HC-SR04

#define trigPin 11
#define echoPin 12
#define led 13
#define led2 10
void setup() {
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(led, OUTPUT);
  pinMode(led2, OUTPUT);
}
void loop() {
  long duration, distance;
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = (duration / 2) / 29.1;
  if (distance < 4) {
    digitalWrite(led, LOW);
    digitalWrite(led2, HIGH);
  } else {
    digitalWrite(led, HIGH);
    digitalWrite(led2, LOW);
  }
  if (distance >= 200 || distance <= 0) {
    Serial.println(distance);
    Serial.println("cm");
  }
  delay(500);

}