int Pr = 0; // will be used for analog 0.
int PrValue = 0; // value of output
int Pr_Input = 100; // value of when light is on
int Y = 0; // map PrValue into light value

void setup() {
  Serial.begin(9600); //start serial Monitor
  pinMode(3, OUTPUT); // pin 3 as output
}

void loop() {
  PrValue = analogRead(Pr);
  Serial.println(PrValue); //prints photoresistor value
  delay(100); // value updated every 0.1 second.

  if (PrValue < Pr_Input) { // if sensor value is less than xx, light will turn on.
    Y = map(PrValue, 0, 300, 255, 0);
    analogWrite(3, Y);//LED on
    delay(15);
  }
  else {
    analogWrite(3, LOW); // LED off
  }
}