Ultrasonic ranging module HC - SR04 provides 2cm - 400cm non-contact
measurement function, the ranging accuracy can reach to 3mm.
includes ultrasonic transmitters, receiver and control circuit. The basic principle
(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