Hi, I'm creating a project where I'm using two HC-SR04 sensors to measure the distance between them. I'm using the receiver from one HC-SR04 and the transmitter from another to communicate. Ideally, the HC-SR04 transmitter would send a signal and the receiver would detect it by triggering its own trigger pin to set the echo pin to high (I covered the trigger pin of the HC-SR04 receiver so its echo pin can't detect the signal it sent). However, the receiver is not detecting any signal (pulseIn(echo, HIGH) = 0) which is where the problem comes in. Initially, I used time delays for the sensors to sync with each other but it resulted in inaccurate and imprecise distances which is why I thought I'd use a radio module (NRF24L01) to sync the sensors. Unfortunately, this time, the receiver echo pin did not read anything and I'm basically getting 0 distances. Here's the code I made for reference:
Transmitter:
#include <nRF24L01.h>
#include <RF24.h>
#include <SPI.h>
RF24 radio (7,8);
const byte address[6] = "00001";
const int trig = 9;
const int echo = 10;
const unsigned long event;
unsigned long prevTime;
void setup() {
pinMode(trig, OUTPUT);
pinMode(echo, INPUT);
radio.begin();
radio.openWritingPipe(address);
radio.setPALevel(RF24_PA_LOW);
radio.stopListening();
}
void loop() {
unsigned long now = millis();
int x = 1;
if (now - prevTime >= 3000){
radio.write(&x, sizeof(x));
trigpin();
prevTime = now;
}
}
void trigpin(){
digitalWrite(trig, HIGH);
delayMicroseconds(10);
digitalWrite(trig, LOW);
}
Receiver:
#include <nRF24L01.h>
#include <RF24.h>
#include <SPI.h>
RF24 radio (7,8);
const byte address[6] = "00001";
const int trig = 9;
const int echo = 10;
long duration;
float distance;
void setup() {
pinMode(trig, OUTPUT);
pinMode(echo, INPUT);
radio.begin();
radio.openReadingPipe(0, address);
radio.setPALevel(RF24_PA_LOW);
radio.startListening();
Serial.begin(9600);
}
void loop() {
if (radio.available()){
int x = 0;
radio.read(&x, sizeof(x));
if (x = 1){
duration = 0;
trigpin();
duration = pulseIn(echo, HIGH);
distance = duration * (0.034 / 2);
Serial.println(duration);
Serial.println(distance);
Serial.println("---");
}
}
}
void trigpin(){
digitalWrite(trig, HIGH);
delayMicroseconds(10);
digitalWrite(trig, LOW);
}
How it works:
The transmitter is supposed to send an ultrasonic wave every 3 seconds to the receiver. But before doing so, it notifies the receiver to send a fake trigger to set the echo pin to high. The receiver then calculates the distance and prints it.
Result:
The printed values are "0" for both duration and distance.
Possible cause(s):
I was thinking that maybe I got zero values was because it took too long for the echo pin to be set to high-maybe it might have missed its window to receive the signal from the transmitter. I tried adding delays before triggering the transmitter trig pin and after sending the radio signal from transmitter to receiver but I got the same results.
Can someone point out what I did wrong and how I can fix it? I hope I didn't take too much of your time. Thanks very much for your help and happy new year! 
P.S. I've only started studying Arduino mainly through YouTube tutorials so please go easy on me 
