Alarm system

Abstract

 9 An alarm system comprises an electrical resistance bridge circuit (R3, D5; R5; R6; R4, D6) in which a first arm (R5) of the bridge comprises a loop incorporating a number of resistive detector units whose resistance varies depending on its state. A second arm (R6) of the bridge comprises a potentiometer operable to balance the bridge. The circuit includes means (IC₁, IC₂) operable to produce an output signal in response to an imbalance of the bridge which results in a potential difference at the balance points (X, Y) which is greater than a pre-set, adjustable value, and which is used to initiate an alarm. This enables the sensitivity of the bridge circuit to be altered either manually or automatically, and is used to facilitate accurate balancing of the bridge which is required if the circuit is not to be prone to false alarm conditions.

Description

[0001] This invention relates to an intruder and/or fire alarm system of the kind consisting of a multiplicity of sensing devices, such as mechanically or magnetically actuated switches, or relay contacts such as are commonly provided by passive or active infra-red or doppler/radar intruder detectors, smoke detectors and the like thereof, whereby such switches or contacts, which may be of the normally open or normally closed variety, are connected in a series and/or series parallel arrangement to form part of a loop or loops that form one or more arms of an A.C. or D.C. Wheatstone Bridge, each said pair of contacts having in parallel therewith a resistor or resistors either of a common value for all such contacts or of a discrete value specific to one pair of contacts and thereby different from the effective resistance in parallel with other contacts. An alarm system of this kind which uses resistors of a discrete value specific to one pair of contacts is described in British Patent No. 1429781 to J.V. Child.

[0002] It will be seen that each loop thus formed will have an electrical resistance that will vary from a low overall value when all of the relevant contacts forming said loop are closed to a much higher figure when all of the said contacts are open, this figure being of course the sum of the effective individual resistors that are thus connected in series. It will also be obvious that each loop thus formed can have any proportion or mix of either open or closed contacts, irrespective of whether the door, window or other protected device is open or closed. Thus, for example, with a multiplicity of windows each protected by a magnet and reed switch or a vibration sensor, some could be of the normally open or off variety when the relevant window is closed, whilst others could be of the normally closed or on variety when the relevant window is likewise closed, thus making the task of anyone desiring to illegally defeat the system doubly difficult, insofar as they have no easy means of discovering which configuration is used. Likewise it would not matter, in this example, which of the said windows were left open or which were closed prior to the alarm system being activated or key switched "on", provided the alarm system could sense, with adequate but not excessive sensitivity, both increases and decreases of the electrical resistance of each arm or arms forming part of the aforementioned bridge circuit, and also providing the said system can accurately determine and set the "null" or balance of the bridge throughout the necessarily wide range required, which may vary from near zero to several thousand ohms. This latter requirement, of being able accurately to determine the balance of the bridge, or the "null point", within a very wide range of electrical resistance and yet be capable of reacting to a change in resistance lower than the smallest value of the resistor of an individual detector unit, is further complicated by the fact that, at the same time, the system must not be over-sensitive and hence vunerable to false alarm conditions caused for example by normal fluctuations in the supply circuit or ambient temperature changes.

[0003] The alarm system herein described meets all of these criteria, with the added advantage of total insensitivity to and consequential immunity from spurious A.F. and R.F. signals or superimposed mains hum, etc., on the loop forming the sensing arm of the bridge, irrespective of how long or meandering said loop is, said interference thus not creating a false alarm condition.

[0004] According to the present invention, there is provided an alarm system comprising an electrical resistance bridge circuit in which a first arm of the bridge is comprised of one or more resistive detector units, each detector unit having more than one state and_!having a resistance which varies in dependence upon its state, and in which a second arm of the bridge comprises a variable rresistance operable to balance the bridge and hence the potential between two balance points of the bridge, means operable to produce an Qutput signal in response to a potential difference between said balance points, and alarm indicating means responsive to said output signal, characterised in that said output signal producing means is arranged to produce an output signal only if the potential difference between said balance points is greater than a preset value, and in that adjusting means operable to vary said preset value are provided whereby the sensitivity of the bridge circuit can be altered.

