// defines pins numbers
const int trigPin = 9;
const int echoPin = 10;
const int ledPin = 3;
// defines variables
long duration;
int distance;
int lastDistanceState = 0;    // previous distance
void setup() {
  pinMode(trigPin, OUTPUT); // Sets the trigPin as an Output
  pinMode(echoPin, INPUT); // Sets the echoPin as an Input
  Serial.begin(9600); // Starts the serial communication
//LED
pinMode(ledPin, OUTPUT);
}
void loop() {
  // Clears the trigPin
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  // Sets the trigPin on HIGH state for 10 micro seconds
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  // Reads the echoPin, returns the sound wave travel time in microseconds
  duration = pulseIn(echoPin, HIGH);
  // Calculating the distance
  distance = duration * 0.034 / 2;
  // Prints the distance on the Serial Monitor
  Serial.print("Distance: ");
  Serial.println(distance);
    //LED
  if (lastDistanceState > 100) {
	  if (distance < 100) {
	  digitalWrite(ledPin, HIGH);
    delay(2500);
	  } else {
	  digitalWrite(ledPin, LOW);
	  }
	  // delay a little bit to avoid bouncing
	  delay (100);
  }
  digitalWrite(ledPin, LOW);
  lastDistanceState = distance;
  delay(100);
}