\#define LDR_PIN     A0
#define LED_PIN     13
#define DARK_LEVEL  400      

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

void loop() {
  int light = analogRead(LDR_PIN);

  Serial.print("light intensity = ");
  Serial.print(light);

  if (light < DARK_LEVEL) {          
    digitalWrite(LED_PIN, HIGH);
    Serial.println("  -> Turn on the light");
  } else {
    digitalWrite(LED_PIN, LOW);
    Serial.println("  -> Turn off the lights");
  }

  delay(500);
}