ELECTRICAL ALARM CIRCUITS AND SYSTEMS

Abstract: Intrusion-detection units (1) which are linked in an alarm system to a central control unit (3) by cable (2), each include a coder circuit (8) responsive to opening of intrusion- and tamper-alarm contacts (5, 6) to change the magnitudes of current flow in respective alarm and tamper signalling lines (14, 15) of the cable (2). Current is supplied to the tamper-signalling line (15) in the circuit (8) via the collector-emitter path of a transistor (16) from the positive supply-line (10) of the cable (2), until the alarm-contacts (5) in its collector circuit open, whereupon current supply of the same magnitude is continued to that line (15) via the base-emitter path from the alarm-signalling line (14). The increase in current in the latter line (14) is detected, to indicate the alarm condition, in a decoder circuit (9) of the control unit (3) by operational amplifiers (36, 40) that respond to the consequent tendency for change in potential of the line (14). The decoder circuit (9) also includes operational amplifiers (22, 23) to detect change in current magnitude in the tamper-signalling line (15) for indicating the tamper condition, such change occurring upon opening of the tamper-contacts (6) of any coder circuit (8) or any breaking or shorting together of the lines (10, 11, 14, 15) of the cable (2). Opening of the tamper-contacts (6) interrupts a current-supply path that is connected to the tamper-signalling line (15) in parallel with the collector-emitter path of the coder-circuit transistor (16), or alternatively interrupts the emitter circuit of that transistor (16). A resistor (20) is connected across the supply-lines (10, 11) to increase current supply to the tamper-line (5) via a diode (21) if the negative supply-line (11) is broken.

Description: Electrical Alarm Circuits and Systems This invention relates to electrical alarm circuits and electrical alarm systems including such circuits, being concerned especially with electrical alarm circuits in which there is electrical-signal change in a first or second of two signalling lines in dependence respectively upon whether a first or second alarm condition exists. Electrical alarm systems for use in providing alarm signals in response to intrusion into an area to be protected, also conventionally include provision for responding to attempts to interfere with or otherwise tamper with, the intrusiondetector units of the system. In general the cabling that is provided to connect the detector units to the relevant central station or alarm-control unit incorporates three pairs of lines, a first pair of lines being used for signalling the intrusion-alarm condition, a second pair for signalling the tamper-alarm condition, and the third pair for supplying electrical power to the detector units. These lines, which may extend over considerable distance, are vulnerable to attack and unless expensive precautions are taken for their protection can represent a weakness in the security of the alarm system as a whole. It is an object of the present invention to provide an electrical alarm circuit which may be used to improve security in this respect. According to the present invention there is provided an electrical alarm circuit of the kind in which there is electrical-signal change in a first or second of two signalling lines in dependence respectively upon whether a first or second alarm condition exists, characterised in that the first signalling line is connected to a device which is operative in one or the other of two currentconducting modes in series with the second signalling line in dependence upon whether the first alarm condition exists, that substantially the same magnitude of current flow is established in a second signalling line via said device in the two operational modes, that the magnitude of current flow in the first signalling line via said device is dependent upon which of the two operational modes is applicable, and that occurrence of the second alarm condition is effective to change the overall magnitude of current flow in the second signalling line whereby the magnitudes of current flow in the first and second signalling lines are indicative of the existence or otherwise of the first and second alarm conditions respectively. By using the electrical alarm circuit of the present invention it ig possible to achieve a high degree of system security especially in regard to attacks on the first and second signalling lines and other lines included in the cabling. The circuit may simply include a transistor having base and emitter electrodes connected to the first and second signalling lines respectively, and arranged such that the collector circuit of the transistor is interrupted for example by the opening of a set of switch contacts in response to the occurrence of the first alarm condition so as to switch main current flow within the transistor from the collector-emitter path to the base-emitter path and thereby increase current flow in the first signalling line. The emitter current of the transistor may be utilized in these circumstances to establish current flow in the second signalling line that remains substantially constant irrespective of the occurrence of the first alarm condition, but which is interrupted or otherwise changed if one or both of the signalling lines is broken or they are shorted together. Accordingly it is possible with the electrical alarm circuit of the present invention to provide distinctive indications of intrusion-alarm and tamper-alarm conditions in a simple and effective manner, and to do this with just the two signalling lines in addition to any power-supply lines required. An electrical alarm system including electrical alarm circuits in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing. The drawing shows an electrical intrusion-alarm system with the electrical alarm circuits incorporated into intrusion-detector units of the system. Referring to the drawing, the alarm system is shown as comprising three detector units 1 that are linked via a multi-core cable 2 with a central, control unit 3. The detector units 1 incorporate conventional detectors 4 that are