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,