[0005] The means enabling adjustment of the sensitivity of the bridge circuit is utilised in various embodiments of the invention to provide an alarm system which has substantial advantages compared with known arrangements. For example, in one embodiement said means enables the band-width of the "null" setting to be set to suit the operational requirements of the system. In another embodiment, as will be described, either alternatively or in addition to the above, said means enables the band-width of the "null" setting to be decreased, either manually or automatically, immediately prior to or simultaneously with null setting irrespective of that part of the potentiometer travel where the null occurs, to provide for more accurate balancing of the bridge circuit and hence more reliable operation of the alarm system.

[0006] Suitably, said adjusting means include first switch means arranged such that operation of said first switch means varies said preset value. This feature enables the sensitivity of the bridge circuit to be varied by simply operating a switch, and is intended to enable the widening of this null window when the alarm is activated or key-switched "on" , as distinct from the initial setting or balancing of the bridge. This is in case the original presetting of the null is inadvertantly a little off-centre, whereby a slight drift due to temperature or other extraneous changes could erroneously trip the alarm. Suitably, second switch means are arranged to selectively connect said output signal to said alarm indicating means, whereby operation of said second switch means is arranged to operate said first switch means; each of said first and second switch means may be a two position switch, said first and second switch means being ganged together for simultaneous operation. When the alarm is key-switched "on" these switches are operable and the trip points are thus automatically taken further away from the manually pre-set points, in respect of both upward and downward changes of resistance in any of the arms of the bridge.

[0007] This automatic de-sensitising of the two alarm trip points minimises the chance of a false alarm whilst retaining an unerring ability to sense the addition or deletion of the lowest single value of fixed resistor used in the protective loop containing the detector units.

[0008] Alternatively or in addition, the adjusting means may simply provide for initial setting of the sensivity of the bridge circuit to a predetermined value which suits the particular configuration of the protective loops and the operational conditions of the system.

[0009] If required, the system may be designed to sense and indicate which of many series connected detector units are disturbed, irrespective of whether the disturbance is the opening of a previously closed switch or the closing of a previously open switch, insofar as a single discrete value of resistance will thereby be added to or deleted from the sensing arm of the bridge, the value of said resistance being unique and particular to a specific detector unit, the location of which is predetermined. To ascertain the value and hence whereabouts of the addition or deletion of a specific resistance it is simply necessary to key-switch the alarm "off" and re-set the null. The angular movement of the null-setting potentiometer, clockwise or anticlockwise, will be proportional to the value of resistance added to or deleted from the sensing arm of the bridge. If pre-calibration is required, an adjustable scale is provided which can be set to zero against a line or pointer on the potentiometer knob and temporarily locked in position, any subsequent re-setting of the knob to a revised null point thus indicating without ambiguity the relevant and specific change of resistance, and hence the location of the detector unit disturbed.

[0010] In another advantageous embodiment, an additional resistor is provided in series with the contacts of each detector unit, this resistor being either of the same or different value to the resistor in parallel with the contacts. Such a pair of resistors, which may for example be encapsulated within a reed switch assembly, would show a change in overall resistance if either the detector unit was actuated, or if the overall assembly was short or open circuited, said change of resistance irreversibly actuating the alarm in the manner described.

[0011] It is a preferred feature of this invention that said loop may terminate at any convenient earth point well away from the control box incorporating the remaining elements of the bridge circuit, the earth return to said control box being conveniently via the earth wiring of the property and the relevant mains plug, removal of which would therefore upset the balance of the bridge and thereby actuate the alarm.

