int photoPin = A0;
int ledPin = 9;

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

void loop() 
{
int lightLevel = analogRead (photoPin) ;
Serial.print ("Light level: ");
Serial.println(lightLevel); 

if (lightLevel < 500) 
{
digitalWrite (ledPin, HIGH) ;
} 
else 
{
digitalWrite(ledPin, LOW) ;
}
delay (100);
}