#include <Keypad.h>

#define ROW_NUM     4 // four rows
#define COLUMN_NUM  3 // three columns

char keys[ROW_NUM][COLUMN_NUM] = {
  {'1', '2', '3'},
  {'4', '5', '6'},
  {'7', '8', '9'},
  {'*', '0', '#'}
};

byte pin_rows[ROW_NUM] = {16, 17, 18, 19};
byte pin_column[COLUMN_NUM] = {25, 26, 27};  // GPIO4, GPIO0, GPIO2 connect to the column pins

Keypad keypad = Keypad( makeKeymap(keys), pin_rows, pin_column, ROW_NUM, COLUMN_NUM );

void setup() 
{
  Serial.begin(115200);
  Serial.println("The sketch has started");
}

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

  if (key) 
  {
    Serial.println(key);
  }

  delay(10);     // helps the simulation
}