HC-SR04 ultrasonic sensor distance measuring module

Description:

1. using IO trigger range, for atleast 10 us high level signal
2. module automatically sent eight40 KHZ square wave, automatically detect whether there is a signalto return
3. have returned signal, through theIO output high level, the duration of the high level is
Ultrasonic from launch to returntime.  test distance= (high level of time * sound velocity(340 m/S)) / 2