Ultrasonic Parking Sensor with Arduino Uno
Wire an HC-SR04 ultrasonic module to an Arduino Uno R3 and drive a buzzer that beeps faster as obstacles get closer. Ideal first project — no libraries needed, pure digitalRead/tone logic.
What You'll Build
A proximity alarm that mimics the parking sensors built into modern cars. The HC-SR04 emits an ultrasonic pulse and measures the echo time to calculate distance. Your Arduino maps that distance to a buzzer tone — slow beeps at 50 cm, rapid beeps at 15 cm, and a solid tone inside 5 cm.
The whole circuit needs no libraries and uses just six wires. It's a reliable first project that teaches pulse timing, pulseIn(), and analog-to-digital mapping in about 40 lines of C++.
💡 Tip
The HC-SR04 runs on 5V and its ECHO pin outputs 5V logic. The Arduino Uno's digital pins are 5V-tolerant, so you can connect ECHO directly. On a 3.3V board (Nano 33, Pico) you'd need a voltage divider.
Components required
Wiring
Keep the connections simple:
| HC-SR04 Pin | Arduino Pin | |-------------|-------------| | VCC | 5V | | TRIG | D9 | | ECHO | D10 | | GND | GND |
Buzzer + → D8. LED anode → D7 through a 220 Ω resistor to GND.
// ── Ultrasonic Parking Sensor ─────────────────────────────────────────────────
// Arduino Uno R3 | HC-SR04 | Active Buzzer | Red LED
// ─────────────────────────────────────────────────────────────────────────────
#define TRIG_PIN 9
#define ECHO_PIN 10
#define BUZZER 8
#define LED 7
// Distance thresholds in centimetres
#define DANGER_CM 5
#define CLOSE_CM 15
#define WARN_CM 30
#define MAX_CM 50
// Returns distance in cm (returns 999 on timeout)
float readDistance() {
digitalWrite(TRIG_PIN, LOW);
delayMicroseconds(2);
digitalWrite(TRIG_PIN, HIGH);
delayMicroseconds(10);
digitalWrite(TRIG_PIN, LOW);
long duration = pulseIn(ECHO_PIN, HIGH, 25000UL); // 25 ms timeout ≈ 430 cm max
if (duration == 0) return 999;
return duration * 0.0343f / 2.0f;
}
void beep(int onMs, int offMs) {
tone(BUZZER, 2400, onMs);
delay(onMs + offMs);
}
void setup() {
pinMode(TRIG_PIN, OUTPUT);
pinMode(ECHO_PIN, INPUT);
pinMode(BUZZER, OUTPUT);
pinMode(LED, OUTPUT);
Serial.begin(9600);
}
void loop() {
float dist = readDistance();
Serial.print("Distance: ");
Serial.print(dist, 1);
Serial.println(" cm");
if (dist >= MAX_CM) {
// Clear — silent
noTone(BUZZER);
digitalWrite(LED, LOW);
} else if (dist >= WARN_CM) {
// 50–30 cm: slow beep every 700 ms
beep(80, 620);
digitalWrite(LED, LOW);
} else if (dist >= CLOSE_CM) {
// 30–15 cm: medium beep every 350 ms
beep(100, 250);
digitalWrite(LED, HIGH);
} else if (dist >= DANGER_CM) {
// 15–5 cm: rapid beep every 120 ms
beep(80, 40);
digitalWrite(LED, HIGH);
} else {
// < 5 cm: solid alarm
tone(BUZZER, 3200);
digitalWrite(LED, HIGH);
delay(50);
}
}Live simulation
Steps
- 1Assemble the circuit on a breadboard following the wiring table above
- 2Open Arduino IDE 2, select board: Arduino Uno, and the correct COM port
- 3Paste the sketch and click Upload
- 4Open Serial Monitor at 9600 baud to watch live distance readings
- 5Move your hand toward the sensor — the buzzer pattern should change at each threshold
- 6Tweak WARN_CM / CLOSE_CM defines to match your use case
Going Further
- Add a 16×2 I2C LCD to display the numeric distance instead of (or alongside) the Serial Monitor
- Replace the active buzzer with a passive one and use the
tone()frequency to create a musical scale — lower pitch for far, higher for near - Mount the sensor in a 3D-printed bracket and power from a 9V battery for a portable device
- Combine two HC-SR04s (front and rear) on D9/D10 and D6/D7 for a dual-axis parking assistant
