Ultrasonic Sensor HC-SR04 + Arduino Calculation? - c++

Question

I'm using this code :
int trigPin = 7;
int echoPin = 8;
void setup() {
Serial.begin(9600);
pinMode(trigPin,OUTPUT);
pinMode(echoPin,INPUT);
}
void loop() {
int duration;
int distance;
digitalWrite(trigPin,HIGH);
delayMicroseconds(1000);
digitalWrite(trigPin,LOW);
duration = pulseIn (echoPin,HIGH);
distance = (duration/2)/29.1;
Serial.print("distance = ");
Serial.println (distance);
delay(500);
}
I understand the concept that we send a pulse of 1000 µsec in this case and we wait for the reflected signal. But I don't understand how in this code, only the pulse width of the reflected signal is being used for the calculation.
I searched around and they say the reflected pulse width is proportional to the distance traveled. Can someone please explain how this happens (the physics behind it) and also where does the 29.1 comes from?
I read this documentation but I still don't understand the pulse width concept.
Many thanks in advance!

Answer 1

Like Chris touched on in the comments, the input pin goes HIGH for the time between it being send and received. I'm not exactly sure if it goes HIGH for a certain amount of time after or during the transmission, but you might be able to find that on a datasheet.
where does the 29.1 come from?
That's the speed of sound through air... you can use that to convert the time to centimeters. You'll have to divide it by two because it goes two ways.
Recap:
Arduino sends pulse to SR04 (1000 microseconds):
digitalWrite(trigPin,HIGH);, delayMicroseconds(1000);, &digitalWrite(trigPin,LOW);.
SR04 emits a ping and changes the signal pin to HIGH
The ping hits an object, bounces back, and goes back to the SR04
The SR04 sets the pin back to LOW
The Arduino measures the length the signal pin is HIGH with this:
duration = pulseIn (echoPin,HIGH);