The keys '0' to 'F' are connected via a 4-bit matrix.
Bill of materials:
- 16-key (hexadecimal) keypad
- PCB 5x7 cm (at least 22 rows)
- 22-pin right-angled male connector
- 2x 4-pin male connector
- 2x 4-wire cable female/female
- DIP-14 socket
- 2x DIP-20 socket
- 74x04 hex inverter
- 2x 74x373 octal latch
- 4x resistor 4k7
- capacitor 100 nF