[0012] A preferred embodiment of the invention further provides for a multiple zone protection loop, from one zone to a dozen or more, with a single common control box, insofar as entire segments of a long continuous loop can be subtracted or added by remotely situated switches, it being a simple matter to decide which zones should or should not be protected and to set zone shorting switches as required prior to adjusting the null and activating or key-switching the alarm "on". Multifarious series, parallel and series/parallel circuits or loops, subdivided into zones if required, are feasible and can be connected to the common control box as necessary. If desired, an installation can start as a simple single loop with a minimum of sensors, and thereafter be extended or added to as required, even to the extent of several hundred detector units in a multiple zone arrangement, without any modification to the original control box. An example of this versatility and zoning would be a simple domestic installation with comprehensive peripheral protection, plus all downstairs internal doors fitted with magnetic reed switches and wired in series as part of the overall loop, but with a conveniently placed shorting switch that could either add this zone to, or delete it from, the overall circuit. Thus during normal day time occupancy the relevant alarm circuit would be peripheral only, simply and easily "set" with the null seeking knob and single key-switch. Prior to retiring for the night, the alarm would have to be momentarily key-switched "off", the switch adding the downstairs doors as a "zone" switched on, the null re-set and the alarm key switched "on" again. A reverse procedure would be applicable on the following morning prior to opening any of the downstairs doors. On the typical exemplary circuit diagram appended hereto the separate zone referred to in the above example could be either part of the overall instantaneous loop or part of the delayed action loop incorporating the low audio level pre-warning of an impending alarm, wired in parallel with the door switch SW3. Note that when the alarm is key-switched "on", any unauthorised opertion of a zoning switch will irreversibly actuate the alarm, irrespective of whether the zone is being added to, or deleted from, the circuits.

[0013] A feature of the preferred embodiment of the invention lies in the use of those silicon chip integrated circuits commonly referred to as operational amplifiers or voltage comparators in a unique back to back configuration for the accurate and repeatable detection of both the null and out of balance conditions of a D.C. bridge, with consequential sensitivities down to one or two milli-volts, coupled with excellent rejection of superimposed spurious waveforms and like A.C. signals. In this embodiment these operational amplifiers or voltage comparators are arranged as a pair, either together encapsulated as a single integrated circuit, or in the form of two separate integrated circuits, with the positive and negative sensitive inputs cross coupled via a resistor network that senses the polarity of the bridge out of balance output, plus the null thereof. Separate D.C. bias voltages are developed across these resistors to set the trip points and "hold on" circuits as desired. The null point, with adjustable band-width set via the bias circuit, is indicated by an on/off L.E.D. or similar light, and/or a buzzer or audible warning device and/or a centre zero meter (either analogue or digital) and/or a bar graph indicator. This unique circuit configuration with its simply and easily set null coupled to a band width that is automatically widened when the key-switch is set to the alarm "on" position, is ideal for use in this intruder/fire alarm mode insofar as it overcomes one of the main disadvantages of most existing alarm systems. Namely, the need to traverse a property shutting all windows etc. before setting the alarm, which people are reluctant to do if they anticipate but a short period of time away from the property - just when a surprising number of burglaries occur. With this alarm system, setting and key-switching "on" is so extraordinarily quick and simple, irrespective of which windows etc. are closed or open, that every-time use, even for short vacations, will become habitual.

[0014] Although the features of this invention are applicable to both A.C. and D.C. bridges used in an intruder detector/fire alarm mode, the advantages of simplicity and cost effectiveness, coupled with an insensitivity to mains induced hum, are with the D.C. bridge, especially as all such alarms need a battery stand-by facility in case of mains failure. In the typical design proposed herewith re-chargeable batteries are used, kept in a fully charged state by integral trickle-charge circuits that allow said batteries to be incorporated within either the Control Unit or within the External Alarm Unit, if fitted. In the latter case the design is tamper proof insofar as short circuiting or open circuiting of the cable interconnecting the two units will immediately initiate an irreversible alarm condition. This is also applicable to the mains cable feeding the Control Unit and, of course, to all wiring forming part of the active sensing loop or loops of the bridge circuit. There is, therefore, no need to hide or make inaccesible ANY wiring forming any part of an installation, irrespective of whether said wiring is within the relevant property, or external thereto. In this design both the internal Control Unit and the External Alarm Unit have either mechanically or magnetically actuated tamper sensing switches arranged to sense any unauthorised attempt to remove a lid and/or to remove a unit from the wall. If either of the lids are removed an irreversible alarm condition will be created, the double skin construction making it impossible to stop the alarm by the use of wire-cutters, etc. The use of a D.C. bridge deletes the need for A.C. generators in the battery stand-by mode. This fact, coupled with the designed high sensitivity of detecting very small out of balance voltages with consequential low bridge currents, provides a unit demanding only small power levels, typically less than one watt, with a resultant low battery drain and long operational duty in the stand-by mode.

