const int pResistor = A0; // Photoresistor at Arduino analog pin A0
const int ledPin=9;       // Led pin at Arduino pin 9

int value;				  // Store value from photoresistor (0-1023)

void setup(){
 pinMode(ledPin, OUTPUT);  // Set lepPin - 9 pin as an output
 pinMode(pResistor, INPUT);
 pinMode(13, OUTPUT);// Set pResistor - A0 pin as an input (optional)
}

void loop(){
  digitalWrite(13, HIGH);
  delay(2000);
  digitalWrite(13, LOW);
  value = analogRead(pResistor);
  
  //You can change value "25"
  if (value > 25){
    digitalWrite(ledPin, LOW);  //Turn led off
  }
  else{
    digitalWrite(ledPin, HIGH); //Turn led on
  }
}