Fantastic write up - thank you! I also love hearing about why each part was chosen. As an electronics hobbyist I often mimic portions of others' designs but I'm not always entirely sure why a certain part was chosen among the various options.
Do you prototype all this on a breadboard before making the PCB and picking specific parts? I'd be curious to hear more about your process. I feel like I always need to test everything I build on a breadboard first since I inevitably miss some small detail if I go straight to schematic + PCB design.
Great question! It really depends. If I'm working with a part I haven't ever worked with before, I'll often make a minimal breakout board that I can talk to with a devboard like an Arduino or Feather.
If I'm pretty familiar with everything, I generally dive into a rough PCB layout and debug from there. If I absolutely can't get anything to work, I'll go back to the drawing board and possibly do some little breakouts, for example: https://twitter.com/theavalkyrie/status/1457845661370568709
Edit: for this specific project I did make breakouts for testing the MOSFETs and solenoid drivers. I'm glad I did, since I was able to try out a couple of different options for each: https://twitter.com/theavalkyrie/status/1550878465876004865
Second the poster above, really enjoyed the reasoning around how and why you picked parts and the intent of various circuits..
Do you have any books you recommend on electronics? I feel I understand the basics, like what various components do in isolation, but would love to level up and understand circuit design like how you are approaching it here
I'll echo sibling's comment about just doing it. Theory only takes you so far, it's important to actually build and experiment, even if it's just in a simulator.
I can recommend Practical Electronics for Inventors as a solid base of projects to experiment with and learn from. If you want to learn how to improve circuits and optimize for specific behaviors, there is a wealth of information in manufacturer application notes. A lot of the protection circuitry used in my article follows advice found in application notes.
IMO the best way is to just pick a project and work on it. Using application notes and datasheets lets you piece together circuits, and you pick up design patterns along the way.
Usually when I read through a build-out description like this, the author writes it for an audience that has a lot more background knowledge than I do. This article, on the other hand, was lucid and the explication helped me follow along.
I got to the part about 3.3V being derived from 5V and thought to myself "why not get it from 24V?" and then just a tiny bit later, there's the bit about USB and how Starfish can just take 5V from the USB host, and it clicked.
I also learned about Sparkfun's QWIIC connector in the aside about the chosen i2c bus switch having two "extra" channels. Love it, thanks for the article, Thea.
Thank you for the kind words! I'm glad it was useful.
> why not get it from 24V?
There are DC-DC converters in that same family that can do 4.75~36V -> 3.3V, but since I needed the 5V anyway for the LEDs and I/Os, tossing an inexpensive and small LDO for 5V -> 3.3V is an easy call.
FWIW, when I was thinking more about text-based literate circuit design, one of the advantages I thought of was to leave ample place to comment upon selection rationale for (and subsequent experience with) particular components.
The interactive schematics and gerbers are subtle but very effective.
Overall a very clear design and tone I really need to follow for my own PCB projects.