You will need:
- 12 button common terminal keypad. 1x
- 1k ohm resistor, 12x
- 12k ohm resistor, 1x
- Protoboard (approx 1.5" x 1.5"), 1x
- Soldering iron & solder
- Hookup wire (red - approx 2"), x1
- Hookup wire (black - approx 2"), x12
- Hookup wire (red,black,yellow - approx 5"), x3
- Molex crimp pins, 3x
- Molex female connector, 1x
- Molex male connector, 1x
- Hot glue gun
- Mini vise
Step 1: Circuit Design
The common terminal of the keypad is held high through a 12k pullup resistor.
Each key terminal of the keypad is connected to a 1k resistor.
That resistor is in turn connected in series with its neighbor in a ladder twoards the common terminal.
The end of the resistor ladder is tied to ground.
The signal line is a voltage divider with its reference point between the 12K resistor and the resistor ladder.
Depressing a key causes a chain of resistors with a unique value to be introduced into the voltage divider, giving each key a unique analog value.
Eagle schematic files are attached for ease of reproduction on a protoboard or single sided etched copper board.