int sensorPin = A0;   
int sensorValue = 0; 
int thresholdValue = 700;
int relayPin = 8;
void setup() {
  pinMode(relayPin, OUTPUT);  
  Serial.begin(9600);         
}
void loop() {
  sensorValue = analogRead(sensorPin);
  Serial.println(sensorValue); 
  if (sensorValue > thresholdValue)  
    digitalWrite(relayPin, HIGH);   
  else digitalWrite(relayPin, LOW);  
  delay(100);
}