Ultrasonic one-way reflective barrier for contact free detection of objects specially in the most diverse applications e.g. with bottles or plastic foils. ews sensors are available in cubical as well as cylindrical housing and are competitive with most through-beam light barriers. One ultrasonic sensor ews, set as emitter, sends cyclic sound impulses, received by another one, set as receiver. If an object interrupts the measuring track between the set transmitter and the receiver, the switched output of the receiver is set.