IES71512B2 - A security circuit and a security sensor - Google Patents

Abstract

A security circuit (1) comprises a central control panel (11) and a plurality of security sensors (2,3,4) of different type connected in series in a loop circuit (5) so that by analysing the current drawn by the loop circuit (5) on a constant voltage being applied across input terminals (8,9) of the loop circuit (5), a determination of which type of three different types of sensor elements (16,17,18) of the security sensors (2,3,4) have been activated. Additionally, each security sensor (2,3,4) comprises a housing having a base (28) and a cover (29) within which the sensor elements (16,17,18) are located. The sensor elements (16,17,18) of the security sensors (2,3,4) are connected into the loop circuit (5) through respective tamper indicating elements, which are provided by plug and socket assembly (21). A plug (39) of the assembly (21) is secured in the cover (29) while sockets (41) of the assembly (21) are located in a terminal block (38) secured to the base (28). Thus, separation of the cover (29) from the base (28) disengages the plug socket assembly (21) which instantly opens the loop circuit (5) and indicates tampering with a security sensor (2,3,4).

Description

The present invention relates to apparatus which comprises an electrical or electronic device or circuit for connecting into a security circuit, for example, a security sensor or a central control panel. The invention also relates to the security circuit comprising one or more of the apparatus, and the invention also relates to a security sensor for use in the security circuit.

Security systems of the type which are typically used in buildings, such as for example, domestic dwellings, offices, factories and the like comprise a central control panel to which a plurality of one or more similar or different types of security sensors are connected by a security circuit. The security sensors, in general, are bi-state devices, for example, monostable devices such as inertia sensors, and bi-stable devices, such as, for example, reed switches. In general, security sensors which comprise an inertia sensor are normally mounted on a window, window frame, door or door frame for detecting a shock or vibration induced in the window or door resulting from an attempted break-in. Security sensors which comprise reed switches, on the other hand, are typically mounted on a window, window frame, door or door frame for coS7 1 512 1 operation with an associated magnet located on the other of the window, window frame or door and door frame, such that on a window or door being opened the reed switch changes state. In general, the reed switch is normally held closed by the associated magnet when the magnet is in close proximity to the reed switch.

Thus, on the window or door being opened the reed switch is opened, thereby indicating an unauthorised entry. Other types of security sensors comprise infrared detectors, which in general, include a bistate switch which changes state on an infrared sensor within the detector detecting a change in heat resulting from movement of an individual or the like in the area being scanned.

However, all such security sensors are vulnerable to tampering. Such security sensors, in general, comprise a housing, which is typically a two part housing having a base and a cover. When assembled the base and cover together define a hollow interior region within which a sensor element is located. By separating the two parts of the housing the sensor element located within the housing may be tampered with in such a way that it r fails to detect a break-in or an attempted break-in.

In order to detect such tampering, a tamper indicating switch is provided, and in general, is located within the hollow interior region of the housing. Such tamper indicating switches, typically, are bi-state switches and are normally held closed while the cover and base are securely attached to each other. However, on separation of the cover and the base, the tamper indicating switch opens thereby indicating an attempt to tamper.

While such tamper indicating switches achieve the objective of indicating tampering, they suffer from a number of disadvantages. Firstly, a tamper indicating switch is an extra component which has to be provided in the security sensor. Secondly, the tamper indicating switches of the security sensors in a security circuit have to be wired by a separate two wire circuit to the central control panel. This is required to allow for separate monitoring of the tamper indicating switches. Indeed, it is desirable that each circuit which connects security sensors to the central control panel should only comprise similar type devices. In order words, only reed switches of security sensors should be connected on one circuit, only inertia sensors, namely, vibration sensing devices should be connected on another circuit, and only tamper indicating switches should be connected on a further circuit. These circuits, in general, are referred to as zones in a security system. Thus, each zone should only comprise similar type sensor elements.

Accordingly, in cases where a security sensor comprises three different types of sensor elements, namely, an inertia sensor, a reed switch and a tamper indicating switch, such a security sensor should be connected into three respective zones by three separate circuits of the security system. This therefore requires the provision of three pairs of wires, in other words, six wires for connecting such a security sensor to a central control panel. The inertia sensor is connected by two wires with a plurality of other inertia sensors, all of which are connected in series in the circuit. A second pair of wires connects the reed switch into a separate circuit for reed switches, while a third pair of wires connects the tamper indicating switch into a separate circuit for tamper indicating switches. Accordingly, it will be appreciated that the wiring of such security sensors to a central control panel is complicated, time consuming, and is particularly prone to installation errors occurring. Additionally, because of the number of circuits required, the cost of connecting such security sensors to a central control panel is relatively high.

