#define ECHOPIN 9
#define TRIGPIN 8
#define RELAY 4

#define MAXIMUMRANGE 100

long duration;
int distanceCM;

void setup() {
  Serial.begin (9600);

  pinMode (TRIGPIN, OUTPUT);
  digitalWrite(TRIGPIN,LOW);
  pinMode (ECHOPIN, INPUT);
  pinMode (RELAY, OUTPUT);
}

void loop() {
  digitalWrite(TRIGPIN, HIGH);
  delayMicroseconds(5);
  digitalWrite(TRIGPIN, LOW);
  delayMicroseconds(5);
  duration = pulseIn (ECHOPIN, HIGH, MAXIMUMRANGE*60);

  distanceCM = duration/58.2;
  delay(1300);

  Serial.println(distanceCM);
  Serial.print("Distanza cm.  ");

 if ((distanceCM >= 3) && (distanceCM <= 35))
  digitalWrite(RELAY, HIGH)
 else
  digitalWrite(RELAY, LOW)
}