The HC-SR04 is used to measure distances by using ultrasonic sounds. It works by sending out a burst of ultrasound and listening for the echo when it bounces off of an object. Here is a picture of the sensor The duration of this second pulse...
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HC-SR04 is an ultrasonic ranging module that provides 2 cm to 400 cm non-contact measurement function. The ranging accuracy can reach to 3mm and effectual angle is < 15°. It.