Interrupts

int ledPin = 11 ;  // a LED
int interruptPin=2; // a presence sensor

#define interruptOffset 2

void setup()
{
    Serial.begin(9600);
    Serial.print("setting up");
    pinMode(ledPin, OUTPUT);
    pinMode(interruptPin, INPUT);
    attachInterrupt(interruptPin-interruptOffset, presence, CHANGE);
      // can be LOW, RISING, FALLING, CHANGE
      // if it is LOW, the interrupt will keep coming as long as the pin is on LOW
}

void loop()
{
}

void presence(){
  Serial.println(digitalRead(interruptPin));
  digitalWrite(ledPin, !digitalRead(interruptPin));
}