Multitool – The Design

It’s been quite a while, but it’s time to share the circuit design with you all. But first, here are some changes I made to the specs listed in the initial post:

  • Switched the screen from the -CSXN-CTP model to the -CSXN-T version: (worse) resistive touch instead of capacitive, but significantly cheaper
  • Voltmeter now capable of -500V to +500V input (instead of 0-300V), which also makes it suitable for AC!
  • Added Ohmmeter functionality for measuring resistance
  • Added user-friendly reset circuit (in case the controller crashes)
  • Added brightness, temperature and humidity sensors (because I can)
  • Extended unused controller pins to a header for external connections:
    • 18 digital I/O pins
    • 4 analog inputs
    • 2 analog outputs (using an additional DAC chip)

With that out of the way, let’s get to the point.
One of the first things I chose is the board size: 150 x 92 mm. The width is based on the size of the screen assembly, and the height is chosen to feel nice when handling it (and also to fit the keyboard without putting the buttons too close together).
Then, with my planned features in mind (and coming up with new ones all the time), I downloaded Autodesk EAGLE (student edition) and started creating a schematic and figuring out the exact specifications and functional requirements of the system. After that was done, I started laying out and routing the PCB. After a few months of work, I ended up with this 4-layer work of art mess:

The PCB, looking “through” it from the top.

I will go over the entire schematic and PCB design in detail, but it won’t fit in a single post. Therefore I will split it into multiple posts and link them here:

Also, if you want to look at the EAGLE design files yourself, here they are:

Be aware that you (probably) can’t open these files in the free version of EAGLE. I’d love to offer a “free-compatible” version of the design, but unfortunately it exceeds all three limitations of the free version: schematic sheet count, PCB layer count and maximum PCB area.
However, if you are a student (like me), you can get the student version of EAGLE for free, which is functionally equivalent to the premium version.

See you around,

The Multitool Project

UPDATE: Design overview here!

After 12 (long) years, I recently finished school. That also meant giving back all of the things that were provided by the school. Most of them were just books, but there’s one thing in particular that I will miss:

The TI-83 Plus graphing calculator. It’s a very useful (and powerful!) tool that helped me a lot in school, but was also quite handy at home and for my past projects. It has a lot of features outside of basic math, like equation solving, integration and differentiation, statistical analysis, matrix operations, programming support (in its own language, TI-Basic) and of course a lot of options for graphical presentation and analysis of functions and graphs. You can even put games on it, if you have a USB-to-calculator link cable. Yes, that was a lot of fun, especially in 8th-9th grade.

But the TI-83+ is definitely not perfect. It’s a very old design based around the Zilog Z80 processor (which was designed in 1976!), its display is monochrome and has a low resolution of 96×64 pixels and most importantly, the device is massively overpriced for its age at around 80$. To put that in context: Nowadays you can get a low-end smartphone for that much money, download some free apps and you have something way more powerful than the 19-year-old calculator with a 42-year-old processor.

But even then, I feel that there is something about having a dedicated calculator with physical buttons that cannot quite be replicated by a smartphone app. And I really miss being able to just reach across my desk and have any basic calculation done in a few button presses without having to unlock my phone, look for the right app and then type something in using a touchscreen keyboard. So I thought: I’m a computer scientist and engineer at heart, so why not build my own calculator? And if I go that far, why not add all kinds of other useful features to it?

That’s how the Multitool project was created.
The idea is to design and build a modern calculator-sized tool that can help me in all kinds of ways in future projects and in everyday life. I am currently still in the middle of planning and designing it, but here are some specs as of now:

  • Processor: Microchip PIC32MZ2048EFH144 (32-bit microcontroller, 200 MHz, 120 I/O-pins, 512 kB RAM)
  • 1 MB of external RAM (yes this seems tiny, but is probably more than I need)
  • Display: NHD-3.5-320240FT-CSXN-CTP (3.5″ diagonal, 320×240 resolution, full RGB, capacitive touchscreen, integrated graphics chip)
  • 50 button keyboard
  • USB micro-B connector (a.k.a. non-Apple-smartphone USB connector)
  • MicroSD card slot (supports cards up to SDHC, so max. 32 GB)
  • Battery: two 18650 Li-Ion cells, USB-charged at 500mA (yes that’s slow)
  • Audio: 0.7W mono speaker
  • DC voltmeter circuit up to 300V

Other than that, I still have a few ideas for modules and additions, but I haven’t looked into how easy those are to implement (and whether I really need them heh). If you have a suggestion for what to add, please comment below 🙂

I’m really excited to finally share my projects with the world, so I will post any updates to the project here and when the design is (almost) done, I will go over the implementation details and schematics here, essentially documenting and explaining the device and my design process.
(for updates see top of post)

See you around,