The invention relates to an electric circuit with a security feature and in particular to a circuit which requires that a transmitted code be accepted by a code receiver in the circuit before an electric circuit can be completed and a component thereof enabled. While the invention can be applied to numerous forms of apparatus, both static and mobile, the invention is seen to particularly good use in relation to the security of vehicles such as those used on land, sea and air. The invention is of particular value in immobilising a vehicle by means of a security code freed device such that only a user informed of the code and/or supplying the required code- programmed key can move the vehicle. For convenience the vehicle will be identified as a car.

The use of coded immobilising devices in a vehicle to provide an authorised user with a release code transmitting device to ensure that only the authorised user can drive the vehicle is well known. These gadgets are evadable. We have discovered that a car thief can identify the gadgets in say the engine compartment of the car merely by identifying the dedicated wiring by which they are incorporated in the vehicle and then disable them by disconnecting them from the electrical circuit of the vehicle. It is an object of this invention to incorporate the code receiving devices in such a way that their presence is so difficult to detect the potential car thief will not be able to steal in this way. Other advantages of the invention will be indicated later.

US patent A-4463340 published in 1984 discloses a control system for a vehicle in which signals are transmitted via the power line. The circuit includes a battery supply, an ignition switch and components to be energised. Transmitting means are

provided together with code receiving means but the components are arranged in an ineffective way.

According to the invention in one aspect there is provided an electric circuit comprising: direct current supply means; at least one component; a circuit switch; code transmitting means; code receiving means arranged on receipt of an authorised code to pass electric current to pass to the component, the code transmitting means being located between the circuit switch and the component and arranged to transmit the code through the circuit to the code receiving means.

With an arrangement as defined not only is the immobilising device concealed but one can provide higher electrical currents through low current wires without undue consumption of power supply.

Preferably the circuit forms part of the wiring harness of a vehicle. The nature of the harness will vary according to the requirement of the vehicle, e.g. a car, motor bike, motor boat; aeroplane; or the like. Typically the harness comprises sheathed conductors defining the circuit and the leads of the code receiving means are typically connected to the conductors in such a way as to become an integral part of the circuit; in other words it is a feature of this invention that it is unnecessary to have extra signal lines just for the immobilising means. Because the receiver is directly incorporated in the wiring harness and not necessarily adjacent the respective component the thief cannot easily locate the code receiver or which component it controls.

In a much preferred variant of the invention the code receiver is miniaturised and disposed within the housing of a component of the vehicle. This concealment of the code receiver within a component will further reduce the risk of a theft.

In another variation the code receiver controlling a component may be incorporated in the circuit at a location remote from the component making identification by a thief more difficult.

Preferably a number of code receivers are distributed about the circuit, each programmed to receive the authorisation code. Preferably each receiver is associated with an individual vehicle component such as an electrical fuel pump, starter motor, gear mechanism, carburettor, distributor or the like; more than one code receiver may be associated with one component. Component assemblies provided with a miniaturised code-receiver at the manufacturing stage could be purchased pre-assembled Off-the-shelf as unprogrammed units. The original vehicle component assembly could therefore be removed and substituted with such, and code-programmed accordingly (code-programming in this way could, → r example, be achieved by the vehicle owner supplying the security-coded key, if used, for the purpose of programming the key-derived security-code into the programmable memory of the device to be installed, using suitable equipment). The code receivers are arranged to sense and accept only a predetermined code which they have been programmed electrically, electronically or by hardware to receive. The security code can be sourced from a programmed key (connectable to the data transmitter prior to actuation by the vehicle ignition key), a keypad or a combination of Key and keypad or by the data transmitter.

Preferably the code is a large binary code which allows many permutations. The code may be transmitted in a variety of forms. Frequency shift keying (FSK) is one. The FSK technique discriminates the required signal information in the presence of any high-level electrical noise from other sources following energisation of the vehicle ignition supply. Pulse-coded modulation (PCM) or pulse-position modulation (PPM), are preferable to maximise the speed by which a security-code may be transmitted and received.

The code transmitter includes a sink switch, preferably in the form of a power transistor across the ignition supply, and preferably includes a transition detector to detect the transitions in a code signal having them.