In general, central control panels for controlling a security circuit comprise a two part housing within which a control circuit for powering and monitoring the security circuit is located. In general, one part of the housing forms a base, and the other a cover. Generally, a tamper indicating element, such as a tamper indicating switch is located within the housing for indicating tampering with the housing. The tamper indicating switch, typically is mounted on the base or the cover, and on an attempt being made to separate the cover from the base the tamper indicating switch goes on open circuit, thus, indicating tampering. The tamper indicating switch is connected into the control circuit, and in general, is connected into the zone into which tamper indicating switches of the security circuit are connected. In other cases, the tamper indicating switch of the central control panel is connected into a separate zone of the control circuit.

This, as discussed above, is inconvenient and unsatisfactory.

There is therefore a need for a security circuit which comprises a plurality of security sensors whereby the security sensors, irrespective of the number of different types of sensor elements within reason contained in the security sensors can be connected to a central control panel by a single two wire circuit, and which facilitates identification of the type of sensor element or tamper indicating element which has changed state. There is also a need for apparatus comprising an electrical or electronic device for connecting into a security circuit which overcomes the problems of known apparatus, for example, a central control panel, a security sensor or any other such apparatus which is suitable for connecting into such a two wire security circuit.

The present invention is directed towards providing such a security circuit, an apparatus and a security sensor.

According to the invention there is provided an apparatus comprising an electrical or electronic device for connecting into a security circuit, the apparatus comprising a housing formed in two parts which are securable together to form a hollow interior region for housing the device, a tamper indicating element being located in the two part housing, the tamper indicating element being responsive to separation of the two parts of the housing and changing state on the two parts of the housing being separated for indicating tampering with the housing, the tamper indicating element being connected with the device so that only two connections are required to the housing for connecting both the « device and the tamper indicating element into the security circuit.

In one embodiment of the invention the electrical or electronic device and the tamper indicating element are connected in series.

Preferably, the tamper indicating element comprises a first electrically conductive element, and a second electrically conductive element, the first and second electrically conductive elements being in electrical contact in a first state of the tamper indicating element for connecting the electrical or electronic device into the security circuit, and being disconnectable into an open circuit state in a second state of the tamper indicating element for disconnecting the electrical or electronic device from the security circuit for indicating tampering with the housing.

Advantageously, the first electrically conductive element is located in one of the parts of the housing, and the second electrically conductive element is located in the other of the parts of the housing so that when the two parts of the housing are secured together the tamper indicating element is in the first state, and when the two parts of the housing are separated the tamper indicating element is in the second state.

Preferably, two first electrically conductive elements and two second electrically conductive elements are provided, respective first and second electrically conductive elements being arranged in pairs of electrically conductive elements, each pair comprising one first and one second electrically conductive element, the pairs of electrically conductive elements being connected in series with the electrical or electronic device for connecting the electrical or electronic device into the security circuit.

Advantageously, one of the pairs of first and second electrically conductive elements is connected to one of the two connections for connecting the electrical or electronic device and the tamper indicating element into the security circuit, and the other of the pairs of first and second electrically conductive elements is connected to the other of the two connections for connecting the electrical or electronic device into the security circuit.

In one embodiment of the invention the first electrically conductive elements are located on the same part of the housing.

Ideally, each first electrically conductive element is provided by an electrically conductive socket, and each second electrically conductive element is provided by an electrically conductive contact pin for engaging the corresponding electrically conductive socket in the first state.

Preferably, each electrically conductive socket is 5 formed in a terminal block, the terminal block having a connecting means electrically connected to a corresponding one of the electrically conductive sockets, each connecting means forming a respective one of the two connections for connecting the electrical or electronic device and the tamper indicating element into the security circuit.

In one embodiment of the invention the terminal block comprises the two electrically conductive sockets, and the two corresponding connecting means.

In general, one of the parts of the housing forms a base, and the other part of the housing forms a cover. Preferably, each first electrically conductive element is located on the base, and each second electrically conductive element is located in the cover.

Advantageously, the electrical or electronic device is mounted in the cover.

In one aspect of the invention the electrical or electronic device is a control circuit for controlling the operation of the security circuit, and for monitoring the security circuit for detecting a change in state in the security circuit, and/or in the tamper indicating element of the apparatus, and in which case, the apparatus is a control panel for controlling and monitoring the security circuit.

