The matrix is controlled by a micro-controller.

For the above 16-button 4×4 matrix, 8 pins of the micro controller will be used.

The first 4 pins will be OUTPUTS and will be connected to the COLUMN wires, while the other 4 pins will be INPUTS and will be connected to the ROW wires.

The OUTPUTS of the micro-controller will NOT all have power at the same time.

The outputs will go high one by one in cycle.

During this time, it will also monitor the inputs for a signal. As long as all inputs are LOW , the Micro-controller will take no action.

Now, suppose that the operator presses the button 3C.

This button has connect the matrix col C, with the matrix row 3.

When the output C of the micro-controller becomes HIGH, the signal arrives also at the input 3 of the micro-controller, through the pressed button.

The micro-controller check the 4 inputs and detects that when the specific output (C) is high, there is a HIGH signal at the input 3.

So, this means that the input C3 is pressed!