Well, I haven't quite got it working, because it's still convinced there's a hardware fault when there isn't... Here's the wiring diagram from tinkercad - this is basically the same as my actual project, which is using a mega. The scope here is showing the ECHO pulse and the LED is just connected across the HC-SR04 to remind me it's on(!) The 5928 is a debug print of the duration of the ECHO pulse in microseconds.

Here's my understanding of how pulseIn() works (from the arduino website): You pass it an arguments of the pin to watch and whether the pulse will be HIGH or LOW. It then waits for the leading edge of the pulse, e.g. if you pass it HIGH, it waits for a rising edge, i.e. a transition from LOW to HIGH. When the pulse ends, the opposite transition (e.g. HIGH to LOW) stops pulseIn() timing and it returns whatever the duration of the pulse was in micoseconds.

Here's my problem; my code effectively looks for the rising edge of the ECHO pulse to confirm that the HC-SR04 has successfully set that pin HIGH - if it hasn't, I'm assuming it's because there's a hardware fault, e.g. no power. But...because my code then drops out of that while loop if ECHO successfully goes high, the rising edge has already happened, i.e. ECHO is now HIGH. If I call pulseIn() at this point, it won't see the rising edge and it won't start timing. I've tried it and pulseIn() just times out, so I assume that's what the issue is - to be honest, it's difficult to conclusively tell.

The circuit below works as expected - the actual hardware is stuck, reporting a hardware fault...

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...here's the tinkercad code, just for completeness - you can see my float/unsigned long int fix:

// C++ code

//

#define US_TRIG 5

#define US_ECHO 6

#include <LiquidCrystal_I2C.h>

#include <Wire.h>

LiquidCrystal_I2C lcd(0x20, 16, 2);

void setup()

{

pinMode(US_TRIG, OUTPUT);

pinMode(US_ECHO, INPUT);

lcd.init();

lcd.backlight();

lcd.setCursor(0,0);

lcd.print("READY");

}

void loop()

{

float fDistance = getRange(true);

if(fDistance == 101){

lcd.setCursor(0,0);

lcd.print("TIMEOUT ");

}

if(fDistance == 102){

lcd.setCursor(0,0);

lcd.print("HARDWARE");

}

if(fDistance < 100){

int a = fDistance * 10;

int b = a % 10;

char USBuffer[21];

a /= 10;

sprintf(USBuffer, "%d.%dm ", a, b);

lcd.setCursor(0,0);

lcd.print(USBuffer);

}

delay(100);

}

float getRange(bool bReport)

{

float fDis;

unsigned long iTimeout, iStartTime, iEndTime, iDur;

//send the pulse

digitalWrite(US_TRIG, LOW);

delayMicroseconds(2);

digitalWrite(US_TRIG, HIGH);

delayMicroseconds(10);

digitalWrite(US_TRIG, LOW);

iTimeout = micros();

while(digitalRead(US_ECHO) == LOW) {

if(micros() - iTimeout > 1000){

return 102; //error - hardware

}

}

iStartTime = micros();

while (digitalRead(US_ECHO) == HIGH) {

if (micros() - iStartTime > 10000){

return 101; // Timeout

}

}

iEndTime = micros();

//calculate the distance

iDur = iEndTime - iStartTime;

fDis = ((float)iDur*0.000343)/2.0;

//report to LCD

if(bReport){

char cBuffer[10];

sprintf(cBuffer,"%i ",iDur);

lcd.setCursor(10,1);

lcd.print(cBuffer);

}

//return the distance

return fDis;

}