long ping() {

      long duration;

      // clear ping pin
      digitalWrite(pingPin, LOW);
      delayMicroseconds(2);

      digitalWrite(pingPin, HIGH);
      delayMicroseconds(10);
      digitalWrite(pingPin, LOW);

      duration = pulseIn(echoPin, HIGH);

      return duration;
    }

    // Helper function to return the distance to an object
    // detected by the ultrasonic sensor in centimeters.
    long msToCm(long microseconds) {
      return microseconds * .034 / 2;
    }