Abstract: A sensor containing at least one piezoelectric transducer element is operated via a common, single signal line in both possible operating modes--first, at low frequencies as a measuring element for a mechanical quantity upon utilization of the direct piezoeffect and, second, at higher frequencies as a piezoelectric resonator upon utilization of the inverse piezoeffect for electrical excitation of mechanical oscillations and of the direct piezoeffect for generating the piezoelectric reaction. A high-frequency signal (u.sup.HF, u.sub.F) that describes the resonant characteristic and a lower-frequency signal (u.sub.NF, u.sub.Q) that describes the mechanical influence are generated from the measured signal on the signal line. The faultless function of the sensor together with the appertaining measuring amplifier, for example, can thus be monitored immediately during a measurement with the sensor or a simultaneous measurement of two different quantities can be undertaken.