I bought some FPC (flexible printed circuit) cables and connector breakout boards from Amazon vendors so I no longer have to solder wires directly to my salvaged Canon Pixma MX340 multi-function inkjet control panel. The first FPC breakout board went on a perforated prototype board to host a Raspberry Pi Pico. Up until this point I had been using Adafruit's KB2040 board. But since they're both based on RP2040 chip and have CircuitPython support, switching my code to run on a Pi Pico is a minor task of changing over a few pin names.

Soldering to the FPC breakout board with its 0.1" (~2.54mm) pitch was far easier than the 1.0mm pitch of soldering directly to the control panel. Which meant I finally got around to connecting five wires that were previously not connected:

  1. 5 volt DC wire to supply WiFi LED

  2. LED+ for Power LED, in series with an 100 ohm current-limiting resistor already on board.

  3. LED+ for Alarm LED, also with resistor.

  4. Power button that is pulled to ground when pressed.

  5. Stop button that is pulled to ground when pressed.

My CircuitPython code had provision to set the appropriate bit flags to manipulate the WiFi LED, but I hadn't tested it until this point due to lack of +5V power. I was happy to see it worked, but I doubt I would find a use for it. As it originally shined into a clear plastic light guide and thus pointed in an opposite direction from the rest of the control panel.

And I hadn't dealt with the remaining four wires at all, as I considered the two direct-wired buttons and two direct-wired LEDs fairly standard microcontroller fare. And indeed, there were no surprises after I declared two CircuitPython digitalio pins for those LEDs and a keypad instance to read (and debounce) those two buttons. All worked as expected on the first try.

I updated my test app to toggle power LED upon presses to the power button, and the same for alarm LED and the stop button. The "In Use/Memory" and WiFi LEDs blink their own separate heartbeat patterns. And the LCD displays status of key matrix scan code activity: every time one of the K13988-scanned button is pressed, its name is displayed on the LCD. It makes for a simple demo to prove I have complete control over all electronic functionality of this salvaged control panel. Now this control panel waits for a project that could put it to good use but while it's waiting, I went ahead with a project that puts it to silly fun use.