[0015] A further feature of this design is the deletion of the need to fit the Control Unit with the usual "test alarm" facility, this being simply and effectively executed by setting the control knob away from the null and momentarily turning the key-switch "on".

[0016] Suitably, the system may include a 24-hour "anti-tamper" facility which is operative when the Control Unit is key-switched either "on" or "off". This is achieved by including a relay coil, or similar current-activated device in the top leg of whichever side or sides of the bridge circuit feeds the external circuit, the current flowing through the device under normal operating conditions varying between pre-determined high and low limits in accordance with the minimum and maximum effective overall resistance of said leg of the bridge, and being insufficient to trigger the device. A cut or break in the protective loop would, however, de-energise the device, the contacts of which are used to initiate an audible or visual alarm. Such an arrangement is also useful to indicate faults caused by any discontinuity in the protective loop. Alternatively a voltage-sensing device such as a zener diode or diodes may be connected to either one or both of the null points x and y, the normal maximum voltage at these points being insufficient to force the appropriate zener diode into its conducting mode. Any discontinuity in the circuits forming the lower legs of the bridge would however cause the voltage at these points to increase to such a value as to force the zener diode(s) into a conducting mode, thereby triggerring audible and/or visual alarms irrespective of whether the system was key-switched "on" or "of".

[0017] The design incorporates two simple cost effective delay circuits to allow for exit and re-entry via a particular door, a special feature thereof being that, for exit purposes - after setting the null and key-switching the system "on" - the relevant door can be safely opened at any time within the countdown period (typically 45 seconds) and then left open for as long as is necessary for the exit of goods and people, the countdown starting again from zero only when the door is finally closed. After the expiration of this delay period, which can be adjustable or pre-set, a re-opening of this door, maybe hours or weeks later, initiates a second countdown operation, again typically 45 seconds, during which a low volume audible warning is given, and at the end of which the alarms, both internal and external, will be irreversibly actuated unless the Key-Switch has meanwhile been turned "off". Other sensors, such as under stair carpet pressure mats, can be included in this double delay circuit if so desired.

[0018] A further advantage and use of these novel delay circuits lies in the fact that the first circuit is automatically primed approximately 45 seconds after setting the null and key-switching the system "on", even if the occupant of the property does not go out. Thereafter if the relevant door is opened, say, to a stranger, the second countdown will be initiated, cancellation of the impending alarm about 45 seconds later only being possible by using the Key-Switch. Thus, if the visitor is unwelcome or has evil intent, help can be summoned by the simple expedient of NOT key-switching "off", so much easier than having to press a "panic button" or the like thereof, which may - in practice - be far from convenient or accessible. The low volume audible pre-warning acts as a reminder, in case of forgetfulness, typically when returning home, that the unit needs key-switching "off" in order to prevent a false alarm. To further inhibit such false alarms, the sensitivity of the bridge out of balance detecting circuit is adjustable and typically pre-set not to respond to changes of resistance of less than about 10% of the value of the lowest resistor bridging any sensor in the detection loop.

