|
(11) | EP 0 356 260 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Control circuit |
| (57) The control circuit, to be connected between a master switch and an electrical circuit,
responds to first and second events. On occurrence of the first event when a first
device (13) of the control circuit is non-conductive and the master switch is closed,
the first device is made conductive through the agency of second and third devices
of the control circuit if and only if the control switch (14) of the control circuit
is closed within a predetermined period. |
[0002] The invention has application to an electrical circuit which is controlled from a master switch and relates to the provision of a control circuit between the master switch and the electrical circuit to resist the possibility of unauthorised activation of the electrical circuit.
[0003] In one form the invention resides in a control circuit for an electrical circuit comprising an input adapted to be connected to a master switch for the electrical circuit and an output adapted to be connected to the electrical circuit said control circuit providing a connection between the input and output having a first device which can be selectively rendered conductive a second device associated with the first device to provide a condition at which said first device is rendered conductive a third device associated with both the first and second devices, a control switch connected between the second device and first device whereby on the occurrence of a first event whereby the first device is non-conductive and the master switch is closed and the control switch is closed subsequently within a first period of time the second device is able to provide the condition to render the first device conductive and on the occurrence of a separate second event whereby the first device is non-conductive and the master switch is closed and the control switch is subsequently closed after the expiration of the first period of time the third device interacts with the second device to prevent creation of the condition.
[0004] According to a preferred feature of the invention said third device is associated with the first device whereby on the occurrence of a third event at which the first device is conductive and said master switch is opened the third device will provide a second condition to maintain the first device conductive for a second period of time.
[0005] According to a further preferred feature of the invention the third device interacts with the second device on the occurrence of a fourth event when subsequent to the occurrence of the second event the master switch is closed the third device prevents creation of said condition by the second device.
[0006] According to a further preferred feature of the invention said second device is associated with a fourth device connected to the opposite side of said control switch from said second device whereby on the occurrence of a fifth event when the control switch is closed prior to the closure of the master switch the second device is prevented from providing said condition.
[0007] According to a preferred feature of the invention a control circuit as claimed at any one of the preceding claims wherein said first device comprises a controlled rectifier and said second device is connected to the gate of the control rectifier.
[0008] According to a preferred feature of the invention said first device is selected from one of the following:-
a silicon controlled rectifier;
a triac; or
a silicon controlled switch.
[0009] The invention will be more fully understood in the light of the following description of one specific embodiment.
[0010] The description is made with reference to the accompanying drawings which is a schematic circuit diagram of the embodiment.
[0011] The embodiment relates to a security control circuit for use with an automobile to prevent unauthorised use of the motor vehicle.
[0012] The embodiment comprises a device which is to be connected between the ignition switch of the vehicle and the ignition circuit of the engine of the vehicle. The device comprises an input terminal which is to be connected to the ignition switch an output terminal which is to be connected to the ignition circuit and an earth terminal. Connection between the input and the output is effected by a pair of relay terminals 11 which are controlled from a relay coil 12 which is part of the control circuit. The current flow through the relay coil 12 is controlled by a silicon controlled rectifier 13 which is connected in series with the relay between the input terminal and the earth terminal. A stabilising circuit comprising a resistor R8 and a capacitor C3 is connected across the gate and cathode of silicon controlled rectifier 13. The gate of the silicon controlled rectifier 13 is connected via one end of the stabilising circuit to a control switch 14 and a first capacitative circuit located on the opposite side of the control switch 14 from the silicon control rectifier 13. The first capacitative circuit comprises a capacitor C2 and a resistor R6 which are connected in parallel to earth. In addition the input terminal is connected via a diode D1 and resistor R2 to a second capacitative circuit which comprises a capacitor C1 and a resistor R4 connected in a parallel array in series with the first capacitative circuit to provide a junction 15 which provides the connection between the control switch 14 and both capacitative circuits. The first capacitative circuit is connected to the relay 12 through a diode D2 and resistor R3. In addition the first capacitative circuit which comprises capacitor C2 and R6 is associated with a resistor R7 to the opposite side of the control switch from the first capacitative circuit whereby the resistor R7 and first capacitative circuit are in parallel on closure of the switch 14. The relative value of the capacitor C2 of the first capacitative circuit and the value of the capacitor C1 of the second capacitative circuit are such that on closure of the ignition switch the first capacitative circuit is substantially fully charged almost instantaneously whereas the second capacitative circuit is nearly completely charged after the expiration of a first period of time (e.g. 10 seconds). In this embodiment the capacitance of capacitor C1 is approximately 50 times the capacitance of C2. While the resistance of resistor R4 is approximately 10 times the resistance of R6, the resistance at which is approximately four times the resistance of R6. According to a specific example of the embodiment R2 has resistance of 1 k ohms, R4 has a resistance of 47 k ohms, R2 has a resistance of 3.9 k ohms, R7 has a resistance of 120 ohms; C1 has a capacitance of 2200 uf and C2 has a capacitance of 47 uf.
