GB2269421A - Motor vehicle ignition immobilisers. - Google Patents

Abstract

A cap 8 secured with security fastenings 10 to the head of an ignition coil 2 contains an immobiliser circuit 16 which acts to make or break the connection between the battery and the low voltage terminal of the coil. A module 22 storing a unique code can be plugged into a socket 20 on the dashboard of the vehicle and when the ignition switch 18 is turned ON it will generate a unique code which the socket 20 will transmit to the circuit 16. The circuit 16 includes a decoder and when the unique code is decoded and identified as identical to a stored code the decoder will activate a relay (52, fig. 2) to close its contacts and so make the connection between the low voltage terminal and the battery. <IMAGE>

Description

MOTOR VEHICLE IMMOBILISERS The present invention relates to motor vehicle immobilisers.

Known motor vehicle immobilisers include systems which use a relay with a set of relay contacts to interrupt the power supply to the fuel pump or the low tension side of the ignition coil. One of the main disadvantages of such systems is that the professional thief finds it fairly easy to hot wire such systems by introducing an electrical conductor to form a bypass loop in parallel with the relay contacts.

The present invention provides an improved immobiliser system.

According to the present invention there is provided a motor vehicle immobiliser comprising a housing for enclosing a terminal of the ignition coil, and arranged to be secured to the ignition coil with a security fastening, an immobilisation circuit located within said housing to make or break the connection of said terminal with a voltage source, a transmitter operable to generate a unique code and a receiver for receiving said code and feeding it to said immobilisation circuit, said immobilisation circuit when energised and in a disabled state acting to break the connection between said voltage source and said terminal until such time as the received generated code corresponds with a stored code whereupon the immobilisation circuit becomes enabled and the connection is remade.

An immobiliser system embodying the present invention, will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a fragmentary view of the dashboard and ignition system of a motor vehicle; and Figure 2 is a circuit diagram of the immobiliser system of Figure 1.

As shown in Figure 1 the ignition coil 2 of a petrol engine (not shown) is clamped to the chassis or body of a vehicle by a bracket 4 using shear-off bolts 6 - bolts which can only be removed by shearing their heads off. A high security housing 8 in the form of a cover of steel or other strong material houses the upper part of the ignition coil 2 and is secured to the body of the coil 2 by a clamping collar 10 secured with a shear-off bolt. The housing 8 includes an opening in its upper extremity to accommodate the high tension lead 12 supplying high tension produced by the coil 2 to the spark plugs of the engine (not shown).

The coil has a low tension terminal 14 which is supplied with a low tension voltage through an electronic immobilisation circuit 16 secured to the inner wall of the housing 8.

The dashboard 30 of the vehicle has an ignition switch 18 which can be operated with a key 20 and an immobilisation switch 18 in the form of an electric socket which can be operated by a module 22 having a plug 24 which can be accommodated by the socket. The dashboard 30 also supports a light bulb (in the form of a light emitting diode) 26 connected to the socket which becomes energised when the immobilisation system is activated.

An output of the switch 18 is connected to the circuit 16 and a multiconductor cable 28 interconnects the circuit 16 with the switch 20.

With the key 20 and the module 22 removed from the dashboard 30, the electronic circuit 16 is disabled and no power is fed to the low tension terminal 14. To enable the circuit 16, first the key 20 is inserted into the ignition switch 18 and turned and then the plug 24 is inserted into the socket 20. The circuit 16 is energised which in turn energises the circuit in the module. The circuit in the module 22, thereupon generates a unique code which is compared with a preset code stored in the circuit 16 and if coincidence is detected the circuit 16 becomes enabled and energises the terminal 14. Once the circuit 16 is enabled the light emitting diode (LED) is extinguished.

Figure 2 shows the electronic circuitry of the module 22 and the immobilisation circuit 16. As shown the plug 24 has three electrically isolated sections which make electrical contact with respective contacts inside the socket 20. The section 24A is earthed, the section 24B is arranged to receive the line voltage and the section 24C is arranged to transmit data.

Inside the module 22 is an encoder 30 arranged to generate a unique code, and a monostable multivibrator circuit 32. The encoder 30 and multivibrator are supplied with power from the sections 24A and 24C of the plug. The output of the monostable circuit 32 feeds the drive input of the encoder 30 to cause the encoder to generate its unique code. The data output of the encoder 30 is connected to the section 24B of the plug.

The socket 20 contains three contacts for making contact with the corresponding sections of the plug.

The LED 26 is connected between the earthed contact and a terminal supplying a signal from the immobiliser circuit 16. The cable 28 connects the contacts and terminal of the socket 20 with the immobiliser circuit 16.

The immobiliser circuit 16 contains a decoder 40 having an input terminal for receiving data transmitted from the encoder 30 via the cable 28 and an output terminal for generating an output signal when equality is detected.

