Abstract: Tyre condition sensing apparatus carried by a vehicle has a relay module (4) in the vicinity of a vehicle wheel. The relay module (4) detects a tyre condition of a tyre in the vehicle such as tyre pressure. Tyre condition indicating apparatus carried by the vehicle has a display module (5) at the vehicle dashboard, and a sounder module (50) arranged in the vicinity of the wheel. The sounder module (50) gives audio indications audible at the wheel corresponding to visual or audio indications given at the dashboard by the display module (5). The indications are permanently off for a normal condition, permanently on for an above-normal condition and intermittently on for a below-normal condition.

Abstract: Signal transmission apparatus for use, for example, in a passive sensor system such as an in-vehicle tyre pressure sensing system, comprises transmitter circuitry and receiver circuitry coupled together by a coupling, preferably a wireless coupling such as two opposed plate-form antennae. The transmitter circuitry includes a resonator having at least one component whose value influences a natural resonant frequency of the resonator and can be changed in use of the circuitry. The effective value is changed, for example, by a physical parameter being sensed or by a control signal to be transmitted. The receiver circuitry includes a driver section for applying to the resonator an excitation signal having a predetermined excitation frequency that is different from the natural resonant frequency. The transmitter circuitry preferably derives its power supply from the excitation signal via a rectifier circuit. The receiver circuitry detects such a change in the effective value via the coupling.