#include <Keypad.h>

const uint8_t NUM_OF_ROWS    = 4;
const uint8_t NUM_OF_COLUMNS = 4;

const char keyChars[NUM_OF_ROWS][NUM_OF_COLUMNS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

const uint8_t ROW_PINS[NUM_OF_ROWS]       = {2, 3, 4, 5};
const uint8_t COLUMN_PINS[NUM_OF_COLUMNS] = {8, 9, 10, 11};

Keypad keypad = Keypad(makeKeymap(keyChars), ROW_PINS, COLUMN_PINS, NUM_OF_ROWS, NUM_OF_COLUMNS);

void setup() {
  Serial.begin(9600);
}

void loop() {
  char key = keypad.getKey();

  if(key != NO_KEY) {
    Serial.print("key: ");
    Serial.println(key);
  }
}