located and arranged to respond to intrusion into the area to be protected; the detectors 4, which may be active or passive devices, respond not only to intrusion but also to attempts to tamper with their operation. In the latter respect and as illustrated in the drawing for one detector 4 only, each detector 4 includes one set of normally-closed contacts 5 that are opened whenever an intrusion alarm is to be given, and a set of normally-closed contacts 6 that are opened whenever a tamper alarm is appropriate. The opening of either set of contacts 5 and 6 is signalled via the cable 2 to control equipment 7 of the unit 3. The equipment 7 processes such signals in accordance with normal practice, to provide the required alarm procedures and responses. To the extent that the system has so far been described with reference to the accompanying drawing, it is generally of conventional form. Systems of this conventional form, whilst including provision for responding to tampering with the detectors themselves, are in general vulnerable to attacks on the cabling linking the detector units to the control unit. More especially, such systems can often be rendered inoperative by shorting together selected leads of the cabling. The system to be described, however, is of improved security in this respect as compared with the known systems and achieves this, together with economy in cabling, by the addition of simple electrical circuitry to the detectors and central, control unit. In particular, a so-called coder circuit 8 is incorporated in each individual detector unit 1 and a so-called decoder circuit 9 is incorporated in the control unit 3. Furthermore, the cable 2 linking the units 1 with the unit 3 involves just two pairs of leads. The leads of one pair provide direct-current supply-lines 10 and 11 to the units 1 from the positive and negative terminals 12 and 13 respectively of the 12volt power supply for the unit 3. The other pair of leads provide lines 14 and 15 interconnecting the circuits 8 and 9 for signalling intrusion-alarm and tamper-alarm conditions to the unit 3. Each circuit 8 includes an N-P-N transistor 16 having its base electrode connected directly to the line 14 and its collector electrode connected to the line 10 via a diode 17 (protecting the transistor 16 against supply-polarity reversal) and the contacts 5 of the respectively-associated detector 4. The emitter electrode of the transistor 16 is connected to the line 15 via a resistor 18. Connection in parallel with the collector-emitter path of the transistor 16 is also made to the line 15 from the line 10 via a resistor 19 that is connected in series with the contacts 6 of the associated detector 4. A resistor 20 and a normally nonconductive diode 21, which are common to the three shuntconnected circuits 8, are connected across the lines 10 and 11 and across the lines 11 and 15 respectively. The line 15 is connected in the circuit 9 to a voltagelevel detector that involves two operational amplifiers 22 and 23. The signal voltage developed across a resistor 24 connected between the lines 15 and 11 is applied to the amplifiers 22 and 23 via a resistor 25. The amplifiers 22 and 23 act respectively to compare this voltage with upper and lower voltage levels which differ from one another by 0.25 volt and which are established in a voltage-divider chain of three resistors 26, 27 and 28 connected across lines 10 and 11. The outputs of the two amplifiers 22 and 23 are connected via respective diodes 29 and 30 to the junction of resistors 31 and 32 in a voltage-divider chain which is connected across lines 10 and 11 and which includes two further resistors 33 and 34. The junction of the resistor 32 with the resistor 33 in this chain is connected to the base electrode of an N-P-N transistor 35 that is connected in the common-emitter circuit configuration for supplying tamper-alarm signals to the control equipment 7. It is to the junction of the resistor 31 with the resistor 34, on the other hand, that line 14 is connected in the circuit 9. The junction of the resistors 31 and 34 in the ciruit 9 is established as a virtual earth (point of constant potential) by negative feedback from an operational amplifier 36. The output of the amplifier 36 is in this respect connected to such junction via a resistor 37 that is shunted by a diode 38, whereas one of its inputs is connected to this same junction via a resistor 39. The other input of the amplifier 36 and one of the inputs of a fourth operational amplifier 40 are connected to the junction of two resistors 41 and 42 that are connected in series chain with two diodes 43 and 44 across lines 10 and 11. The amplifier 40 compares the output of the amplifier 36 with the potential at the junction of resistors 41 and 42 to derive intrusion-alarm signals for supply to the control equipment 7. In the normal condition in which the contacts 5 and 6 of all detectors 4 are closed and tere is no cause for either intrusion- ortamper-alarm operation of the equipment 7, lines 14 and 15 are maintained respectively at about 6 volt and 1 volt positive with respect to terminal 13. Transistor ] of each circuit 8 is fully conductive at this time such that the magnitude of current flowing in the resistor 18 equals that flowing in the resistor 19. Base current is in each case drawn via line 14 from the virtual-earth junction of resistors 31 and 34, the magnitude of current in resistor 34 being double that in resistor 31 with the result that the output of the amplifier 36 is low. The amplifier 40 compares this low output with the potential at the junction between resistors 41 and 42, so as to apply a normal, low-output signal to the equipment 7. A low-output signal is also applied to the equipment 7 by the normallyconductive transistor 35, the transistor 35 in this respect remaining conductuve while both diodes 29 and 30 are nonconductive. Both diodes 29 and 30 remain non-conductive while the potential of line 15 with respect to terminal 13 remains at its normal value within the upper and lower limits of comparison established for the amplifiers 22 and 23 in the chain of resistors 26 to 28. The opening of the alarm contacts 5 in any of the detectors 4, breaks the collector circuit of the transistor 16 in the associated circuit 8. The emitter current of the transistor 16 in these circumstances, however, remains constant, and so the opening of contacts 5 switches the transistor 16 into a condition in which there is a significant increase in its base current drawn via line 14 from the resistor 34. The amplifier 36 responds to oppose the consequent tendency for change in potential of line 14, by increasing current flow in the resistors 37 and 31. The output signal of the amplifier 36 applied to the amplifier 40, therefore rises above the potential at the junction of resistors 41 and 42. The amplifier 40 in its turn responds to this to provide a hlgE.