Description About Product Ultrasonic ranging module HC-SR04 provides 2cm - 400cm non-contact measurement function, the ranging accuracy can reach to 3mm. The modules includes ultrasonic transmitter, receiver and control circuit The basic principle of work - Using IO trigger for at least 10us high level signal - The Module automatically sends eight 40 kHz and detect whether there is a pulse signal back - If the signal is back, through high level, time of high output I/O duration is the time from sending ultrasonic to returning. Test distance=(high level time x velocity of sound (340m/s) /2