Air pressure indicator for a vehicle tyre

The present invention relates to a device which indicates the safety limits of the air pressure in each one of the pneumatic tires of a vehicle by transmitting appropriate information from the wheels to the driving control board. Transmission of information is based on the capacitive coupling between rotating and non-rotating conductive annular surfaces (2-5, 12-19, 72-75) which are mounted into channels of insulated bases (6-9) fixed in the available space between the bearing flange, the braking system and the wheel bearings (66) and are forming cylindrical and plane capacitors. An electronic circuit incorporates these capacitors as basic elements for its operation, and also an AC bridge (20) to give an automatic compensation by measuring the differential capacitance for each pair of the wheels to avoid faulty indication due to causes like humidity, temperature, etc. Also a simple electronic circuit can be used for each one of the wheels. A tire pressure sensor (1) comprises a semiconductor membrane (30) with an electrode (33) which is shorting out the rotating conductive surfaces (2, 3, 12, 13, 16, 17, 72, 73) when the tire pressure deviates from the safety limits resulting in a change of the original frequency and giving visual display or audio indication.