-output signal to the equipment 7 indicative of the intrusion-alarm condition signalled by the opening of contacts 5. The opening of the set of contacts 5 of any detector 4 does not affect the magnitude of current flow in line 15, since the emitter current of the associated transistor 16 remains constant in spite of such opening. Accordingly the potential of line 15 is unchanged and the transistor 35 in the circuit 9 continues to conduct as normal. However (and indeed irrespective of whether the contacts 5 are open or closed) opening of the contacts 6 in any of the detectors 4 breaks current flow to line 15 through the resistor 19 of the associated circuit 8. This break in current flow causes the potential of line 15 to fall below the lower comparison limit established for the amplifier 23. The response of the amplifier 23 to this condition causes the diode 30 to conduct and this in its turn causes the transistor 35 to become non-conductive. A high-output signal is thus applied in the equipment 7 from the collector circuit of the transistor 35 to indicate the tamper-alarm condition signalled by the opening of the contacts 6. The tamper-alarm condition is also signalled in the system if there is tampering with the cable 2, or more particularly with any of lines 10, 11, 14 and 15. If line 10 is broken, this again cuts off current flow to line 15 through the resistor 19, reducing the potential of line 15 below the lower comparison limit and producing the high-output signal from the transistor 35 in the same manner as for the opening of contacts 6. Reduction in potential of line 15 to produce the same outcome also takes place if line 15 itself, or line 14, is broken; in the latter case the transistor 16 loses its base reference and current supply to line 15 via the resistor 18 is in consequence reduced significantly. On the other hand, if line 11 is broken, the potential of line 15 increases by virtue of the current supplied to line 15 via the resistor 20 and ttie now-conductive diode 21. The potential of line 15 rises in this way above the upper comparison limit established for the amplifier 22, and the response of the amplifier 22 to this causes the diode 29 to conduct. Conduction of the diode 29 renders transistor 35 non-conductive so as to indicate the tamper condition to the equipment 7 as before. The system also responds to any of the conditions in which lines 10, 11, 14 and 15 are shorted together rather than being broken. More especially, shorting of line 14 to line 10 increases current flow to line 15 in each circuit 8 so that the potential of line 15 rises above the upper comparison limit for the amplifier 22. The same applies if line 14 is shorted to line 15 or line 15 is shorted to line 10, the response of the amplifier 22 in all three cases bringing about indication of the tamper condition to the equipment 7. Shorting of line 14 or line 15 to line 11 similarly brings about an indication of the tamper condition, the current flow in line 15 being in each case reduced with a consequent fall in potential of line 15 below the lower comparison limit for the amplifier 23. Finally, shorting together of lines 10 and 11 will be effective, in the blowing of fuses in the power-supply source to terminals 12 and 13 or otherwise, to indicate the tamper condition to the equipment 7 in the normal manner for power failure. Nuisance actuation of the alarm system is reduced by minimizing the effects of high-frequency pick-up in the cable 2. In this respect a capacitor 45 is connected with the resistor 39 to decouple high-frequency components on line 14 from the input to the amplifier 36. Similarly, a capacitor 46 is connected with the resistor 25 to decouple high-frequency components on line 15 from the inputs to the amplifiers 22 and 23. The diodes 43 and 44 serve to ensure that the potential of the junction of resistors 41 and 42 and used as the reference for operation of the amplifiers 36 and 40, is appropriately related to the establishment of a potential of 6 volt positive with respect to terminal 13 for the emitter electrode of the transistor i6 in each circuit 8. The voltage drop across each diode 43 and 44 is substantially equal to the base-to-emitter voltage of the transistor 16 and remains so with temperature change, thereby providing for temperature compensation in the operation of the amplifiers 36 and 40. Although the system has been described above as including three detector units 1, more or fewer, and in particular just one, may in principle be provided. Where as described a single resistor 20 is used it is preferably located at the furthest extremity of the lines 10, 11, 14 and 15. For convenience the circuits 8 and 9 may be provided in the form of devices that can be readily added into existing systems to up-grade them, and in these circumstances each circuit 8 may incorporate the resistor 20 and diode 21; where more than one circuit 8 is to be installed in such a system the resistor 20 may then be disconnected in all but the most remote installation. In order to achieve maximum security it may be found preferable to connect the detector units 1 in cascade with one another (the lines 10, 11, 14 and 15 being extended through each successive unit 1 to the next) rather than in the particular shunt-connection illustrated. Furthermore it may be found preferable to omit the resistor 19 and connect the contacts 6 in the emitter circuit of the transistor 16, in all except the most remote unit 1.

