const int sensorPin=A0;//choose the pin for phtot resistor
const int ledPin=9;//choose the pin for LED
int lightCal;//reading the intial value of photo resistor
int lightVal;//reading for the current photo sensor
void setup() {
  pinMode(ledPin,OUTPUT);// declare LED as a output
  lightCal=analogRead(sensorpin);
}

void loop() {
  lightVal=analogRead(sensorPin);//read input value
  if(lightVal<lightCal-50){//check the input is less than threshold
  digitalWrite(9, HIGH);//turn LED ON
  else{
    digitalWrite(9, LOW);//turn LED OFF
  }  
}