Abstract: A passive electrically operable monitoring means has an inherent resistance and a characteristic capacitance and inductance only, so that the monitoring means has a predetermined natural frequency of oscillation. Variations in a physical parameter being monitored by the monitoring means will cause the characteristic capacitance and/or inductance to vary thereby varying the natural frequency of oscillation of the monitoring means. A transducer includes an excitation means for exciting the monitoring means and a sensing means for sensing the natural frequency of oscillation of the monitoring means. The excitation means includes a pulse generating means for exciting the monitoring means with a string of pulses having a repetition rate suitably lower than the natural frequency of oscillation of the monitoring means so that damped oscillations occur in the monitoring means.

Abstract: A tire pressure indicating device includes a coil, and mounted in a tire forming with a capacitor to form a passive oscillatory circuit. The circuit is energized by pulses supplied via a coil positioned outside the tire and secured to the vehicle and the frequency in the passive oscillatory circuit is varied with tire pressure due to changes caused to the capacitance value of the capacitor. The frequency in the passive oscillatory circuit is sensed via a coil positioned outside the tire and secured to the vehicle.