Tiny Calculator: New Hardware

After figuring out how to build calculator keypads with my new laser cutter, one of the first things I worked on was new hardware with a new keypad for my Tiny Calculator project. The original keypad was made by printing color labels on overhead transparency plastic then cutting those out and gluing them to 3D printed keys. Since the labels were printed in reverse and glued with the ink side down to protect the ink, the glue caused the ink to smudge in some cases. The laser-cut keys, on the other hand, are very crisp and look much better. The only disadvantage is that they come in just one color. Each key is glued down individually to a 3D printed grid, so it would be possible to make some of the keys a third or fourth color as long as there is no more than two colors per key. One thing I want to experiment with in the future is making keys out of two different pieces to get up to four colors on one key.

The original hardware was two circuit boards stacked together with headers that made everything pretty thick. The new version is just one circuit board with everything mounted on top. One way to squeeze everything in such a small space was using magnet wires which take up even less room than the wire-wrap wires on the original. Both sides of the LCD are encased in a 3D printed housing that is glued directly to the battery holder and microcontroller. This makes it impossible to make any changes to the circuit but is worth it to make the body thin. A small header above the LCD allows for programming.

The LCD on the original hardware was 128x32 which is a great resolution for this project since the stack is four levels deep. That display is about $12 and has the downside of needing 8 capacitors to function. I distinctly remember the frustration of soldering directly to the pins of the LCD module for all those capacitors since the pins aren't the same pitch as the protoboard. The new LCD, on the other hand, has a ribbon cable that was a lot less trouble to solder to and doesn't need any capacitors. Amazingly, it only cost $0.78 in quantity one compared to the $12 of the old display. The lack of a backlight on the new LCD isn't a drawback since it wasn't hooked up on the old one. Another great thing about the new display is that it's 132 pixels wide rather than 128 which would be useful if I ever use one to  emulate an HP calculator like the HP-42S which has a 131 pixel wide screen.

The reason I wanted to get the calculator so thin was to put it on the back of my badge holder at work to have a calculator with me all the time. The bottom surface of the circuit board isn't smooth, so I 3D printed a spacer to glue between the flat parts of the circuit board and a blank CR80 card that fits in the badge holder. I wore it to HHC 2023 and noticed that it swings forward and bangs into the table when I sit down if I'm not careful. So far, the firmware only prints a test pattern to the LCD and recognizes key presses. My plan now is to rewrite most of the existing firmware. The decimal floating point routines will be abstracted into a separate library, and the rest of the firmware will be rewritten from assembly to C.