System for detecting an abnormal internal tyre pressure

A detector system for detecting an abnormal internal air pressure of a tyre of a motor vehicle produces an alarm when an abnormal change in the air pressure in the tyre is detected. The system comprises a first device which is arranged on a wheel (or on a rotating part) of the motor vehicle and a device for detecting an abnormal pressure using a crystal resonator and an aerial for transmitting the detection information into the air; and a second device which is arranged in the vicinity of a vehicle seat (or on a non-rotating part) and has a transmitter/receiver arrangement which is provided with an aerial. The crystal resonator is driven by excitation energy which is emitted by the transmitter component of the transmitter/receiver. Directly after the termination of the transmission of the excitation energy, a signal is discharged by the crystal resonator, specifically on the basis of oscillation energy collected in the resonator, and is received by the reception part of the transmitter/receiver in order to indicate an abnormality in the internal pressure of the tyre if one is present. The crystal resonator is either used simply as a transmitting device for information (abnormal reduction in the air pressure in the tyres) in combination with an abnormality detector switch (an element which responds to pressure) or is utilised both to detect an abnormal pressure and to transmit the information.