int photocellPin = A0; 
int ledPin = 3; 
int val = 0;    

void setup() { 
   pinMode(ledPin, OUTPUT);  
   pinMode(photocellPin, INPUT);  
} 

void loop() { 
   val = analogRead(photocellPin); 
   if(val<=512) {
     digitalWrite(ledPin, HIGH); 
    }
   else {
     digitalWrite(ledPin, LOW); 
    }
}