This PCB encodes a keypad to output a binary value DCBA (D is the MSB and A is the LSB). The DATA output briefly toggles HIGH when a keypad button is pressed.

Pinout

Note that the empty header pin (all the way to the right in the diagram) has no connection.

Hardware

This PCB uses a 12-key keypad and the 74C922 keypad encoder in asynchronous mode.

Schematic

Schematic of the keypad encoder printed circuit boards.