const int ldrPin = A0;       
const int ledPin = 9;        
int ldrValue = 0;            
const int threshold = 500;    

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

void loop() {
  ldrValue = analogRead(ldrPin);  
  Serial.println(ldrValue);
  
  if (ldrValue < threshold) {
    digitalWrite(ledPin, HIGH);   
    digitalWrite(ledPin, LOW);    
  }
  
  delay(100);  
}