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.

Calculator Keypads: Laser Cutter

My last post on building calculator keypads explained all the methods I tried then presented on at the 2023 Hewlett-Packard Handheld Conference. Last year, I bought an Elegoo Phecda Laser Cutter as the next step in building keypads, and the results are really impressive! This particular model uses a laser diode which is much cheaper than other types of lasers like CO2. Common power ratings for diode lasers are 5.5W and 10W although some as low as 1W are used for etching rather than cutting. My laser cutter is a 10W model and also comes in a 20W version that is a lot more expensive. As it turns out, 10W is more than powerful enough for building keypads. Diode lasers like this one have a couple limitations - they don't cut blue material well since it absorbs the laser wavelength and they can't mark or cut harder materials like metal. They also aren't supposed to be able to cut white acrylic but mine punched straight through 3mm of black acrylic and 3mm of white acrylic underneath it.

The first thing to figure out was the software to run the laser. A lot of people swear by LightBurn which is $99 for the license plus an extra yearly fee to access updates. Instead of that, I went with LaserGRBL which is free and works on Windows. Since then, I switched my laptop over to Ubuntu, so LaserGRBL needs to run in a virtual machine. My laser cutter was shipped to my next work destination, so I'll need to wait a few months to test that everything works. Setting up LaserGRBL was really easy. It includes test images for cutting to figure out the ideal number of passes and laser strength to use on a particular piece. For etching images, the default resolution was pretty low, so I changed it to 16 lines per millimeter since the focal size of the laser is 0.06 x 0.06mm. This was enough to get it cutting and etching exactly how I wanted. Generating images for etching was easy using Python to create an SVG then converting to PNG as with my other keypads. The laser's cutting mode uses SVG images directly.

Calculator Keypads

As described in my 2023 goals post, my plan for the year was to finish enough projects to give a presentation at the 2023 Hewlett-Packard Handheld Conference (HHC). Even though I didn't finish everything on my list, there were still enough interesting things to give a presentation on including my Blinky Robot and 6507 Calculator Forth Virtual Machine. The largest part of the presentation was about building calculator keypads.

While building a circuit board or writing the firmware for a calculator are achievable goals for a hobbyist or amateur, building a keypad tends to be much more challenging. The keys themselves can be made on a 3D printer, but getting good quality labels on the keys is very difficult. Since none of my calculator projects so far have had really good keypads, I decided to try every method available for making keys to compare the results.

3D Model
The 3D model for the test consists of eight rows of five keys each designed in OpenSCAD for my still unfinished 6507 Graphing Calculator. OpenSCAD has been my go-to program for modeling since I started with 3D printing a few years ago. The biggest advantage of this program is that the entire model is specified by a script so nothing needs to be manipulated or placed by hand. This works well for people like me who don't have a lot of artistic talent but feel comfortable with programming. It's cheap and relatively quick to print out a set of these keys on my Ender 3.

PIC32/6502 Blinky Robot

The PIC32 robot from my 2022 Project Goals is finally finished! Building it was a fun way to learn about the MIPS architecture and programming the PIC32. It was also a chance to work on a 6502 emulator that will be useful for other projects.

As far as robots go, this is not a very useful one since all it does is blink its eyes every few seconds. The interesting part is how it does the blinking. Rather than switching the LEDs on then delaying a while before switching them off, the robot runs several layers of emulation with the innermost emulated system blinking the LEDs as fast as possible. All the layers of emulators within emulators combine to produce enough overhead to slow the blinking down to a rate that is visible.

Kitty Salon

For Christmas this year, my girlfriend and I made a talking toy cat for her nephew. It was a fun opportunity to build something together and practice some fabrication skills that are useful for calculator projects.

The idea for this project came from her nephew pretending to be a cat and calling himself "Kitty Salon." The toy version of Kitty Salon has five recorded messages saying things like "Meow! Meow!" and "Rise and shine!" A button on the bottom plays one of the messages every time it's pressed, cycling through all five. Two LEDs for eyes glow to show that the cat is on and fade on and off while the cat is talking.

Body

The first step to building the cat was sculpting a model out of clay. The innermost part of the model is a small box made out of foamcore which is large enough to hold all the electronics. The first try at making the cat out of clay from the craft store failed. The box the clay came in said that it dries when exposed to water. In our case, the model cracked in several places after drying and the feet fell off. The second try used Sculpey which doesn't require water to set and has the added benefit of working well with silicone rubber molds. You can see in the picture above that the back end of the cat is a little square and fat since the Sculpey is molded around the box for the electronics.

Tiny Calculator: Keypad

The last few weeks I have spent a lot of time on the keypad for my Tiny Calculator project and the results are a little surprising! After considering a lot of different ways to make caps for the keys, I finally bought a 3D printer, specifically an Ender 3. Before that, I tried using an Xacto knife to cut out a base and a slightly smaller square post from PVC I bought at the craft store. The result was pretty uneven and there didn't seem to be any way to make them all exactly the same size, so I bought the 3D printer the same day. It was easy to put together and has produced great prints from the very first time I tried it!

The first thing I worked on was designing a button cap with a skirt that would keep it from falling out of a grid overlay. It was really easy to do in OpenSCAD. I printed several versions to get the dimensions right and design a hole in the base so it would fit over the posts of the tactile switches. The first version was black and looked really nice. Next, I started on the overlay and experimented with several versions. One that was only 0.5mm thick fell apart when it came off the raft it was printed on, and I eventually settled on 1mm for the thickness. As I experimented with improving it, I added supports that fit over the header pins on the keypad to keep everything straight and holes for longer pins to keep the grid attached. This worked really well to keep the keys straight and attached but the grid was still a little flexible because it was so thin, so I added a 3mm wall around the edge which made the print very rigid.

The buttons themselves were printed with the holes for the posts facing down. The slicer program inserts tiny supports to keep the hole from collapsing while the button is being printed. It took a long time to carve the supports out and the results ended up being a little uneven. Some of the holes were slightly too deep and the entire button post fit in them with no clearance for the button to be pressed. This led some buttons to have a stronger click and some to stick up farther than the others. One thing I tried to make them more even was to print them in rows of five with the hole facing up. The hole depths all turned out extremely uniform but the lip holding them together came out garbled and they easily broke apart since the lip had to basically be printed parallel to the ground with no support. The next time, I turned the row of five on its end, which worked since I had removed the lower skirt edge to make room for the lip holding them together. Printing five of them in a row like this makes the button presses more uniform and the rows straighter.

The design for the keys labels was done in Google Sheets and copy and pasted into Paint. This worked really well! It was way easier to make modifications and keep all the text uniformed and aligned than doing it in Gimp, which I tried a few months ago. Once I had the design, I went to Staples and had them printed on overhead transparencies. Unfortunately, there is no way to print in white on a standard printer, so you couldn't see anything when the labels were put on black buttons. I bought a roll of white plastic and now the labels look exactly how I planned. For the next iteration, I bought a cheap inkjet printer and transparency paper so I can make the labels myself. They are printed in reverse so I can glue the ink side down and avoid smudges from finger presses. I designed and printed a little bracket for cutting out the labels at exactly the right size, but it turned out not to be useful. Super gluing all of the labels on took a couple tries to get right, but I'm very happy with the result! Now that I have the 3D printer, I plan to make similar keypads for some of my other projects.

Test button made from PVC	Different versions of the button cap	Under side of the button caps. The middle one still has supports.
Failed 0.5mm overlay print	1mm overlay print	Overlay with added wall for stability
Failed print of five connected buttons	Successful print of five connected buttons	Bracket for cutting out labels

Keypad labels