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    The author (on hack-a-day) basically just links to a repo on github.

    That repo doesn’t contain much… Surprisingly little. I skimmed both source files (different versions of the same thing) and the readme and I still couldn’t figure out how it was actually reading the keypad matrix with GPIO…

    Oh, there it is: https://github.com/witnessmenow/arduino-switcheroonie/blob/master/switcheroonie/switcheroonie.ino#L30-L35

    #include <Keypad.h>
    // This library is for interfacing with the 4x4 Matrix
    // 
    // Can be installed from the library manager, search for "keypad"
    // and install the one by Mark Stanley and Alexander Brevig
    // https://playground.arduino.cc/Code/Keypad/
    

    …Ok, that link in the comment is pretty old, and it leads here: https://github.com/Chris--A/Keypad

    And here we go, this contains the magic I was looking for: https://github.com/Chris--A/Keypad/blob/master/src/Keypad.cpp

    That said… The witnessmenow repo DOES contain very useful information. I am glad it is here.

    Back to hack-a-day, well, they did provide that cool video, by Brian Lough. …Who turns out to be witnessmenow on GitHub. Cool.

    Summary: this content is really cool, but I’m not sure the hack-a-day post was the right introduction to it. But, it was better that, than just a link to the youtube video. I always skip SLYT posts.