Distance detection using ultrasonic sensor works on the principle of time-of flight principle which is based on measuring the time taken for the ultrasonic signal to travel from source to an object (whose distance has to be measured) and back to the receiver. For this application, both Monostatic and Bi-static configuration of the sensor can be used but for this demonstration purpose, this article shows monstatic configuration. The same hardware setup shown in material detection can be used here as well. The difference is that, here instead of measuring the amplitude of echo signal we measure the time taken for the transmitted signal to reach back the receiver after getting reflected from the object. Here the total distance the signal travels is twice the distance between the sensor and the object.

The total time taken is called the time of flight and is denoted by Δt in Figure 4-10. The first pulse here is the excitation or transmitted signal and the second one is the echo signal. Speed of sound in air is around 343m/s, total time of flight for round trip of the signal is Δt, so the distance of the object from the sensor can be given by d= (343m/s x Δt)/2