int ldrPin = A0;
int ledPin = 9;

void setup() {
  pinMode (ledPin, OUTPUT) ;
  Serial.begin (9600);
}

void loop() {
  int lightLevel = analogRead (ldrPin) ;
  Serial.print ("Light level: ");
  Serial.println(lightLevel); 

  if (lightLevel < 500) {
    digitalWrite (ledPin, HIGH) ;
  } 
  else {
    digitalWrite(ledPin, LOW) ;
  }
  
  delay (100);
}