🎾 Tennis Ball Parking Aid

An Arduino sketch to help with parking in the garage.

Are you concerned you might pull too far forward when pulling into the garage, potentially causing damage to your home or vehicle? Are you running short on tennis balls but have a spare Arduino laying around? Well if that's the case this project might be for you! This Arduino device / sketch provides a digital alternative to the old tennis-ball-on-a-string-hanging-from-the-ceiling.

This Arduino sketch is a simple mashup of the default Ping and LED examples. The device consists of an Arduino, a sonar distance sensor, a push button and an array of LEDs/resistors. When everything has been wired up correctly, a push button is used to set the optimal distance from the device (mounted on your garage wall). This sets the point at which the red LED will light up. From there, a LED will unlight for every foot you back up. When you pull in to your garage you should get a nice visual representation of how close your car's front end is to the garage wall.

Photo of device in use

Tennis ball device running on garage wall

I'd love to clean this up with a 3D-printed case, but so far so good. It has been working like a charm for 6+ months and I'm yet to run into the wall!

Schematic and source code

Arduino wiring diagram for tennis ball device
// our pin for the sonar
const int pingPin = 4;
// our pin for the 'set distance' button
const int buttonPin = 2;
// the number of inches in a foot
const int foot = 12;
// our 'set distance' button is not pressed by default
int buttonState = 0;
// default stop distance (the distance at which the red light should light)
int stopDistance = foot;

void setup() {
  // initialize serial communication:

  // initialize LEDs
  pinMode(6, OUTPUT); // green
  pinMode(7, OUTPUT); // yellow
  pinMode(8, OUTPUT); // yellow
  pinMode(9, OUTPUT); // yellow
  pinMode(10, OUTPUT); // yellow
  pinMode(11, OUTPUT); // yellow
  pinMode(12, OUTPUT); // yellow
  pinMode(13, OUTPUT); // red

  pinMode(buttonPin, INPUT);

void loop() {
  // establish variables for duration of the ping, and the distance result
  // in inches and feet:
  long duration, currentDistanceInInches, feet;

  // The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
  // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW);
  digitalWrite(pingPin, HIGH);
  digitalWrite(pingPin, LOW);

  // The same pin is used to read the signal from the PING))): a HIGH pulse
  // whose duration is the time (in microseconds) from the sending of the ping
  // to the reception of its echo off of an object.
  pinMode(pingPin, INPUT);
  duration = pulseIn(pingPin, HIGH);

  // convert the time into a distance
  currentDistanceInInches = microsecondsToInches(duration);
  feet = currentDistanceInInches / foot;

  Serial.print("in, ");

  buttonState = digitalRead(buttonPin);

  // check if the push button is pressed. If it is, the buttonState is HIGH:
  if (buttonState == HIGH) {
    // set stop distance to current distance
    stopDistance = currentDistanceInInches;

  // bunch of if statements is a little ugly but keep-it-simple-stupid
  // lights / unlights LEDs based on the currentDistanceInInches value
  // in relation to the current set stopDistance

  if (currentDistanceInInches <= stopDistance) {
    // light the red LED
    digitalWrite(13, HIGH);
  } else {
    digitalWrite(13, LOW);

  if (currentDistanceInInches <= (stopDistance + (1*foot))) {
    digitalWrite(12, HIGH);
  } else {
    digitalWrite(12, LOW);

  if (currentDistanceInInches <= (stopDistance + (2*foot))) {
    digitalWrite(11, HIGH);
  } else {
    digitalWrite(11, LOW);

  if (currentDistanceInInches <= (stopDistance + (3*foot))) {
    digitalWrite(10, HIGH);
  } else {
    digitalWrite(10, LOW);

  if (currentDistanceInInches <= (stopDistance + (4*foot))) {
    digitalWrite(9, HIGH);
  } else {
    digitalWrite(9, LOW);

  if (currentDistanceInInches <= (stopDistance + (5*foot))) {
    digitalWrite(8, HIGH);
  } else {
    digitalWrite(8, LOW);

  if (currentDistanceInInches <= (stopDistance + (6*foot))) {
    digitalWrite(7, HIGH);
  } else {
    digitalWrite(7, LOW);

  // always light the green LED
  digitalWrite(6, HIGH);


long microsecondsToInches(long microseconds) {
  // According to Parallax's datasheet for the PING))), there are 73.746
  // microseconds per inch (i.e. sound travels at 1130 feet per second).
  // This gives the distance travelled by the ping, outbound and return,
  // so we divide by 2 to get the distance of the obstacle.
  // See: https://www.parallax.com/package/ping-ultrasonic-distance-sensor-downloads/
  return microseconds / 74 / 2;

long microsecondsToCentimeters(long microseconds) {
  // The speed of sound is 340 m/s or 29 microseconds per centimeter.
  // The ping travels out and back, so to find the distance of the object we
  // take half of the distance travelled.
  return microseconds / 29 / 2;