Claims: Claims: 1. An electrical alarm circuit ir. which there is electricai- signal change in a first or second of two signalling lines in dependence respective Qibon zither a first or second alarm condition exists, characterised in that the first signalling line is connected to a device which is operative in one or the other of two current-conducting modes in series with the second signalling line in dependence upon whether the first alarm condition exists, that substantially the same magnitude of current flow is established in the second signalling line via said device in the two operational modes, that the magnitude of current flow in the first signalling line via said device is dependent upon which of the two operational modes is applicable, and that occurrence of the second alarm condition is effective to change the overall magnitude of current flow in the second signalling line whereby the magnitudes of current flow in the first and second signalling lines are indicative of the existence or otherwise of the first and second alarm conditions respectively. 2. An electrical alarm circuit according to Claim 1 further characterised in that the said device is a transistor having base and emitter electrodes connected to the first and second signalling lines respectively. 3. An electrical alarm circuit according to Claim 2 further characterised in that the collector circuit of the transistor is interrupted in response to the occurrence cf the first alarm condition so as to switch main current flow within the transistor from the collector-emitter path to the base-emitter path and thereby increase current flow in the first signalling line. t A' alarm circuit according to Claim 2 or Claim 3 her characterised in that the emitter current of the transistor establishes a first component of current 4 ir: trie second signalling line, and that a second cpsnent of current flow in the second signalling line sblished via resistance that is connected to the found signalling line in parallel with the collector nutter path of the transistor. 5 An alarm circuit according to Claim 4 further characterised in that switching means is included in series with the said resistance to interrupt the second component of current flow in response to the occurrence of the second alarm condition. 6. An alarm circuit according to any one of Claims 2 to 5 further characterised in that resistance is connected across two power-supply lines to the circuit, that the collectoremitter path of the transistor is connected between a first of the power-supply lines and the second signalling line, and that a diode is connected between the second powersupply line and the second signalling line to conduct in response to interruption of the said second power-supply line. 7. An electrical alarm system in which one or more alarmdetector units are linked to an alarm-control unit by first and second signalling lines, and in which each of the one or more alarm-detector units responds distinctively to first and second alarm conditions to produce electricalsignal change in the first or second signalling line in dependence upon whether the first or second alarm condition exists, characterised in that each of the one or more alarmdetector unit includes an electrical alarm circuit according to any one of Claims 1 to 6, that the alarm-control unit includes an electrical circuit for connecting the two signalling lines to the two sides respectively of an electrical power supply and for providing respective output responses to change in current-flow magnitude in th. first and second slgnlling lines. 8. An electrical alarm system according to Claim 7 further cheracterised in that the said electrical circuit of the alarm-control unit includes resistance connecting the first signalling line to one side of the power supply and means for responding to tendency for change in potential of the junction between the said resistance and the first signalling line such as to counteract such tendency, and that the output response of this said circuit to change in currentflow magnitude in the first signalling line is provided in accordance with the said response of said means. 9. An electrical alarm system according to Claim 7 or Claim 8 further characterised in that the said electrical circuit of the alarm-circuit includes resistance connecting the second signalling line to one side of the power supply and means for responding to departure of the voltage across this resistance from a predetermined range, and that the output response of this said circuit to change in currentflow magnitude in the second signalling line is provided in accordance with the said response of the latter means.

Inventors: WOOLVIN, TREVOR JOHN
Applicants: CHUBB ELECTRONICS LIMITED
Application year: 1979
Main classification: G08B25
IPC codes: G08B25