Ultrasonic Sensor: HC-SR04

Working Principle

The HC-SR04 measures distance using time-of-flight: it emits an ultrasonic burst at 40 kHz, waits for the echo reflected by an object, and outputs a pulse whose width encodes the round-trip time. Distance is calculated as:

distance_cm = (pulse_us / 2.0) / 29.1

Analogy: It works exactly like shouting into a canyon and timing the echo — just at a frequency far above human hearing.

Electrical Characteristics

Parameter	Value
Supply Voltage	5 V
Current	15 mA
Range	2 cm – 400 cm
Accuracy	± 3 mm
Beam Angle	~15°
Trigger Pulse	10 µs HIGH

Interfacing

Two GPIO pins are needed: TRIG (output, sends 10 µs pulse) and ECHO (input, measures return pulse width). If connecting to a 3.3 V MCU, use a voltage divider on the ECHO pin.

Calibration

The speed of sound varies with temperature. For precise measurements, apply temperature compensation:

speed_of_sound = 331.3 + (0.606 * temperature_c) m/s

Code Example

/*
 * HC-SR04 Ultrasonic Distance Sensor — Arduino
 * Wiring: TRIG → Pin 9, ECHO → Pin 10, VCC → 5V, GND → GND
 */
#define TRIG_PIN 9
#define ECHO_PIN 10

void setup() {
    Serial.begin(9600);
    pinMode(TRIG_PIN, OUTPUT);
    pinMode(ECHO_PIN, INPUT);
}

float measureDistance() {
    /* Send 10µs trigger pulse */
    digitalWrite(TRIG_PIN, LOW);
    delayMicroseconds(2);
    digitalWrite(TRIG_PIN, HIGH);
    delayMicroseconds(10);
    digitalWrite(TRIG_PIN, LOW);

    /* Measure echo pulse duration */
    long duration = pulseIn(ECHO_PIN, HIGH, 30000); /* 30ms timeout */
    if (duration == 0) return -1.0;  /* No echo = out of range */

    /* Convert to cm (speed of sound ≈ 343 m/s at 20°C) */
    return (duration / 2.0) / 29.1;
}

void loop() {
    float dist = measureDistance();
    if (dist < 0) {
        Serial.println("Out of range");
    } else {
        Serial.print("Distance: ");
        Serial.print(dist, 1);
        Serial.println(" cm");
    }
    delay(100);  /* Min 60ms between measurements */
}

Real-World Applications

Limitations

• Soft surfaces: Fabric and foam absorb sound, producing weak or no echoes.
• Narrow angles: Objects outside the 15° cone are invisible.
• Crosstalk: Multiple HC-SR04 modules can interfere; stagger trigger timing.
• Minimum range: 2 cm dead zone means you cannot measure very close objects.