In one aspect of the invention the electrical or electronic device is a sensor element. In another aspect of the invention at least two sensor elements are located in the hollow interior region.

In a further aspect of the invention each sensor element in the hollow interior region is different to the other sensor elements therein.

Preferably, the sensor elements are connected in 15 series.

Ideally, the one of the sensor elements is connected in series with one of the pairs of first and second electrically conductive elements, and the other sensor element is connected in series with the other of the pairs of first and second electrically conductive elements.

In another aspect of the invention the sensor element comprises an inertia sensor.

In a still further aspect of the invention the sensor element comprises a reed switch, and preferably, an impedance means is connected in parallel with the reed switch for facilitating differentiation between a signal caused by the reed switch going into an open circuit state and a signal caused by the tamper indicating element going into the second state.

In another aspect of the invention the sensor element 10 comprises an infrared sensor.

In one embodiment of the invention the apparatus is a security sensor.

Additionally the invention provides a security sensor for connecting into a security circuit, the security sensor comprising a housing formed in two parts which are securable together to form a hollow interior region, a sensor element located in the hollow interior region for connecting into the security circuit, a tamper indicating element located in the two part housing, the tamper indicating element being responsive to separation of the two parts of the housing and changing state on the two parts of the housing being separated for indicating tampering with the housing, the tamper indicating element being connected with the sensor element so that only two connections to the security sensor are required for connecting the security sensor into the security circuit.

Preferably, the sensor element and the tamper indicating element are connected in series.

Advantageously, the tamper indicating element comprises a first electrically conductive element, and a second electrically conductive element, the first and second electrically conductive elements being in electrical contact in a first state of the tamper indicating element for connecting the sensor element into the security circuit, and being disconnectable into an open circuit state in a second state of the tamper indicating element for disconnecting the sensor element from the security circuit for indicating tampering with the housing.

Preferably, the first electrically conductive element is located in one of the parts of the housing, and the second electrically conductive element is located in the other of the parts of the housing so that when the two parts of the housing are secured together the tamper indicating element is in the first state, and when the two parts of the housing are separated the tamper indicating element is in the second state.

Ideally, two first electrically conductive elements and two second electrically conductive elements are provided, respective first and second electrically conductive elements being arranged in pairs of electrically conductive elements, each pair comprising one first and one second electrically conductive element, the pairs of electrically conductive elements being connected in series with the sensor element for connecting the sensor element into the security circuit.

Advantageously, one of the pairs of first and second electrically conductive elements is connected to one of the two connections for connecting the security sensor into the security circuit, and the other of the pairs of first and second electrically conductive elements is connected to the other of the two connections for connecting the security sensor into the security circuit.

Ideally, the first electrically conductive elements are located on the same part of the housing.

Preferably, each first electrically conductive element is provided by an electrically conductive socket, and each second electrically conductive element is provided by an electrically conductive contact pin for engaging the corresponding electrically conductive socket in the first state.

In one embodiment of the invention each electrically conductive socket is formed in a terminal block, the terminal block having a connecting means electrically connected to a corresponding one of the electrically conductive sockets, each connecting means forming a respective one of the two connections for connecting the security sensor into the security circuit. Preferably, the terminal block comprises the two electrically conductive sockets, and the two corresponding connecting means. in general, one of the parts of the housing forms a base, and the other part of the housing forms a cover. Preferably, each first electrically conductive element is located on the base, and each second electrically conductive element is located in the cover.

Advantageously, the sensor element is mounted in the cover.

In one aspect of the invention at least two sensor elements are located in the hollow interior region.

In another aspect of the invention each sensor element in the hollow interior region is different to the other sensor elements therein.

In a further aspect of the invention the sensor 5 elements are connected in series.

In a further aspect of the invention one of the sensor elements is connected in series with one of the pairs of first and second electrically conductive elements, and the other sensor element is connected in series with the other of the pairs of first and second electrically conductive elements.

In another aspect of the invention the sensor element comprises an inertia sensor. In a further aspect of the invention the sensor element comprises a reed switch, and preferably, an impedance means is connected in parallel with the reed switch for facilitating differentiation between a signal caused by the reed switch going into an open circuit state and a signal caused by the tamper indicating element going into the second state.

In another aspect of the invention the sensor element comprises an infrared sensor.