The security-code may be retained in a code-receiver in a variety of ways. Electronic programming of PROM, EPROM or EEPROM memory devices are preferred. The use of such devices allows codes to be easily down-loaded with the aid of suitable programming equipment, and additionally facilitating subsequent re- programming if EPROM or EEPROM is employed for this purpose. Alternative, technically simpler methods involve either electrical or hardware programming principles in defining an electrical code-pattern, such as:

Electrically blowing fuses or printed circuit tracking. The systematic insertion, or removal, of wire links. Manual switch setting of a multiple-switch arrangement

Preferably the code receiving means includes a set time device arranged so that there is a short time period in which the receiving means can accept a code after which it will be disabled or go inactive for a time period. This will prevent a thief from trying repeatedly until he identifies the predetermined code, (of which there are potentially billions of permutations).

In another aspect of the invention there is provided an electric circuit in which the code signals are pulse modulated, preferably pulse position modulated. Accordingly in this aspect the invention provides an electric circuit comprising: direct current supply means; at least one component; a circuit switch; means for transmitting a pulse modulated code; code receiving means arranged on receipt of an authorised pulse modulated code to pass direct current to the component, the code transmitting

means being arranged to transmit the code through the circuit to the code receiving means.

In another aspect the invention provides an electric circuit comprising: direct current supply means; at least one component; a circuit switch; means for transmitting a code; code receiving means arranged on receipt of an authorised code to pass electric current to the component, the code transmitting means being arranged to transmit the code through the circuit and the code receiving means being arranged to receive the code through the circuit, and wherein a component control switch is associated with the code receiving means for the or each component and the component control switch is arranged to be normally closed to allow electric current to reach the component but to be open when the circuit switch is first closed until the code receiving means has accepted the transmitted code whereupon the component control switch closes, whereby the code receiving means consumes electric current only when the transmitted code is being verified by the code receiving means.

In a much preferred feature a component control switch is associated with the code receiving means for a component and the component control switch is normally closed to allow current to reach the component. When the ignition is actuated, the component control switch is opened to prevent flow of current to the component until the associated code receiving means has accepted the transmitted code. Preferably the component control switch is a bistable relay, transistor, thyristor or the like.

In an alternative construction the harness may include dedicated signal cables in order to conduct code data from code-transmitting means included in the circuit to the code receiving means.

According to the invention in yet another aspect there is provided a method of activating a component in an electric circuit comprising direct current supply means, a circuit switch, at least one component, code transmitting means and code receiving means, the code transmitting means being located between the circuit switch and the component, the method, comprising:

transmitting a code through the circuit to the code receiving means;

comparing the code in the code receiving means with a predetermined code; and

enabling the component if the received code is the predetermined code.

In a specific aspect the invention provides a vehicle including a battery power supply in circuit with an ignition system for igniting fuel and so mobilising the vehicle, the circuit being the loom of wiring supplying electric current to electrically controlled components and including an ignition switch and incorporating security immobilising apparatus comprising code transmitting means and code receiving means to receive a transmitted code from the code transmitting means and means to verify the transmitted code and complete the circuit in the case of an authorised code only, the code transmitting means being located between the ignition switch and the components, the code receiving means being associated with one or more components incorporated in the circuit.

In order that the invention may be well understood it will now be described by way of example only with reference to the accompanying diagrammatic drawings, in which

Figure 1 is a schematic diagram of a vehicle supply loom, including a code data transmitter and code data receiver;

Figure 2 is a block diagram of the code transmitter shown in Figure 1 ;

Figure 3 is a block diagram of the code data receiver shown in Figure 1 ;

Figure 4 shows a signal wave forms associated with the code;

Figure 5 is a circuit diagram showing the configuration for a starter motor solenoid control; and

Figure 6 is a circuit diagram showing the configuration of a fuel pump control.

In the schematic circuit shown in Figure 1 , the battery 1 of a vehicle supplies direct current into a wire loom or harness L via an ignition switch 2. The loom consists of the usual colour coded sheathed conductors providing direct current to the components. The circuit includes a code transmitter circuit 3 associated with the ignition switch 2, a fuse box 4 and the motor of a fuel pump 5 shown as a representative component, connected to which is a code receiver 6. Other circuits pass through the fuse box 4 to other components, not shown.

The code transmitter circuit 3 comprises a hard wired multi-pole key 10 which supplies parallel data to a parallel-to-series converter 1 1 from which it is passed to a logic transition detector and drive interface circuit 12 (shown in detail in Figure 2). This is connected in turn to a transistor device 13 connected across the supply from the switch 2 and to ground. The interface circuit 12 consists of an input line 14 leading to one input line 25 of an X-OR gate 16. The other input line 17 of the gate 16 is capacity coupled to voltage substrate Vss and the two input lines 15,17 are bridged by a resistor 18. The output from the X-OR gate 16 leads to a drive

amplifier 19 which leads to a current switching transistor 20, one side of which is connected to the ignition switch 2 and the other to a resistor 21 and then to ground.

