// External pullup circuit
void setup()
{
  pinMode(18, OUTPUT);
  pinMode(33, INPUT);
Serial.begin(9600);
}

void loop()
{
int x=digitalRead(33);
if(x==1)
{
  digitalWrite(18, LOW);
Serial.println("switch OFF");
}
else{

  digitalWrite(18, HIGH);
Serial.println("switch ON");
}

}