Further, the invention provides a security circuit comprising a single loop circuit having a pair of input terminals for connecting the loop circuit to a central control panel for monitoring the state of the loop circuit, and a plurality of security sensors according to the invention, the security sensors being connected in series by the loop circuit, each security sensor being connected into the loop circuit by its two connections so that a change in state of a tamper indicating element or a sensor element in any of the security sensors is detectable by the central control panel, and may be differentiated one from the other by type of element by the control panel.

In one aspect of the invention the sensor elements and the tamper indicating elements of the security sensors are connected in series in the loop circuit.

In another aspect of the invention the loop circuit goes into an open circuit state on any one of the tamper indicating elements being in the second state.

In a still further aspect of the invention the security circuit comprises the central control panel, the loop circuit being connected to the central control panel by the two input terminals, the central control panel comprising an analysing means for differentiating between a signal received from the loop circuit on a tamper indicating element changing state, and a signal received from the loop circuit on a sensor element changing state.

In one embodiment of the invention the analysing means differentiates between signals received from different types of the sensor elements.

In another embodiment of the invention the central control panel is provided by the apparatus according to the invention.

The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a block representation of a circuit diagram of a security circuit according to the invention, Fig. 2 is a circuit diagram of the security circuit of Fig. 1, Fig. 3 is a perspective view of a security sensor also according to the invention for use in the security circuit of Fig. 1, Fig. 4 is a perspective view of the security sensor of Fig. 3 with a portion of the security sensor illustrated in outline, Fig. 5 is a perspective view of a part of the security sensor of Fig. 3, Fig. 6 is a partly cutaway perspective view of another part of the security sensor of Fig. 3, and Fig. 7 is a cross-sectional end elevational view of the security sensor of Fig. 3.

Referring to the drawings there is illustrated a security circuit according to the invention indicated generally by the reference numeral 1. The security circuit 1 is suitable for installation in a building, such as, for example, a domestic dwelling house, office, factory or the like, and comprises a plurality of apparatus according to the invention which in this case comprise a plurality of security sensors which are indicated generally by the reference numerals 2, 3 and 4 for detecting unauthorised or attempted unauthorised intrusion into the building. The security sensors 2, 3 and 4 are connected in series in a loop circuit 5 formed by two wires 6 and 7 of the security circuit 1. The loop circuit 5 terminates in input terminals 8 and 9 which are connected to another apparatus also according to the invention, namely, a central control panel 11 for powering and monitoring the state of the loop circuit 5, and in turn, the sensors 2, 3 and 4.

The central control panel 11 is of the type which applies a continuous DC voltage signal across the input terminal 8 and 9 of the loop circuit 5, and monitors the current drawn by the loop circuit 5 for detecting a change in state in any of the security sensors 2, 3 and 4. This is described in more detail below.

Referring now in particular to Figs. 3 to 7 each security sensor 2, 3 and 4 comprises a housing 15 within which one or more bi-state sensor elements 16, and 18 are located. A tamper indicating element for indicating if the housing 15 of the security sensors 2, 3 and 4 is being tampered with comprises a plug socket assembly 21 which is also located in the housing 15, and as will be described below connects the sensor elements 16, 17 and 18 into the loop circuit 5. The central control panel 11 comprises a control circuit 22 illustrated in block representation in Figs. 1 and 2 for powering and monitoring the loop circuit 5. The control circuit 22 is located within a housing 26, which is also illustrated in block representation in Figs. 1 and 2. Although being significantly larger than, and being of shape different to the housing 15, the housing 26 in principle, is similar to the housing 15 of the security sensors 2, 3 and 4. A tamper indicating element, also provided by a plug socket assembly 21 which is located in the housing 26 connects the control circuit 22 through the two terminals 8 and 9 to the loop circuit 5. The housing 26 of the central control panel 11 is not illustrated or described in detail, however, its similarity in principle to the housing 15 of the security sensors 2, 3 and 4 is discussed in detail below. Before describing the housing 15 and the plug socket assembly 21 of the security sensors 2, 3 and 4 in detail, the sensors 16, 17 and 18 will first be described.

