The keypad uses an onboard ATMega328P to scan the matrix for button presses, decode them, and send them via UART or I²C to an Arduino. [ELECTRONOOBS] has the PCB files available via Patreon for ...

and are emulating the keypad using an Arduino and a set of transistors. They started by reverse engineering the keypad matrix using a continuity tester. Once they worked out the column and row ...