Ultrasonic Transducers & HC-SR04 Timing Physics

1. Overview & Concept Definition

An active acoustic ranging transducer that measures distance by emitting an ultrasonic sound burst and calculating the round-trip flight time of the reflected echo.

2. Engineering & Industrial Significance

Non-contact distance measurement unaffected by target color, optical transparency, or ambient lighting conditions (e.g. liquid level sensing, mobile robot obstacle detection).

3. Physical Working Principle & Transducer Dynamics

A 10µs TTL pulse to the Trigger pin causes internal electronics to pulse the transmitter piezoelectric crystal with an eight-cycle 40 kHz square burst. The receiver captures the mechanical wave reflection. The Echo pin stays HIGH for the exact acoustic transit duration $t$. Distance is computed using the velocity of sound.

// Arduino C++: Non-blocking or hardware-timer HC-SR04 driver
const int TRIG_PIN = 5;
const int ECHO_PIN = 18; // Use 1k/2k divider for 3.3V ESP32

float readDistanceCm(float ambientTempC) {
    digitalWrite(TRIG_PIN, LOW);
    delayMicroseconds(2);
    digitalWrite(TRIG_PIN, HIGH);
    delayMicroseconds(10);
    digitalWrite(TRIG_PIN, LOW);

    // Measure echo pulse width (timeout 30000us ~ 5 meters max)
    unsigned long duration = pulseIn(ECHO_PIN, HIGH, 30000UL);
    if (duration == 0) return -1.0f; // Out of range or timeout

    float speedOfSound = 331.3f + (0.606f * ambientTempC); // m/s
    float speedCmPerUs = (speedOfSound * 100.0f) / 1000000.0f;
    return (duration * speedCmPerUs) / 2.0f;
}

6. Common Engineering Pitfalls & Troubleshooting

7. Applications & Physical Constraints