Transmitter and receiver are integrated in one housing. The sensor switches as soon as the sonic signal is reflected back by an object within the adjusted sensing range (background suppression principle).
Advantage: simple installation, low cabling efforts
Transmitter and receiver are integrated in the same housing. The sensor detects the sound reflected by a fixed reflector. The sensor switches as an object interrupts the distance between sensor and reflector.
Advantage: Independent of object attributes, reliable signal even with extremely weak or diffuse reflective or sound-absorbing objects