int knop = 11;
int led = 9;

void setup() {
  // put your setup code here, to run once:
  pinMode(knop, INPUT);
  pinMode(led, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  if (digitalRead(knop) == LOW)
    digitalWrite(led, HIGH);
  else
    digitalWrite(led, LOW);
}