Device for monitoring and wirelessly indicating a pressure or a pressure change in pneumatic tires mounted on vehicles

1. Device for monitoring and wirelessly signaling a pressure, or a change in pressure, in pneumatic tires on vehicles comprising:

a pressure sensor for measuring the pressure or the change in pressure;

a transmitter for transmitting a signal derived from the measuring signal of the pressure sensor;

a control circuit that activates the transmitter intermittently to submit said derived signal;

a battery as current source;

and a back-up capacitor connected in parallel to the battery; and

wherein a controllable switching unit is provided which is so controlled such that the back-up capacitor is connected in parallel to the battery intermittently before the control circuit activates the transmitter and for a limited period of time during the transmitting operation of the transmitter.

2. The device as defined in claim 1 , wherein the controllable switching unit has an input for control signals arriving from the control circuit by which the back-up capacitor is brought into the circuit from time to time for said limited period of time only.

3. The device as defined in claim 2 , wherein the control circuit is arranged to bring the back-up capacitor into the circuit shortly before it activates the transmitter.

4. The device as defined in claim 1 , wherein the control circuit is arranged to bring the back-up capacitor into the circuit shortly before it activates the transmitter.

5. The device as defined in claim 4 , wherein the control circuit is arranged to activate the back-up capacitor every time shortly before it activates the transmitter.

6. The device as defined in claim 4 , wherein the control circuit is arranged to activate the back-up capacitor not every time the transmitter is activated, but only if and when necessary.

7. The device as defined in claim 6 , wherein the control circuit comprises a comparator.

8. The device as defined in claim 7 , wherein the control circuit is connected to a device for measuring the voltage of the battery and is arranged such that it will determine that the need exists when the voltage of the battery has dropped below a given limit value.

9. The device as defined in claim 8 , wherein the control circuit determines that the need exists when the voltage of the battery has been determined to be below its limit value in one or more preceding activations of the transmitter.

10. The device as defined in claim 7 , wherein the control circuit is connected to a sensor and is arranged such that it will determine that the need exists when the measured value supplied by the sensor is below or above a given limit value.

11. The device as defined in claim 10 , wherein the measured value is the temperature.

12. The device as defined in claim 7 , wherein the control circuit is connected to a timer and is arranged such that it will determine that the need exists when a predetermined period of time has lapsed from the time the timer has been started.

13. The device as defined in claim 7 , wherein the control circuit is connected to a counter, which continuously counts the number of the transmitting operations, and that the control circuit is arranged such that it will determine that the need exists when a predetermined number of transmitting operations has been exceeded.

14. The device as defined in claim 7 , wherein the control circuit is arranged such that it will determine that the need exists when the pressure measured is determined to be below a given limit value.

15. The device as defined in claim 7 , wherein the control circuit is arranged such that it will determine that the need exists when the variation in pressure over time exceeds a given limit value.

16. The device as defined in claim 6 wherein the transmitter is coupled back to the control circuit and that the control circuit is arranged such that it evaluates a signal transmitted from the sensor through the feedback system to determine any information indicating that an attempted transmission has failed and that the control circuit determines that a need exists to bring the back-up capacitor into the circuit by determining that an attempted transmission has failed.

17. The device as defined in claim 6 , wherein the control circuit is arranged such that when a corresponding need has been determined to exist it will bring the back-up capacitor into circuit for a given number of transmitting operations and will then once more check the criterion or criteria that led to the determination that a corresponding need exists.

18. The device as defined in claim 1 , wherein the switching unit comprises a current limiter through which the back-up capacitor is charged, but not discharged.

19. The device as defined in claim 18 , wherein the switching unit comprises two switching elements controlled by the control circuit the first of which serves to cut in the back-up capacitor via the current limiter, while the second serves to bridge the current limiter.

20. The device as defined in claim 1 , wherein the switching unit comprises as switching elements a field effect transistor.