int sensorPin=A0;  
int led=4;
int sensorValue;

void setup()
{
  pinMode(led,OUTPUT);
  pinMode(sensorPin,INPUT);
}

void loop()
{
  sensorValue = analogRead(sensorPin);  
  if (sensorValue >500)
  {
    digitalWrite(led,HIGH);
  }
  else
  {  
    digitalWrite(led,LOW);
  }  
}