The output signal is fed via an inverter 42 to one input of a bistable circuit 44. The output of the bistable circuit 44 is fed to one input of an AND gate 46. The other input of the bistable circuit 44 and the other input of the AND gate 46 are both connected to the output of an inverter 48 which in turn is supplied with power from the ignition switch 18 via a signal processing circuit 50. The ignition switch 18 also supplies power to the ignition coil 2 via the normally closed contact set 52A of a relay 52. The relay 52 has an energisation coil 52B which is supplied with power from the output of the AND gate 46 via an inverter 54.

A diode is connected in parallel with the relay coil 52B to suppress unwanted induced voltages.

A second output of the bistable circuit 44 is connected to the terminal of the socket supplying the LED 26 via an inverter 56.

The bistable circuit 44 includes two NOR gates 44B and 44C in which the output of one is connected to the input of the other and vice versa. The remaining input of the gate 44B forms the reset input and the remaining input of the gate 44C forms the set input.

The set input is fed by the output of yet a further NOR gate 44A which acts to ensure that the bistable is reset as soon as the ignition is turned on.

The immobiliser circuit 16 also includes a regulator and filter 58 for supplying the various components of the circuit 16 with power.

In operation with the plug 24 and the key 20 withdrawn the relay 52 is de-energised and so the contacts 52A are closed.

With the key switched ON and the plug 24 inserted the input to the inverter 48 is held low so that the consequent high generated at the output is fed to the second input of the AND gate 46.

If at this time the decoder 40 has not generated a signal, the output of the inverter 42 will be high which acts to reset the bistable circuit 44 so that its output will be low.

The first input at the AND gate 46 is thus low and so the output of the AND gate 46 will also be low which will cause the output of the inverter 54 to go high and energise the relay 52. This in turn will break the contact set 52A and so disconnect or immobilise the ignition coil.

In the meantime the encoder 30 will have been energised and caused to generate its unique code. The unique code will be detected by the decoder 40 and if equality is detected, the output of the decoder 40 will generate a high and cause the output of the inverter 42 to fall to a low. With two lows applied to the inputs of the NOR gate 44A of the bistable circuit 44 the contact will turn high and this will set the bistable.

Setting of the bistable causes a high to appear at the first input of the AND gate 46 with a high now at each input the output of the AND gate 46 will turn high, and so cause the inverter to de-energise the relay 52. The contact set will revert to being closed and the ignition coil thus becomes enabled.

The output of the NOR gate 46C is connected via the inverter 56 to the LED 26 so that the LED is energised when the immobiliser is enabled.

The arrangement of the NOR gates is such that if the vehicle battery is disconnected and reconnected the bistable circuit is always triggered initially to reset and so this ensures that the relay 52 will always start energised.

It will be appreciated that instead of the module having a plug it can have an ultrasonic, optical or radio frequency transmitter for transmitting the unique code to a receiver. The receiver would act in place of the socket to store the received code and to feed it to the decoder.

While the security fastenings used to secure the housing to the coil and the coil to the chassis are shear-off bolts, it will be appreciated that other security fastenings such as padlocks, adhesives or weldings can be used to secure these components in place so that they cannot readily be removed without considerable difficulty and without involving the expenditure of considerable time. This acts as a significant deterrent to the prospective thief.

Claims (10)

1. A motor vehicle immobiliser comprising a housing for enclosing a terminal of the ignition coil, and arranged to be secured to the ignition coil with a security fastening, an immobilisation circuit located within said housing to make or break the connection of said terminal with a voltage source, a transmitter operable to generate a unique code and a receiver for receiving said code and feeding it to said immobilisation circuit, said immobilisation circuit when energised and in a disabled state acting to break the connection between said voltage source and said terminal until such time as the received generated code corresponding with a stored code whereupon the immobilisation circuit becomes enabled and the connection is remade.

2. An immobiliser according to Claim 1, wherein said transmitter comprises a module having a plug, and wherein the receiver comprises a socket arranged to receive the plug and to electrically connect the module to the immobilisation circuit.

3. A circuit according to Claim 1 or to Claim 2, including a light source, said light source being energised only in response to said immobilisation circuit being enabled.

4. A circuit according to any preceding claim, wherein said immobilisation circuit includes a relay providing contacts for making or breaking said connection.

5. A circuit according to Claim 4, wherein said immobilisation circuit comprises a decoder connected to receive said unique signal and to compare it with a stored code and when equality is detected to trigger a bistable circuit controlling said relay.

6. A circuit according to Claim 5, including means connected to the ignition switch and operable to ensure that said bistable device is immediately reset upon the decoder being triggered after the immobilisation circuit is energised.

7. A circuit according to Claim 1, wherein said transmitter and receiver operate in the optical frequency range.

8. A circuit according to Claim 1, wherein said transmitter and receiver are a radio frequency transmitter and receiver.

9. A circuit according to Claim 1, wherein said transmitter and receiver are an ultrasonic transmitter and receiver.

10. A motor vehicle immobiliser substantially as hereinbefore described, with reference to the accompanying drawings.