initial commit

README.md

@@ -0,0 +1,140 @@
+# Homemade IV-12 VFD tube calculator
+
+I have always been interested in working with microcontrollers, and I recently decided to start a project that incorporates them. I chose to build a simple calculator, using IV-12 VFD (Vacuum Fluorescent Display) tubes for the display. These tubes, with their vintage look, offer a unique alternative to modern displays. In this post, I will walk you through my process of creating this calculator, from the initial idea to the finished product.
+
+## The Idea
+
+I have been fascinated with [Nixie tubes](https://en.wikipedia.org/wiki/Nixie_tube) ever since I first learned about them in a [YouTube video](https://youtu.be/s4-pgzyT0No?si=MB0ZcV2Ts6cme0ZN) by Techmoan. Today, I own a beautiful Nixie clock with four IN-14 tubes and a Sharp Compet 23 desk calculator from the 1960s. I have wanted to build my own device using Nixie tubes for a long time, but as someone with very little experience in electrical engineering, I have always found the high voltages required to drive them somewhat daunting. So I began searching for alternative display technologies with a similar charm and came across VFD tubes. These largely replaced Nixie tubes in the 1970s and required much lower operating voltages. VFD tubes from old stock are still widely available in various shapes and sizes on eBay.
+
+I didn't want to build another clock, and I thought a calculator might be a bit more involved but still a manageable project. After some research, I settled on using Soviet-made IV-12 tubes, mainly because I liked their size. They appear to be very similar to the more popular (?) IV-11 tubes but lack the segment for a decimal point and have proper pins that can be used with a tube socket, so I wouldn't have to solder them in place. That does mean, however, that my calculator wouldn't be able to display decimal numbers which I thought was fine.
+
+## Driving an IV-12 Tube
+
+Before I went ahead with the project, I wanted to make sure I knew how to drive the tubes. Some research revealed the following information.
+
+The pins of an IV-12 tube are keyed so they can only be socketed in the correct orientation. The image below shows the pinout of an IV-12 tube when looking at it from above.
+
+<img src="assets/iv12_pinout.png" width="200px">
+
+Pins `a`-`f` are connected to the anodes that form the digit segments. They follow the conventional segment names still used in modern 7-segment LCDs. To enable a segment, a positive potential of 25 volts has to be applied to its corresponding pin. To turn it off, a slightly negative potential should be applied instead but in my testing, leaving a segment floating reliably disables it as well.
+
+When  looking into a tube, you will notice a fine mesh formed from thin wire in front of the segments. It is connected to the `grid` pin and, when connected to positive potential of 25 volts, will enable the tube. When it is left floating or connected to a negative potential, all segments will remain disabled even when a positive voltage is applied to the anodes. This enables multiplexing the tubes.
+
+When looking even closer, you may also notice two extremely thin wires in front of the grid. This is the so-called filament. When connected to 1.5 volts (polarity does not matter), the filament will heat up and start emitting electrons. The positively charged grid will accelerate them towards the segments behind it. In front of the anodes that form the segments lies a phosphor-coated substrate. When a positive potential is applied to an anode, the electrons will be further accelerated towards it until they hit the phosphor and cause it to light up (similar to a CRT). The segment is now illuminated.
+
+No current-limiting resistors are required to drive a tube. IV-12 tubes require very little current (110mA for the filament, 12mA for the grid, 4mA for each segment) so driving them from a low-power source like a AAA battery is possible when the voltage is properly regulated.
+
+### Multiplexing the Tubes
+
+When driving multiple tubes simultaneously, it is generally recommended to use multiplexing to reduce the number of data pins required to display a number. Normally, the seven segment pins of each tube would need to be connected individually to the data source (in this case, a microcontroller). With multiplexing, however, the same segment pin on every tube is connected in parallel with the corresponding pin on every other tube. For example, all the `a` pins would be connected together. This way, only seven data lines need to be connected to the microcontroller.
+
+To prevent all tubes from displaying the same digit, the grid pin of each tube must be connected to the microcontroller. The microcontroller can then scan through the tubes by setting only one `grid` pin to 25 volts at a time and writing the digit it should display to the data lines. When driven at a sufficiently high frequency it will appear as if all tubes are on all the time to the human eye.
+
+## Designing the and Building the Keyboard
+
+I started by designing the calculator's keypad for which I wanted to use regular key caps and Cherry MX switches. After a bit of research, I found [this tutorial](https://www.instructables.com/Arduino-Mechanical-Keypad-1/) on building a keypad using an Arduino. I followed the instructions and designed a backplate which I had 3D printed.
