HC-SR04 Ultrasonic Sensor uses sonar to determine distance to an object like bats or dolphins do. It provides 2cm - 400cm non-contact measurement function, the ranging accuracy can reach to 3mm. The module includes ultrasonic transmitters, receiver and control circuit. Its operation is not affected by sunlight or black material like Sharp rangefinders are (although acoustically soft materials like cloth that can be difficult to detect). Similar in performance to the SRF005 but with the low-price of a Sharp infrared sensor. The basic principle of work: Using IO trigger for at least 10us high level signalThe Module automatically sends eight 40kHz and detect whether there is a pulse signal backIf 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 x velocity of sound (340M/S)) / 2 Parameters: Thickness: 2.8~3.1mmInner diameter: 16mmFasten hole: 3.8mmWorking voltage: 5V (DC)Output signal: Electric frequency signal, high level 5V, low level 0VSensor angle: Not more than 15 degreesDetection distance: 2cm ~ 450cmHigh precision: Up to 3mmMode of connection: VCC / trig(T) / echo(R) / GNDThe 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.If you are sourcing a ultrasonic ranging module, the HC-SR04 is a good choose . Its stable performance and high ranging accuracy make it a popular module in electronic market.Compared to the Shape IR ranging module, HC-SR04 is more inexpensive than it. But it has the same ranging accuracy and longer ranging distance.