Pro Micro upgrade - Making two more pins available
- Tools needed: soldering iron
- Time needed: 1-5 minutes
- Goal: 2 additional pins on your Pro Micro
While the heart and soul of the Pro Micro board, the Atmega32u4 MCU, features 26 GPIO pins, the Pro Micro makes only 18 of them easily accessible. If this is a limiting factor in your keyboard project, you can buy a controller with more pins (like the Teensy, Proton C, Elite C etc.), but for much more money. Or you can grab your soldering iron and make two more pins available.
Let there be no light
The Pro Micro wastes the B0 and D5 pins of the MCU for two onboard LEDs indicating TX/RX communication. For a keyboard project two more pins seem much more useful than this LED feature, so we target the resistors before those LEDs.
Resistance is futile
We can solder directly to the resistors of the LEDs or remove them altogether. Because I couldn't really see any part of the pads exposed, I decided to remove them.
The process is way easier than you might think. The resistors come off easily after a few seconds of heating. Use a pointy tip and keep the soldering iron to the bottom end of the resistor. While heating, push it firmly in the opposite direction. After a few seconds the tiny resistor gives up all resistance (muhhahha!) and comes off revealing the shiny solder pad.
Adding YUKI©®™ (Your Ultimate Keyboard Interface)
You could solder your stuff to these pads directly. But because the pad is originally intended for tiny SMD parts, I felt even a 28 AWG wire too bulky for this task. Instead, I soldered a piece of really thin wire from an old FDD ribbon cable (ca. 0.24 mm diameter core) to it. The thicker wires of the matrix or the encoder can be soldered to this.. khm.. interface.
The 20 pin Pro MicroThat's it! Your Pro Micro has now 20 easily accessible pins, which is enough for driving a keyboard with up to 100 keys. Well, theoretically.
If you are interested in seeing this technique in action, check my Steel Tormentor build, which features 46 keys, 2 rotary encoders and backlighting with a single humble Pro Micro.