The Dit and Dah lines from your paddle are connected to digital pins on the Arduino.
Elias sighed and pulled up the official GitHub repository for the K3NG Keyer on his laptop. He scrolled past the massive keyer.h file and opened the PDF schematic. k3ng keyer schematic
The K3NG keyer schematic is more of a "choose your own adventure" than a rigid blueprint. You can start with just an Arduino and a transistor and eventually scale up to a full-featured station controller with a display and memory buttons. The Dit and Dah lines from your paddle