int sensorPin = A0;    
int ledPin = 2;      
int sensorValue = 0;  
void setup() {
  pinMode(ledPin, OUTPUT);
}
void loop() {
  sensorValue = analogRead(sensorPin);
  if (sensorValue >=400 )
  {   
  digitalWrite(ledPin, HIGH);
  }
  else 
  {
  digitalWrite(ledPin, LOW);
}  
}