http://www.bytecreation.com/blog/2013/10/13/raspberry-pi-ultrasonic-sensor-hc-sr04 if lucky, get a web page vs a data sheet. - if unlucky, mostly works but doesn’t. - cross reference.
check stupid stuff. - wasted 5 minutes b/c unaligned jumper. - swap in 3v for 5v ? - swap the pins. - 5v out 3v - not resisted? (1) Using IO trigger for at least 10us high level signal, (2) The Module automatically sends eight 40 kHz and detect whether there is a pulse signal back. (3) IF the signal back, through high level , time of high output IO duration is the time from sending ultrasonic to returning. Test distance = (high level time×velocity of sound (340M/S) / 2,