const int TRIG_PIN = 2;
const int ECHO_PIN = 4;
const int RELAY_PIN = 15;
long duration, distance;
void setup()
{
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
}
void loop()
{
  distance = getDistanceInCentimeter();
  if(distance <= 100)
  digitalWrite(RELAY_PIN, HIGH);
  else
  digitalWrite(RELAY_PIN, LOW);
  Serial.println("Distance = "+ String(distance));
  delay(1000);
}
long getDistanceInCentimeter()
{
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  duration = pulseIn(ECHO_PIN, HIGH);
  return (duration * 0.034 / 2) + 1;
}