Pro Micro pinout

The Arduino Pro Micro is cheap (~3$) and perfect for small boards (less than 81 keys). With a little hack though, it can drive a keyboard with up to 100 keys.

Pro Micro & Fio V3 Hookup Guide - SparkFun Learn

What are the Arduino micro and Pro micro board?

diymore Arduino Pro Micro 16MHz 5V with 2-Row Pins Header Module Board ATmega32U4 Module Board Micro USB Pro Micro Development Board Microcontroller : Electronics

Bit-C PRO

Pro Micro: Arduino Pinout - tinkerBOY

Quickpin

Arduino Pro Micro connections pinouts.

Arduino Pro Micro Pinout: Connection Pins for the ATmega32U4-Based Microcontroller

Arduino Pro Micro – Voltaat