Here’s how to turn a Nintendo controller (USB Retropad) in to a wireless version using a targus wireless keypad. The NES controller uses 8 inputs and one ground for it’s switch state input. The keypad uses a 4×5 matrix decoder. So I split the ground trace on the control pad into 2 parts (you can see this cut in the second image below) and gave each ground to a column on the keypad. Then I jumpered pairs of buttons on the control pad and connected each pair to one of four rows on the keypad, effectively giving me / 8 2 0 for A B Sel Start and * 9 3 . for Up Rt Lf Dn. Link.