn in my workshop. The lettering and the recessed key matrix were once again machined on a CNC mill:
Behold, typography!<br>Here’s a photo of the glue-up:
No such thing as too many clamps.<br>The keypad uses sixteen relatively fancy 18×18 mm NKK JF series tactile switches (catalog here). I made custom vinyl decals for each key.<br>Here’s the photo of the finished enclosure:
Calcumator 2000 in all its glory.<br>The final portion of the project is the control circuitry. Some folks egged me on to implement analog calculations, but that would make the calculator even less practical — and if you want to take that route, purely-mechanical designs are more fun. So, the brains of the operation is an 8-bit AVR128DA28 MCU.<br>The chip is powered directly from a 5 V wall wart. It uses pulse-width modulation on seven digital lines (PD0-PD6) to drive the meters, a 4×4 sense-drive grid (PA0-PA3 / PA4-PA7) to scan the keypad, and two lines (PC0, PC1) for the indicator lamps.<br>I’ll spare you the walkthrough of the software architecture because it’s fairly straightforward. About the most interesting part is the implementation of fixed-point (6+5 digit) arithmetic to avoid the accuracy issues related to floats. You can download the source here; it’s short and commented well.<br>As discussed earlier on this blog, calculator UI is hell and the code makes several choices related to that. It allows repeated operations by pressing “+”, “×”, or “÷” twice, but reserves “-” as a prefix for changing sign, except if pressed right after the equals key. It also interprets dual “=” as an instruction to clear calculator state because the keypad doesn’t have a dedicated “C” button.<br>And now, the moment you’ve all been waiting for. It shows the handling of fractions, negative numbers, and error conditions:
That’s all.
Subscribe
Share
Discussion about this post<br>CommentsRestacks
TopLatestDiscussions
No posts
Ready for more?
Subscribe
© 2026 lcamtuf · Publisher Privacy<br>Substack · Privacy ∙ Terms ∙ Collection notice
Start your SubstackGet the app<br>Substack is the home for great culture
This site requires JavaScript to run correctly. Please turn on JavaScript or unblock scripts