[0013] The control switch 14 comprises a magnetically sensitive switch which will close on a magnetic field being brought into close proximity to the switch whereby the magnetic field may be provided by a small permanent magnet. In addition the circuit is provided with two terminals A and B which may be located to either side of the control switch 14 and which may be used with any other form of switching device if desired.
[0014] The operation of the control circuit is such that on closure of the ignition switch there is no current flow through the relay coil 12 since the silicon controlled rectifier 13 is non-conductive.
[0015] Immediately on closure of the ignition switch both capacitors begin to charge, but because of C1 has a much greater capacitance in value, C2 charges almost instanteously to a voltage nearly as high as the voltage supplied from the ignition switch. It is able to do this since C1 presents a low impedance initially in the circuit. As C1 charges, the voltage across C2 falls (ie at junction 15).
[0016] When the voltage across C2 drops to a particular value the silicon controlled rectifier 13 cannot be triggered by closure of control switch 14. The period of time during which the voltage across C2 reaches the particular value at which the silicon controlled rectifier 13 cannot be rendered conductive constitutes the first period of time. The second capacitative circuit incorporating C1 is almost fully charged at this point in time. Beyond this point in time the voltages across C1 and C2 reach an equilibrium. Beyond the expiration of the first period of time it is not possible to trigger the silicon controlled rectifier 13 as the voltage across C2 is too low.
[0017] On the closure of the ignition switch, and closure of the control switch 14 during the first period of time the voltage condition which then exists at the junction 15 causes activation of the gate of the silicon controlled rectifier 13 to render it conductive. As a result current is caused to flow through the relay coil 12 which causes closure of the contacts 11 of the relay and thus activation of the ignition circuit. The stabilising circuit which comprises capacitor C3 and resistor R8 in parallel serves to stabilise the voltage at the gate of silicon controlled rectifier 13 to prevent inadvertent activation of the gate and thus false triggering of the silicon controlled rectifier 13 to the conductive state. Once triggered the silicon controlled rectifier 13 will remain conductive while the ignition switch remains closed irrespective of whether the control switch 14 remains closed. If the control switch 14 is not closed within the first period of time the second capacitative circuit and in particular capacitor C1 will be fully charged which results in a voltage being expressed at the junction 15 which is insufficient to activate the gate of the silicon controlled rectifier 13. As a result the control circuit provides a security device for the operation of the ignition switch whereby on the ignition switch being closed the control switch 14 must also be closed within the predetermined first period of time.
[0018] When the ignition switch is turned OFF after the controlled rectifier has been rendered conductive the second capacitative circuit will be caused to discharge through diode D2, resistor R3, the relay coil 12, and the silicon controlled rectifier 13 to maintain a holding current in the silicon controlled rectifier 13 during that discharge of the capacitor C1 which will take place over a second period of time. On the expiration of that second period of time when the capacitor C1 is substantially discharged the silicon controlled rectifier 13 will be rendered non-conductive. As a result in the event that the ignition switch is turned ON during the second period of time after having been turned to the OFF position a direct connection is provided to the ignition circuit of the vehicle. This provides a security where the ignition switch is turned OFF inadvertently or its turned OFF subsequent to the engine having stalled and it being necessary to switch the ignition switch to the OFF position before re-starting the engine.
[0019] In the event that the ignition switch has been turned to the ON position and the control switch 14 is not closed until after the expiration of the first period of time it is not possible to again attempt to start the vehicle until the ignition switch is turned off and the second capacitative circuit C1 has been allowed to discharge through resistor R4 which will take place over a third period of time. Due to the high restrainer of the resistor R4 this is the only time at which it becomes relevant to the action of the circuit. As a result in the event of a unsuccessful attempt to start the engine it is not possible to again attempt to start the engine until after the expiration of the third period of time.
