Abstract: A piezoelectric device for measuring the pressure of a gas like medium comprises two oscillators disposed opposite to each other in a substantially closed system by intermediate introduction of the gas to be measured. The measuring is performed by defining a transfer factor from the first piezoelectric oscillator to the second piezoelectric oscillator, whereby the first piezoelectric oscillator is definitely excited and the measuring signal is picked up by the second piezoelectric oscillator.

Abstract: To determine if a pulsed beam system, typically an ultrasonic pulsed beam system, which is located externally of a motor vehicle is in operative condition, a transmitter (14) projects a beam to a fixed reflector (16) located on the vehicle, and one (19) of a group of timing stages (18; 19, 20, 21) evaluates the transit time to the reflector. If the transit time to the reflector is slightly less than a predetermined time interval (Tc), so that the threshold timing level (Tc) is passed, a first indication, for example a frequency at a first tone (f1), generated by an acoustic tone generator (30) is reproduced in a loudspeaker; if, then, an obstruction is sensed by the beam, and the distance between the transmitter (14) and the obstruction (17) which forms a reflecting object drops below a first predetermined distance represented by a time period (T2), a second one of the transit time stages responds, and, through logic gates (22-24, 28, 29) provides a second discrete output frequency (f2) in the loudspeaker.