Membrane keypad
From embeddedTS Manuals
Keypad
The 4 button membrane keypad allows for a 4 button input. These are set up on GPIO pins, and are connected to the system as a standard input event device. The four keys are connected as arrow keys. An example binary is included in order to demonstrate the input capabilities of these buttons:
/usr/local/bin/keypad-test
Will draw a box around the screen, with some text. But, whenever a button is pressed it will display a block on the LCD above the button that has been pressed. Please note that the LCD device must first be set up and operational before this binary will launch.
See the sources for more information on this demos and how it operates.