[0020] In the event that the control switch 14 is closed before the ignition switch is turned to the ON position and is held closed the junction 15 assumes a voltage which is formed by resistors R2, R6 and R7, R7 being significantly low in resistance to render R6 irrelevant. As a result the voltage present at junction 15 is not sufficient to enable activation of the gate of the silicon control rectifier 13. As a result it is not possible to close the control switch 14, hold at closed and then subsequently close the ignition switch to start the engine.
[0021] The control switch 14 which comprises a magnetically sensitive switch is intended to be located at some "hidden" location on the vehicle which is known only to authorised persons and which is hidden behind a wall formed of a material which does not provide a magnetic shield. On an authorised person activating the ignition switch and locating a magnet adjacent the control switch 14 within the first period of time such that the control switch 14 is closed the ignition circuit and associated circuits of the vehicle can be activated. It is difficult for any authorised person to attempt to by-pass the security device which is provided by the control circuit which ensures that the control circuit is always active. It should be appreciated that the output of the circuit may be associated with any other form of electrical circuit in the vehicle for activation of those other circuits in accordance with a similar procedure to that set out above.
1. A control circuit for an electrical circuit comprising an input adapted to be connected
to a master switch for the electrical circuit and an output adapted to be connected
to the electrical circuit said control circuit providing a connection between the
input and output having a first device which can be selectively rendered conductive,
a second device associated with the first device to provide a condition at which said
first device is rendered conductive and a third device associated with the first and
second device, a control switch connected between the second device and the first
device whereby on the occurrence of a first event whereby the first device is non-conductive
and the master switch is closed and the control switch is closed subsequently within
a first period of time the second device is able to provide the condition to render
the first device conductive and if on the occurrence of a second event whereby the
first device is non-conductive and the master switch is closed and a control switch
is subsequently closed after expiration of the first period of time the third device
interacts with the second device to prevent creation of the condition.
2. A control circuit as claimed in claim 1 wherein said third device is associated
with the first device whereby on the occurrence of a third event at which the first
device is conductive and the master switch is open the third device will provide a
second condition to maintain the first device conductive for a second period of time.
3. A control circuit as claimed in claim 1 or 2 wherein the third device interacts
with the second device on the occurrence of a fourth event when subsequent to the
occurrence of the second event the master switch is closed the third device prevents
creation of said condition by the second device to render the first device conductive.
4. A control circuit as claimed at claim 1, 2 or 3 wherein said second device is associated
with a fourth device connected to the opposite side of said control switch whereby
on the occurrence of a fifth event when the control switch is closed prior to the
master switch the second device is prevent from providing said condition.
5. A control circuit as claimed at any one of the preceding claims wherein said first
device comprises a controlled rectifier and said second device is connected to the
gate of the controlled rectifier.
6. A control circuit as claimed at claim 5 wherein the second device comprises a first
capacitative circuit and the third device comprises a second capacitative circuit
said second and third devices being connected in series, the junction between the
first and second devices being connected to said gate whereby the charging of said
second device is substantially instantaneous on closure of the master switch and the
charging of the third device extends over the first period of time after closing the
master switch.
7. A control circuit as claimed at claim 6 as dependent on claim 2 whereby on the
occurrence of the third event the discharge of the third device extends over the second
period of time.
8. A control circuit as claimed at claim 6 and 7 as dependent on claim 3 whereby on
the occurrence of the fourth event the discharge of the third device extends over
the fourth period of time.
9. A control circuit as claimed at any one of claim 6, 7 or 8 as dependent on claim
4 whereby said fourth device comprises a resistive element in parallel with the second
device which on occurrence of the fifth event prevents the charging of the second
device on closure of the master switch.
10. A control circuit as claimed at any one of the preceding claims wherein a voltage
stabilising circuit is provided between the first and second devices.
11. A control circuit as claimed in any one of the preceding claims wherein the control
switch is sensitive to the presence of a magnetic field.
12. A control circuit as claimed at claim 11 whereby said control switch is located
in a hidden location on a housing associated with the electrical circuit.
13. A control circuit as claimed at any one of the preceding claims wherein said connection
between the input and the electrical circuit accommodates a relay coil which closes
set of relay contacts on said first device being rendered conductive to provide the
connection between the input and output.
14. A control circuit as claimed at any one of the preceding claims wherein said first
device is selected from one of the following:-
a silicon controlled rectifier;
a triac;
a silicon controlled switch.
a silicon controlled rectifier;
a triac;
a silicon controlled switch.
15. A control circuit as claimed at any one of the preceding claims wherein the electrical
circuit comprise the ignition circuit of a motor vehicle.