#define trigPin 9
#define echoPin 10
#define buzzerPin 13

void setup() {
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(buzzerPin, OUTPUT);
}

float readDistanceCM() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  int duration = pulseIn(echoPin, HIGH);
  return duration * 0.034 / 2;
}

void loop() {
  float distance = readDistanceCM();

  bool isNearby = distance < 100;
  digitalWrite(LED_BUILTIN, isNearby);

  Serial.print("Measured distance: ");
  Serial.println(readDistanceCM());
  delay(100);

  if      (distance>200 && distance<250) {tone (13, 1000, 200); delay(400);}
  else if (distance>100 && distance<201) {tone (13, 1000, 200); delay(200);}
  else if (distance<101)                 {tone (13, 1000, 200); delay(100);}
  else    {noTone(13);}
}