The HC-SR04 ultrasonic sensor is one of the best first projects for any Arduino beginner. It measures distance by bouncing sound off objects — just like a bat or a car’s parking sensor — and it costs very little. In this guide you’ll wire it up and read the distance in centimetres on the Serial Monitor.

What you’ll need

Arduino Uno R3 (with USB cable) — the brain of the project
HC-SR04 Ultrasonic Sensor — measures the distance
Solderless Breadboard — to build the circuit without soldering
Male-to-Female Jumper Wires — to connect the sensor to the board
Active Buzzer (optional) — to turn it into a distance alarm

How it works

The HC-SR04 has two “eyes”: a transmitter and a receiver. When you send a short pulse to the TRIG pin, it emits an ultrasonic burst. The sound hits an object, bounces back, and the ECHO pin goes HIGH for as long as the round-trip takes. Since sound travels at about 343 m/s, we convert that time into distance.

Wiring

VCC → Arduino 5V
GND → Arduino GND
TRIG → Arduino D9
ECHO → Arduino D10

The Arduino code

const int trigPin = 9;
const int echoPin = 10;

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

void loop() {
  // send a 10 microsecond pulse
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  // read the echo time and convert to cm
  long duration = pulseIn(echoPin, HIGH);
  float distanceCm = duration * 0.0343 / 2;

  Serial.print("Distance: ");
  Serial.print(distanceCm);
  Serial.println(" cm");
  delay(300);
}

Upload the sketch, open Tools → Serial Monitor at 9600 baud, and move your hand in front of the sensor. The distance updates live.

Project ideas

A parking / reverse alarm that beeps faster as you get closer (add the buzzer).
A contactless “auto” dustbin lid or hand-sanitiser dispenser.
An obstacle-detecting robot.

Get the parts

Everything for this project is in stock at Smart Home | Arduino Corner — Shaheen Market, DAV College Road, Rawalpindi — with fast, cash-on-delivery shipping across Pakistan. Grab the HC-SR04 and an Arduino Uno to get started. Browse the full store →

How to Use the HC-SR04 Ultrasonic Sensor with Arduino (Beginner Guide)