HCSR04 is an Ultrasonic ranging module (Sonar Sensor) which consists of a transmitter,receiver and control circuit. It has four pins for VCC, GND, Trigger and Echo.You can easily interface it with micrcontrollers and Arduino boards.
The Basic Principle of Ultrasonic Sensor:
(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.