#define echoPin 7        // Echo Pin
#define trigPin 8        // Trigger Pin
#define LEDPin 13        // Onboard LED

int maximumRange = 200;  // Maximum range
int minimumRange = 0;    // Minimum range

long duration, distance; // Used to calculate distance

void setup() {
  Serial.begin(9600);           // Initialize serial communication
  pinMode(trigPin, OUTPUT);     // Set trigPin as output
  pinMode(echoPin, INPUT);      // Set echoPin as input
  pinMode(LEDPin, OUTPUT);      // Set LEDPin as output (Use LED indicator)
}

void loop() {
  digitalWrite(trigPin, LOW);             // Set trigPin LOW to start pulse
  digitalWrite(trigPin, HIGH);            // Set trigPin HIGH to send pulse
  delayMicroseconds(10);                  // Pulse duration 10 microseconds
  digitalWrite(trigPin, LOW);             // Set trigPin LOW again to end pulse

  duration = pulseIn(echoPin, HIGH);      // Measure time taken for pulse to return

  distance = duration * 0.0344 / 2;       // Calculate distance (speed of sound = 0.0344 cm/us)

  if (distance >= maximumRange || distance <= minimumRange) {
    /* Send a negative number to the computer and turn LED ON to indicate "out of range" */
    Serial.println("-1");
    digitalWrite(LEDPin, HIGH);   // Turn on LED
  } else {
    Serial.println(distance);      // Print distance
    digitalWrite(LEDPin, LOW);     // Turn off LED
  }

  // Delay 50ms before next reading
  delay(50);
}