In this embodiment of the invention the security circuit 1 comprises three security sensors 2, each of which comprises a single sensor element 16 which is an inertia sensor of the type comprising a pair of electrically conductive supports 19 which support an inertia mass provided by an electrically conductive inertia bar 20. The supports 19 and the inertia bar 20 are illustrated in diagrammatic form only in Figs. 1 and 2. Such inertia sensor elements will be well known to those skilled in the art, and further description should not be required. The inertia bar 20 closes a circuit between the supports 19. On the inertia sensor element 16 being subjected to a shock or vibrations, the inertia bar 20 is caused to vibrate on the supports 19, thereby, rapidly and alternately opening and closing the loop circuit 5, and this is detected by the central control panel 11 as will be described below. Such shock or vibration may be caused by glass of a window being broken, a window or a door being jemmied, or by other such attempts to break into a building.

The security circuit 1 comprises two security sensors 3. Each security sensor 3 is also provided with a sensor element 16 which comprises an inertia sensor, however, as well as being provided with an inertia sensor element 16, each security sensor 3 is also provided with a sensor element 17, which comprises a reed switch 23. Such reed switches will be well known to those skilled in the art, and it is not intended to describe them in further detail. Each reed switch 23, is retained normally closed by an associated adjacent magnet 24, see Figs. 1 and 2. Reed switches 23 are typically used for determining when a door or window has been opened. The magnet 24, typically, is mounted on the door while the security sensor 3 comprising the reed switch 23 is mounted on the frame of the window or door adjacent the magnet 24. For so long as the reed switch 23 and magnet 24 are adjacent, the reed switch 23 is held closed. However, on the reed switch 23 and magnet 24 moving apart, the reed switch 23 opens. Each reed switch 23 is connected in series with its corresponding inertia sensor element 16. A resistor Rl is connected in parallel with each reed switch 23, and in series with the sensor element 16 so that in the event of a reed switch 23 opening electrical continuity is maintained through the loop circuit 5. The resistance values of the resistor Rl of the respective security sensors 3 are similar.

One security sensor 4 is provided, and the sensor element 18 of the security sensor 4 is what is commonly referred to as a panic button switch. It is a normally closed push button switch 25 which can readily easily be pushed to open in the event of an emergency. A resistor R2 is connected in parallel across the push button switch 25 for maintaining electrical continuity through the loop circuit 5 in the event of the switch 25 being opened. The value of the resistor R2 is different to the value of the resistors Rl to facilitate identification of the type of sensor element 17 or 18 which has gone into an open circuit state.

An end of line resistor R3 is provided in the loop circuit 5 to avoid a short circuit between the input terminals 8 and 9. The value of the end of line resistor R3 is also different to the values of the respective resistors Rl and R2, likewise, for facilitating identification of a sensor element 17 or 18 which has gone on open circuit.

The housing 15 of the security sensors 2, 3 and 4 are substantially similar and only the security sensor 2 will be described in detail with reference to Figs. 3 to 7. The housing 15 comprises two parts, namely, a base 28 and a cover 29, both of which are of injection moulded plastics material. A screw 30 through a bore 31 in the cover 29 which engages a threaded bore 32 of a post 33 extending from the base 28 secures the cover 29 and base 28 together. When secured together the base 28 and the cover 29 form a hollow interior region 35 within which the sensor element or elements of the sensor elements 16, 17 and 18 and plug and socket assembly 21 are located. In the case of the security sensor 2 the sensor element 16 and the plug and socket assembly 21 are located in the hollow interior region. The sensor elements 16, 17 and 18 and their associated resistors Rl and R2 are secured to and are located within the cover 29. A sub-housing 36 mounted in the cover 29 houses the one or more sensor elements and their associated resistors Rl and R2.

The plug and socket assembly 21 comprises a terminal block 38 of electrically insulating plastics material, which is mounted on the base 28 and a plug 39 which is mounted to and located within the cover 29. The terminal block 38 comprises six connecting means, namely, six electrically conductive connector elements 40, only two of which, namely, the connector elements 40a and 40b are used to form two connections for connecting each security sensor 2, 3 and 4 into the loop circuit 5 in series with the other security sensors 2, 3 and 4. Associated with each connector element 40 is a first electrically conductive element, namely, an electrically conductive socket 41 which is electrically connected to the corresponding connector element 40, and each socket 41 and its corresponding connector element 40 are electrically insulated from the other sockets 41 and connector elements 40. For convenience the sockets 41 which are electrically connected to the connector elements 40a and 40b are identified by the references 41a and 41b, respectively.