The circuit of the code receiver 6 is shown in Figure 3. The line of the loom from the ignition switch key 2 includes a capacitor 22 and the circuit includes a signal amplifier 23 leading to a- signal squaring inverter 24 to redefine the signals and then to a divide-by-two counter 25 and then to a code comparator 26 in which the received code is compared to a preprogrammed reference code 27 which is ground to Vss. The output from the code comparator 26 leads to a latch 28 controlled by a "set enable" timer circuit 29 and then to the relay drive transistor 30 one side of which is ground to Vss and the other leads to one side of a relay device 31. The relay device is a bistable switch including a coil 32 and having a normally open contact 33, a change over contact 34 and a normally closed contact 35. The output line from the relay device 31 is tapped into the line leading to the fuel pump motor 5. Because, as shown in Figure 1 , the data receiver 6 interrupts the line of the loom L beyond the fuse box 4 and leading to the pump 5 and is connected to ground (as is the pump motor 5), the circuit is complete when the contacts are closed, as is normally the case.

When the ignition switch 2 goes high the coil 32 of the relay 31 is activated and the normally closed contacts 34,35 separate so immobilising the component 5 until the received code is verified against the preprogrammed code following which (if the comparison is positive), the relay is deactivated again and the movable change over contact moves to the closed condition. The "set enable" timer circuit controls the time period in which the code receiver is active to receive a code following which, if the wrong code is transmitted, it goes inactive for a predetermined period. This arrangement ensures security. In addition the only period in which the code receiver 6 consumes electric current is during the comparison period when the

normally closed contacts of the relay are open. The receiver 6 therefore does not drain current from the battery supply.

The transmitter circuit 3 is connected to the switched ignition supply so that the outputted ripple-pulse signals may be conducted through the main fuse-box 4 following the closure of key-switch 2 (thereby allowing the data-transmitter 3 to commence transmission). The resultant induced ripple-pulses, (the paths of which are identified by arrows in Figure 4), pass through the fuse box 4 and, in addition to the four unidentified fused supplies shown, are conducted to the data-receiver 6 connected to the vehicle petrol pump 5.

The code signals are shown in Figure 4. At stage A, from the 'off' state, the ignition supply circuit is energised following the closure of the vehicle ignition key-switch 2 to provide a data stream as a step B. At stage C, the transition detector 12 within the data-transmitter circuit 3, generates a marker pulse for each and every logic level transition of the data stream. Each marker pulse closes the current 'sink' switch 20 in step C thereby generating inverse supply ripple pulses on the ignition supply circuit as in step D. The pulses are amplified by amplifier 23 and then inverted (squared) by the inverter 24 in the receiver to re-constitute the marker pulses in step C. As shown in step F a 'divide-by-two' counter 25 re-constitutes the original data stream from step B by acting upon the leading edge of the detected marker pulses in step E. The recovered security-code at step F is compared to that with which the data-receiver 6 has been pre-programmed in the comparator 26, and then the relay control circuit de-energises the relay coil 32 and latches to maintain this condition whilst the ignition supply is maintained. By controlling the relay so that high voltage is needed to actuate it, one guards against the risk that a low battery voltage would cause undesirable vehicle immobilisation to take place.

94/16924

In the embodiment of Figure 5, there is shown a 4 wire configuration for a starter motor solenoid control and in accordance with this invention. The line to the relay from the switched ignition supply leads to the relay and the contacts are connected to the starter switch and the starter solenoid. In the embodiment of Figure 6, there is shown a three terminal configuration for a fuel pump control in accordance with this invention where the normally closed contact leads to the supply to the fuel pump and there is a link between the switched ignition supply and the open contact.

Other features and advantages of the invention are that there is no need for dedicated wiring. (If required or advantageous dedicated lines may be included in the loom). The code is very secure as the network or circuit is closed and therefore neither radiating nor responding to wireless transmissions. The system is versatile because the quantity of code receivers may be varied.

The invention is not limited to the embodiment shown. The vehicle may be a motor bike; it is of particular value in that context because zero current is consumed when the circuit is inactive. Other vehicles may be trucks, trailers, aircraft, snow mobiles. The component may be a vehicle accessory, e.g. a car radio or stereo system. The connections of the receiver to the loom may be made by twist on wire wrap connectors. The component control switch may be arranged to immobilise components other than those mentioned, e.g. an electromechanical fuel valve, an accessory and/or to sound an alarm. A gain control feature, preferably automatic, may be included to better define the transmitted and received code signal thereby improving the ability to discriminate against high level supply noise.