Code sample for my article on HC-SR04 on https://www.atrakeur.com/blog/art30-comment_utiliser_un_capteur_a_ultrasons_hcsr04_avec_arduino

PingHCSR04.ino

//Define the echo pin (receive the echo from the ultrasound)
#define SENSOR_ECHO_PIN 7
//Define the trig pin (trigger the ultrasonic emitter)
#define SENSOR_TRIG_PIN 8
//On some sort of sensors, echo and trig can be on the same pin

//Precalculate some values
const float SpeedOfSound = 343.2; // ~speed of sound (m/s) in air, at 20°C         
const float MicrosecondsPerMillimetre = 1000.0 / SpeedOfSound; // microseconds per millimetre
const float MicrosecondsToMillimetres = (1.0 / MicrosecondsPerMillimetre); //distance by time elapsed
const float MicrosecondsToMillimetresDistance = MicrosecondsToMillimetres / 2.0; //because the ping make twice distance to object

void setup() {
	// initialize serial communication to debug
	Serial.begin(9600);
	
	//Set pin modes
	pinMode(SENSOR_ECHO_PIN, INPUT);
	pinMode(SENSOR_TRIG_PIN, OUTPUT);
	
	//Enable internal pull down for echo (to force it to zero when not in HIGH)
	digitalWrite(SENSOR_ECHO_PIN, LOW);
}

void loop() {
	//Send a high pulse on the trigger for a given amount of time
	digitalWrite(SENSOR_TRIG_PIN, HIGH);
	delayMicroseconds(10); // Some sensor need more than 10 us
	digitalWrite(SENSOR_TRIG_PIN, LOW);
	
	//Sound wave is in transit, wait for it to come back (ECHO pin goes to LOW)
	long microseconds = pulseIn(SENSOR_ECHO_PIN, HIGH)

	long millimetres = MicrosecondsToMillimetresDistance * microseconds;
	
	//Print result
	char buff[32];
	sprintf(buff, "Distance: %d cm", millimetres);
	Serial.println(buff);
	
	//Add a delay to avoid flooding Serial, and avoiding soundwave collisions
	delay(1000);
}