+
+<img src="assets/keypad_model.png" width="500px">
+
+I ordered Cherry MX Brown switches from Amazon and a set of key caps from AliExpress. Since there is no key which only contains an equals sign on a regular US keyboard, I had to substitute it with a one that had a cute little rocket on it.
+
+<img src="assets/keypad_keys.jpg" width="500px">
+
+## Designing the Case
+
+Next, I designed a case for the calculator. Having no experience in 3D CAD design, I initially experimented with Fusion 360. However, I found it to be extraordinarily cumbersome and buggy. So, I decided to give OnShape a try, which I had initially avoided simply because so many YouTube videos were being by it. To my surprise, it turned out to be great. With the help of [this tutorial](https://www.youtube.com/watch?v=YPoJ484-7tI), I was quickly able to create a 3D model of a case which would fit all the required parts.
+
+<img src="assets/case_model.png" width="500px">
+
+I layed out the pieces my case would be constructed from and sent them off to a laser cutting company to be cut from 3mm plywood.
+
+<img src="assets/case_parts.png" width="500px">
+
+## Building the Case
+
+My laser cut parts arrived a few days later. After getting my okay, the pieces were re-arragnged by the supplier to fit smaller plywood plates since they ran out of the larger ones I designed the cutting instructions for.
+
+<table>
+    <tr>
+        <td>
+            <img src="assets/laser_orig.jpg">
+        </td>
+         <td>
+            <img src="assets/laser_pieces.jpg">
+        </td>
+    </tr>
+</table>
+
+All the pieces fit together nicely and I was soon left with a fully assembled case.
+
+<img src="assets/case_assembled.jpg" width="500px">
+
+I did not feel like the aesthetic of the raw plywood was quite what I was going for so I decided to try something I had always been hesitant about: veneering. I ordered a few sheets of 0.7mm walnut veneer from Amazon and looked up some veneering tutorials. When they arrived a few days later, I promptly got started. My plan was to cover one side of the box with a thin layer of wood glue, clamp on the veneer, let it cure, and then trim the excess with a scalpel and sandpaper. This method worked surprisingly well.
+
+<table>
+    <tr>
+        <td>
+            <img src="assets/case_veneer_1.jpg">
+        </td>
+         <td>
+            <img src="assets/case_veneer_2.jpg">
+        </td>
+    </tr>
+</table>
+
+I repeated the process for all six sides, ensuring the grain patterns aligned. After a few days of work, I had a fully-veneered case, though it still needed holes for the keypad and tubes. Once I cut those out, I was quite satisfied with the result.
+
+<table>
+    <tr>
+        <td>
+            <img src="assets/case_veneered.jpg">
+        </td>
+         <td>
+            <img src="assets/case_veneered_holes.jpg">
+        </td>
+    </tr>
+</table>
+
+The only thing left to do now was finishing the case with a few coats of linseed oil varnish. The oil beautifully enhanced the wood grain.
+
+<img src="assets/case_finished.jpg" width="500px">
+
+## Wiring the Tubes
+
+Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
+
+<table>
+    <tr>
+        <td>
+            <img src="assets/wiring_1.jpg">
+        </td>
+         <td>
+            <img src="assets/wiring_2.jpg">
+        </td>
+    </tr>
+</table>
+
+
+## Designing and Building the PCB
+
+After a few failed attempts at wiring the grid for the keypad by hand, I abandoned the idea and decided to design a custom PCB instead. Although I had never done anything like this before, it turned out to be quite straightforward. Using [EasyEDA](https://easyeda.com/), I took advantage of the community-provided footprints to design a board that would not only hold the keys but also the microcontroller, the DC-DC voltage converters required for the tubes, and the transistor arrays that allow the Arduino to supply the necessary voltages.
+
+<img src="assets/pcb_logic.png" width="500px">
+
+After sketching out the logic, I converted it into a PCB of the required dimensions and with holes through which the entire assembly could be mounted to the case. I ensured the connectors for the keyboard switches were on the top and all the other components would be mounted on the bottom of the circuit board.
+
+<img src="assets/pcb_board.png" width="500px">
+
+<!-- I sent my files off to [JLCPCB](https://jlcpcb.com/) for production. Because I selected the transistor arrays from their library, they were also able source the chips and reflow solder them to the board. About two weeks later, the partly assembled PCBs arrived at my doorstep. -->
+
+<table>
+    <tr>
+        <td>
+            <img src="assets/pcb_final_bottom.jpg">
+        </td>
+         <td>
+            <img src="assets/pcb_final_top.jpg">
+        </td>
+    </tr>
+</table>
+