The plug 39 of each security sensor 2, 3 and 4 comprises a pair of second electrically conductive . elements, namely, electrically conductive contact pins 43a and 43b which are mounted in and extend from a plug base 44 of electrically insulating plastics material which is secured to the cover 29. The pins 43a and 43b are engagable with the corresponding sockets 41a and 41b, respectively, with electrically conductive engagement for connecting the sensor elements 16, 17 and 18 and their associated resistors R1 and R2 as the case may be into the loop 5 when the cover 29 and the base 28 are secured together. The contact pins 43a and 43b are electrically connected to the corresponding sensor element or elements of the sensor elements, 16, 17 and 18 of each security sensor 2, 3 and 4 by two electrical conductors 45a and 45b, respectively, which extend between the plug 39 and the sensor element 16, or 18 through the sub-housing 36 so that the respective corresponding sensor element or elements 16, 17 and 18 are connected in series with the respective corresponding contact pins 43a and 43b and the sockets 41a and 41b into the loop circuit 5.

In the case of the security sensors 2, the contact pins 43a and 43b, are electrically connected to the respective supports 19 of the inertia sensor of the sensor element 16 for connecting the sensor element 16 in series into the loop circuit 5. In the case of the security sensors 3, one contact pin 43a is electrically connected to one of the supports 19 of the inertia sensor of the sensor element 16, while the other contact pin 43b is electrically connected to one terminal of the reed switch 23 of the sensor element 17. The other terminal of the reed switch 23 is connected to the other support 19 of the sensor element 16 so that the contact pins 43a and 43Jb connect the sensor elements 16 and 17 in series into the loop circuit 5. In the case of the security sensor 4, the contact pins 43a and 432? are electrically connected to the terminals of the push button switch 25 of the sensor element 18 for connecting the push button switch 25 in series into the loop circuit 5.

The plug and socket assembly 21 is so arranged that as the cover 29 and the base 28 are being moved apart relative to each other on an attempt being made to disengage the cover 29 from the base 28, the contact pins 43 disengage the corresponding respective sockets 41, thus, providing an open circuit condition in the loop circuit 5. Such an open circuit condition is readily detectable by the central control panel 11, which thus determines that an attempt to tamper with one of the security sensor 2, 3 or 4 has occurred.

The housing 26 of the central control panel 11 is also of two part construction, having a base (not shown), and a cover (also not shown) which when securred together form a hollow interior region. The control circuit 22 is located in the hollow interior region and is mounted within the cover (not shown). The plug socket assembly 21 is identical to the plug socket assembly 21 of the housing 15 of the security sensors 2, 3 and 4 and is located in the hollow interior region of the housing 26. In the case of the housing 26 a pair of electrically conductive sockets 41a and 41b are provided in a terminal block 38, similar to the terminal block 38 of the security sensors 2, 3 and 4. The terminal block 38 is mounted on the base of the housing 26, and within the hollow interior region defined by the cover and the base. Connector elements, similar to the connector elements 40a and 40b are provided for receiving the input terminals 8 and 9 of the loop circuit 5. A plug 39, which is similar to the plug 39 of the security sensors 2, 3 and 4 is located in the hollow interior region and is mounted in the cover of the housing 26. The plug 39 comprises a plug base 44 and a pair of contact pins 43a and 43b for engaging the sockets 41a and 41b, respectively, of the terminal block 38 when the cover and base of the housing 26 are secured together. The plug base 44 and the contact pins 43a and 43b of the plug 39 of the housing 26 are similar to the plug base 44 and the contact pins 43a and 43b of the security sensors 2, 3 and 4. The two contact pins 43a and 43b of the plug 39 are electrically connected to the control circuit 22 by conductors 45, which are similar to the conductors 45 of the security sensors 2, 3 and 4. Thus, the control circuit 22 is connected into the loop circuit 5 in series through respective contact pins 43a and 43b and their corresponding sockets 41a and 41b of the plug socket assembly 21. The plug socket assembly 21 is so mounted in the housing 26 that on the cover being separated on the base of the housing 26 the contact pins 43a and 43b disengage the sockets 41a and 41b, respectively, which thereby forms an open circuit which is detected by the control circuit 22 indicating tampering with the housing 26.

The control circuit 22 of the central control panel 11 comprises a means for applying a continuous constant DC voltage to the input terminals 8 and 9 of the loop circuit 5, and an analysing means which comprises electronic hardware which is suitably configured by software for analysing current being drawn by the loop circuit 5. Thus, on the loop circuit 5 changing state as a result of a change In state of any of the sensor elements 16, 17 or 18 or the plug and socket assembly 21 of any of the security sensors 2, 3 and 4 or the central control panel 11, the central control panel 11 identifies which type of sensor element 16, 17 or 18 or plug and socket assembly 21 has changed state by analysing the current signal being drawn by the loop circuit 5.

