PING sensor on RPI, start counting when less than 40cm? - c

Question

I have a PING sensor (HC-SR04) hooked up to my RPI. I want it to start counting when the sensor sees less than 40 cm. It should stop counting again when the sensor again sees less than 40 cm, print out the time and start counting again.
How can I do this?
This is my code:
#include <stdio.h>
#include <stdlib.h>
#include <wiringPi.h>
#define TRUE 1
#define TRIG 5
#define ECHO 6
void setup() {
wiringPiSetup();
pinMode(TRIG, OUTPUT);
pinMode(ECHO, INPUT);
//TRIG pin must start LOW
digitalWrite(TRIG, LOW);
delay(30);
}
int getCM() {
//Send trig pulse
digitalWrite(TRIG, HIGH);
delayMicroseconds(20);
digitalWrite(TRIG, LOW);
//Wait for echo start
while(digitalRead(ECHO) == LOW);
//Wait for echo end
long startTime = micros();
while(digitalRead(ECHO) == HIGH);
long travelTime = micros() - startTime;
//Get distance in cm
int distance = travelTime / 58;
return distance;
}
int i;
int main(void) {
setup();
while (1)
{
printf("Distance: %dcm\n", getCM());
delay(250);
}
return 0;
}

Answer 1

The code doesn't measure the distance. It only measures the duration of the reflected signal. To get the distance you need to measure the delay between the beginning of ping, and the beginning of echo:
startTime = micros();
digitalWrite(TRIG, HIGH);
delayMicroseconds(20);
digitalWrite(TRIG, LOW);
//Wait for echo start
while(digitalRead(ECHO) == LOW);
long travelTime = micros() - startTime;
Of course, this is just a skeleton: you need to debounce the echo to guard yourself against background noises and other glitches.