There are 4 pins out of the module: VCC , Trig, Echo, GND . So it's a very easy interface for controller to use it ranging. The all process is : pull the Trig pin to high level for more than 10us impulse , the module start ranging ; finish ranging , If you find an object in front , Echo pin will be high level , and based on the different distance,it will take the different duration of high level. So we can calculated the distance easily :
Distance = ((Duration of high level)*(Sonic :340m/s))/2
NOTE: The link is the documents about Ultrasonic Module HC-SR04 Distance Sensor.
https://s3-ap-northeast-1.amazonaws.com/sain-amzn/20/20-019-100/HC-SR04.rar
Price: $12.00