volatile bool state = LOW;

void setup ()
{
  pinmode (34, INPUT);
  pinmode (21, OUTPUT);
  Serial.begin(9600);

  attachInterrupt(34, interrupts, FALLING);
}

void loop()
{

Serial.println("Serial wordt uitgevoerd");
delay(50000)
}

void interrupts()
{
state = !state;
  digitalWrite(21, state);
}