Which type of sensor operates based on the Doppler principle?

The correct answer is the microwave sensor, which operates based on the Doppler principle. Microwave sensors emit microwave radar waves and analyze the reflection received after these waves bounce off moving objects. The Doppler effect refers to the change in frequency or wavelength of waves in relation to an observer moving relative to the source of the waves. In the case of the microwave sensor, when an object moves towards or away from the sensor, the frequency of the reflected waves changes. This frequency shift is then detected and processed to determine the presence, distance, and speed of the object.

In contrast, capacitance sensors work by measuring changes in capacitance caused by a change in proximity of an object. Electromechanical sensors tend to rely on physical movement or presence of an object to activate a mechanism. Photo sensors detect changes in light levels and are not related to frequency shifts in waveforms. Each of these other types of sensors utilizes different principles and mechanisms that do not involve the Doppler effect, thus reinforcing why microwave sensors are uniquely tied to this phenomenon.