How Does an Ultrasonic Sensor Like HC-SR04 Measure Distance?

Question

Hi everyone,I'm trying to understand the workings of the HC-SR04 ultrasonic sensor. I know that the transmitter sends ultrasonic waves, which are then received by the receiver. The sensor measures the time the waves return and calculates the distance based on this time. However, I'm curious about how it performs this measurement.Can someone explain the process in more detail? Specifically, how does the sensor calculate the time and convert that into a distance measurement?

Alfred Alonso · Accepted Answer

HiTo start a measurement, the sensor needs a trigger pulse, which is a short, 10-microsecond pulse sent to the 'Trig' pin of the sensor. This pulse activates the transmitter to emit a burst of 8 ultrasonic pulses at a frequency of 40 kHz. These pulses travel through the air. The transmitted ultrasonic waves propagate through the air until they hit an object, and when the waves encounter an object, they reflect back towards the sensor.Immediately after the sensor transmit the signal, the Echo pin goes high. This signifies the start of the sensor's listening period. The sensor now awaits the return of the reflected ultrasonic wave. If an object is within range, the receiving signal will be detected, causing the Echo pin to transition from high to low. The duration the Echo pin remains high corresponds to the total time taken for the ultrasonic wave to travel to the object and back to the sensor. So, if you have the microcontroller like arduino is connected to the sensor it will measures the duration for which the 'Echo' pin remains high, using an internal timer. This duration (T) represents the time it takes for the ultrasonic waves to travel to the object and back to the sensor.