Arduino parking sensor — HC-SR04 and a traffic light that says STOP

#What you will build

An HC-SR04 ultrasonic sensor watches the space in front of it about ten times a second. A traffic-light module shows how much room is left:

• green — more than 30 cm: plenty of room,
• yellow — 16 to 30 cm: slow down,
• red — 15 cm or less: STOP.

The Serial Monitor prints the distance too. Real cars do the same with four to eight ultrasonic sensors in the bumper, and robot vacuums use it to slow down before they touch a wall.

#How it works

This project puts together two things you already know. If you have built Measure distance, the sensor part is the very same code.

#1. Measure the distance

The Arduino sends a 10 µs pulse to TRIG. The sensor sends out a burst of 40 kHz ultrasound and holds ECHO HIGH until the echo comes back. Sound needs about 58 µs to go 1 cm and back, so the echo time divided by 58 is the distance in centimetres.

#2. Choose a light

Three comparisons decide which light is on. Each one is either true or false, and they are written so that exactly one is true for any distance:

&& means and: both sides must be true.

#The traffic-light module

The module holds a red, a yellow and a green LED, and each already has its own 220 Ω resistor on the board — so you wire it straight to the pins. Each LED takes about 13–14 mA:

#The wiring

The distance sensor (standing on the table, facing the open space):

1. VCC → 5V, TRIG → pin 9, ECHO → pin 10, GND → GND.

The traffic light (on the breadboard):

1. Push the module's four pins into row c: R in 15c, Y in 16c, G in 17c, GND in 18c.
2. Wires: pin 4 → 15a (red), pin 3 → 16a (yellow), pin 2 → 17a (green), 18a → GND.

#The code, line by line

const int TRIG = 9, ECHO = 10;
const int RED = 4, YELLOW = 3, GREEN = 2;

long distanceCm() {
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  return pulseIn(ECHO, HIGH) / 58;
}

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(RED, OUTPUT);
  pinMode(YELLOW, OUTPUT);
  pinMode(GREEN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  long cm = distanceCm();
  digitalWrite(GREEN, cm > 30);
  digitalWrite(YELLOW, cm > 15 && cm <= 30);
  digitalWrite(RED, cm <= 15);
  Serial.print(cm);
  Serial.println(" cm");
  delay(80);
}

• const int RED = 4, YELLOW = 3, GREEN = 2; — names for the pins, so the rest of the code reads like English.
• long distanceCm() { … } is your own function: a named block of code that does one job and gives back (returns) an answer — here, the distance as a long. loop() stays short and clear.
• pulseIn(ECHO, HIGH) / 58 times the echo in microseconds and turns it into centimetres. Whole-number division drops the fraction, so 1790 µs gives 30 cm.
• digitalWrite(GREEN, cm > 30) — a neat trick. A comparison is true (1) or false (0), and digitalWrite() treats 1 as HIGH and 0 as LOW. So the light simply follows the comparison — no if needed.
• digitalWrite(YELLOW, cm > 15 && cm <= 30) — yellow is on only when both are true.
• delay(80) — a short rest between measurements so old echoes can fade.

#Common mistakes

• Red stays on, and the readings say 0 cm. No echo arrives, so pulseIn() gives up after one second and returns 0 — and 0 is "15 or less". Check the TRIG and ECHO wires, and that VCC has 5 V.
• Two lights are on at once. The comparisons overlap, for example cm >= 30 for green and cm <= 30 for yellow. Each distance must fit exactly one rule.
• The wrong colour lights. The R, Y and G wires are swapped. Pin 4 is red, 3 is yellow, 2 is green.
• No light at all. The module's GND pin is not connected. All three LEDs return through it.

#Try these challenges

1. Make yellow start at 40 cm (change both 30s).
2. Add a buzzer that beeps on red — see play a tune with a buzzer.
3. Make the beeps get faster as the obstacle gets closer, like a real car: wait cm * 10 ms between beeps.
4. In the 3D lab, drag the obstacle toward the sensor and watch the light change at 30 cm and 15 cm. Then type a distance in the sensor's window.

#Check yourself

#Questions people ask

#How do I add a beeping buzzer to an Arduino parking sensor?

Put a passive buzzer between a free pin (for example pin 8) and GND, and call tone(8, 1000, 50) when cm <= 15. For beeps that speed up as you get closer, make the pause between them depend on the distance.

#Can I use separate LEDs instead of a traffic-light module?

Yes. Use three LEDs, each with its own 220 Ω resistor, from pins 2, 3 and 4 to GND — see Blink an LED. The code does not change at all.

#Why does my parking sensor flicker between two colours?

Near a limit the reading wobbles by a centimetre. Add hysteresis — two limits, one for each direction — just like in the thermostat project.

#How far can an Arduino parking sensor see?

The HC-SR04 measures from about 2 cm to 4 m, so it is fine for a garage. Soft or angled objects reflect less sound and may be missed.

#What's next

You have finished all sixteen projects — well done! 🎉 Pick your next challenge from all the tutorials, or open the Build bench and invent your own circuit.

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Last updated 2026-10-29