In the event of any one of the sensor elements 16 which comprises an inertia sensor being subjected to a shock or vibration, a high frequency current pulse signal is induced in the loop circuit 5. By reading the mark space ratio of the pulse signal, the software in the central control panel 11 readily easily determines if a sensor element 16 has been activated.

In the event of a reed switch 23 having changed state, in other words, having gone from the closed circuit state to an open circuit state, an additional resistor R1 is shunted into the loop circuit 5 which thereby reduces the current drawn by the loop circuit 5, and by analysing the level of current being drawn in the loop circuit 5 the software in the central control panel 11 determines a change in state of the loop circuit 5 caused by a reed switch 23 having changed state.

On the push button switch 25 of the sensor element 18 going into an open circuit state, the resistor R2 is shunted into the loop circuit 5, thereby reducing the current drawn by the loop circuit 5. The central control panel 11 by analysing the level of current drawn in the loop circuit 5 determines a change in state of the sensor element 18. Furthermore, the central control panel 11 readily differentiate between a reed switch 23 or the push button switch 25 going into an open circuit state, since the values of the resistors Rl and R2 are different. The resistors Rl and R2 are so chosen that should the security circuit comprise one, two or more reed switches 23, even should some or all of the reed switches 23 go into an open circuit state, the resistance of the loop circuit 5 caused by the sum of the resistances of the resistors of some or all of the resistors Rl is different to the value of the resistor R2, and is also different to the value of the resistor R3.

Disengagement of the contact pins 43 from the sockets 41 of any of the plug and socket assemblies 21 is readily determined by the software in the control panel 11 by virtue of an open circuit state being caused in the loop circuit 5. Accordingly, although all the security sensors 2, 3 and 4 are connected in series in a single loop circuit 5, in other words, by two wires to the central control panel 11, the type of sensor element or tamper indicating element which changes state can readily easily be determined. Thereby the two wire security circuit of the invention provides the same effect as if only sensor elements of the same type and the tamper indicating elements were all connected into respective different zones of a central control panel.

In use, with the security sensors 2, 3 and 4 connected into the loop circuit 5 as illustrated in Figs. 1 and 2, and the input terminals 8 and 9 of the loop circuit 5 connected to the central control panel 11, the security circuit 1 is ready for use. The central control panel 11 applies a continuous constant DC voltage supply, typically, a five volt DC supply across the terminals 8 and 9 of the loop circuit 5 and monitors the current drawn by the loop circuit 5 for determining a change of state of any of the sensor elements 16, 17 or 18 or of the plug socket assemblies 21 of the security sensors 2, 3 and 4 or the plug socket assembly 21 of the central control panel 11, and also for determining which type of element of the security sensors 2, 3 and 4, or if a plug socket assembly 21 of any of the security sensors 2, 3 and 4 or the central control panel 11 has changed state.

The advantages of the invention are many. A particularly important advantage of the invention is that it enables a plurality of different types security sensors to be connected in series in a single loop circuit formed by two wires, and at the same time provides for tamper indication for indicating an attempt to tamper with any of the security sensors, or the central control panel, and the tamper indicating elements and sensor elements of the respective security sensors are connected in series in the single loop circuit formed by the two wires. Thus, the sensors can be connected in series by a two wire circuit to a central control panel, and at the same time the central control panel can detect both tampering with any security sensor or the central control panel itself and a change of state of a security sensor caused by an attempted break-in and differentiate between the type of sensor element which changed state, and also differentiate between a sensor element changing state and tampering.

It will of course be appreciated that once a security sensor has been tampered with, and the contact pins and sockets of the plug and socket assembly of the tamper indicating element have been disengaged, the central control panel is no longer able to monitor the security sensors in the loop circuit. However, by knowing that a security sensor or the housing of the central control panel has been tampered with, corrective action may be taken. It will of course be appreciated that this latter problem could be overcome by providing an impedance element, typically, an electrical resistor in series with the respective pins and sockets of the plug socket assembly. The resistor would maintain electrical continuity in the loop circuit on the contact pins and sockets being disengaged. The fact that electrical continuity is being maintained through a resistor would alter the current being drawn from the central control panel, and by suitably selecting the resistance value of the resistors, an indication of tampering would be detected identified by virtue of a drop in current in the loop circuit.

A further advantage of the invention is that it provides a security sensor and a central control panel which can readily easily be installed and serviced.

