const int SENSOR_PIN = 2;
const int SENSOR_PIN2 = 3;
const int SENSOR_PIN3 = 4;
const int SENSOR_PIN4 = 5;

void setup ()
{
  Serial.begin ( 115200 );
  pinMode ( SENSOR_PIN, INPUT_PULLUP );
  pinMode ( SENSOR_PIN2, INPUT_PULLUP );
  pinMode ( SENSOR_PIN3, INPUT_PULLUP );
  pinMode ( SENSOR_PIN4, INPUT_PULLUP );
}

void loop ()
{
  int state = digitalRead ( SENSOR_PIN );
  Serial.println ( state );
  delay ( 20 );
}