Mapping and wiring the keyboard key matrix: A cheap keyboard have two pieces of transparent plastic foils with the wirings and contacts printed. In order to get the correct contacts in the PCB, I had to paint the printed wires with different colors and then did a list with the corresponding contacts per character. I noticed that there is a main set of contacts and a secondary set. The foil with less contacts is the main.

In order to check functionality, I connected the PCB to the computer and some wires to a breadboard according to the aforementioned distribution, then checked two characters sharing one single secondary line and a different main line (1 and Q) and it worked. Then I checked two characters sharing one single main line and two different secondary lines (1 and 2). It worked too! So, the thickness of the cables I was using won’t affect the result (obviously I would use the thinnest ones possible).