Installation of the security sensors and the central control panel merely requires securing the base of the security sensor or panel to the appropriate part of the structure, and then wiring the base into the loop circuit by connecting the wire of the loop circuit into the respective connector elements 40a and 40b. The cover which incorporates the relevant sensor element or elements or the control circuit may then readily easily be plugged into the base by engaging the contact pins of the plug with the sockets of the terminal block.

The cover is then readily easily secured to the base by the screw. Servicing of the security sensors and the control panel merely requires unscrewing the screw and unplugging the cover from the base. The sensor element or elements or the control circuit can then be serviced, and the cover with the sensor element or control circuit located therein plugged back into the base, or the cover with the sensor element or control circuit may he replaced with a new cover and sensor element or control circuit, which can likewise be plugged into the base by engaging the contact pins in the corresponding sockets.

While the security sensors have been described as comprising sensor elements provided by inertia senors, reed switches and a panic switch, the security sensors may comprise any other type of sensor element, for example, an infrared sensor element, a reed switch sensor element on its own without an inertia sensor, or any other type or combination of sensor element.

While the apparatus according to the invention has been described as being a security sensor and a central control panel for connecting into a security circuit, it will be readily appreciated that the apparatus may be any other type of apparatus which incorporates an electrical or an electronic device or circuit, and in which case the housing is provided in two parts, and a tamper indicating element is located within the housing for detecting tampering. For example, it is envisaged that the apparatus may be an alarm box for use with a security circuit. Such alarm boxes, are of the type which comprise a two part housing, which defines a hollow interior region within which an alarm is located. Such alarm boxes, typically, are mounted on an exterior wall of a building. It is envisaged that the tamper indicating element would be located within the hollow interior region of the two part housing of the alarm box for connecting the alarm to the central control panel, so that tampering with the housing of the alarm box would cause the tamper indicating element to go into an open circuit state which could be detected by the central control panel.

It will also of course be appreciated that the security circuit may be provided with any number of different types of security sensors.

Claims (7)

1. CTATMS

1. Apparatus comprising an electrical or electronic device for connecting into a security circuit, the apparatus comprising a housing formed in two parts 5 which are securable together to form a hollow interior region for housing the device, a tamper indicating element being located in the two part housing, the tamper indicating element being responsive to separation of the two parts of the housing and changing 10 state on the two parts of the housing being separated for indicating tampering with the housing, the tamper indicating element being connected with the device so that only two connections are required to the housing for connecting both the device and the tamper 15 indicating element into the security circuit.

2. Apparatus as claimed in Claim 1 in which the tamper indicating element comprises a first electrically conductive element, and a second electrically conductive element, the first and second 20 electrically conductive elements being in electrical contact in a first state of the tamper indicating element for connecting the electrical or electronic device into the security circuit, and being disconnectable into an open circuit state in a second 25 state of the tamper indicating element for disconnecting the electrical or electronic device from the security circuit for indicating tampering with the housing, the first electrically conductive element being located in one of the parts of the housing, and the second electrically conductive element being located in the other of the parts of the housing so that when the two parts of the housing are secured together the tamper indicating element is in the first state, and when the two parts of the housing are separated the tamper indicating element is in the second state.

3. Apparatus as claimed in Claim 2 in which two first electrically conductive elements and two second electrically conductive elements are provided, respective first and second electrically conductive elements being arranged in pairs of electrically conductive elements, each pair comprising one first and one second electrically conductive element, the pairs of electrically conductive elements being connected in series with the electrical or electronic device for connecting the electrical or electronic device into the security circuit, one of the pairs of first and second electrically conductive elements being connected to one of the two connections for connecting the electrical or electronic device and the tamper indicating element into the security circuit, and the other of the pairs of first and second electrically conductive elements being connected to the other of the two connections for connecting the electrical or electronic device into the security circuit.

4. Apparatus as claimed in any preceding claim in 5. Which the electrical or electronic device is a sensor element.

5. A security circuit comprising a single loop circuit having a pair of input terminals for connecting the loop circuit to a central control panel for

6. 10 monitoring the state of the loop circuit, and a plurality of apparatus as claimed in any preceding claim, the apparatus being connected in series by the loop circuit, each apparatus being connected into the loop circuit by its two connections so that a change in

7. 15 state of a tamper indicating element or an electrical or electronic device in any of the apparatus is detectable by the central control panel, and may be differentiated one from the other by element or device by the control panel.