const byte TRIGGER_PIN = 2;
const byte ECHO_PIN = 3;
const float SOUND_SPEED = 340.0 / 1000;
int a = 0;
void setup() {
  // put your setup code here, to run once:
  pinMode(13, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(TRIGGER_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  digitalWrite(TRIGGER_PIN, LOW);

  Serial.begin(9600);


}

void loop() {
  digitalWrite(TRIGGER_PIN,HIGH);
  delay(10);
  digitalWrite(TRIGGER_PIN,LOW);
  long measure = pulseIn(ECHO_PIN, HIGH, 25000UL);
  float distance_mm = measure / 2.0 * SOUND_SPEED;
  Serial.println(distance_mm);


  if(distance_mm<300){
    tone(7, 400, 5000);
    delay(500);
  }
  else{
    noTone(7);
  }














  //tone(7, 440, 2000);


}