[0019] Thus, by way of an example, if the detection loop comprises one hundred either on or off switches wired in series, each bridged by a ten ohm resistor connected in parallel thereto, the overall loop would have a minimum effective resistance of near zero ohms and a maximum effective resistance of about one thousand ohms. The sensitivity of the detection and trip circuits would be typically set to give a null window width equivalent to about two ohms, with near equispaced widening to the equivalent of about four ohms when the alarm is key-switched "on". Thus if the null is set truly centrally, the alarm trip points will be at plus two ohms and minus two ohms. If the null is offset negatively at say minus one ohm (worst possible case), the alarm trip points will be minus one ohm to plus three ohms. Likewise if the null is offset positively at say plus one ohm (worst possible case), the alarm trip points will be minus three ohms to plus one ohm. Thus small drifts (within + or-1 ohm, or 0.1% of 1000 ohms) caused by extraneous influences will not create a false alarm condition, but a genuine alarm will be reliably initiated if any switch is actuated or if the wiring is open or short circuited, etc.

[0020] In order to promote a more complete understanding of the above, an embodiment of the invention will now be described, by way of example only, with reference to the accompanying schematic circuit diagram.

[0021] Referring to the drawing, a mains transformer T1 feeds, via a centre-tapped secondary, a pair of diodes D1 and D2 that form a conventional full wave rectifier, the output voltage of which is developed across a reservoir capacitor Cl, Rl and C2 give ripple smoothing, R1 and the zener diode Z1 also providing a degree of voltage stabilisation. Trickle charging of the battery B1 is limited by the resistor R2. The diode D3 prevents the battery discharging via R2. In the event of mains failure the battery B1 maintains the HT + rail by discharging via D4, this diode being normally reverse biased by virtue of the voltage set by Z1 being slightly greater than the maximum on charge battery voltage. R3, D5 and R5 form two legs of a Wheatstone Bridge, R5 being in effect multiple switches and resistors forming the intruder alarm protection loop, normally connected to terminals 1 and 2. R4, D6 and R6 form the remaining two legs of the Wheatstone Bridge. R6 is adjustable and capable of balancing the bridge or setting a null between points X and Y. The voltage dropped across D6 is approximately constant throughout the entire range of adjustment of R6. A proportion of this voltage is tapped off by the pre-set potentiometer R7, the resulting small current passing through R8, R9 and R10, and thence through R11 and R12, thus positively biasing the relevant inputs of the Voltage Comparators IC1 and IC2. Note that adjustment of the potentiometer R7 sets the effective width of the null window when the unit is key-switched "off". This in itself is a useful feature which enables the sensitivity of the null setting potentiometer R6 to be varied depending on user requirements. The value of R8 compared with R9, R10, determines the increase in effective width of the null window when the unit is key-switched "on", and have the two alarm trip points. It will be appreciated that if you required R8 may also be made adjustable to give further control over the operation of the system. The quadruple resistor network R11, R12, R13 and R14 is cross coupled back to the Wheatstone Bridge mid points X and Y. The diodes D7 and D8 limit the maximum voltage that can be developed between points X and Y, even in the event of R5 and/or R6 being totally either open or short circuited. These two diodes could be omitted if the 24-hour anti-tamper facility described is included. The capacitors C3 and C4 smooth out any remaining HT ripple, and also, together with R7, R4, R5 and R6, give time constants that slug or delay the voltage inputs to the circuits ICl and IC2, so as to meet the requirements of B.S. 4737. C3 also removes any superimposed mains hum or spurious signals picked up by the loop connected to terminals 1 and 2. by offering a very low impedance to earth, compared to the high impedance of the quadruple resistor network R11 to R14 inclusive. The Voltage Comparators IC1 and IC2 switch the output terminals (connected to anodes of D9 and D10) to earth ONLY when the negative inputs (the junctions of R13 and R16 and R14 and R17) equal or exceed the positive inputs (the junctions of R9 and R11 and R10 and R12). Point X of the Wheatstone Bridge is connected to the negative input terminal IC1 via R13 and the positive input terminal of IC2 via R12. Point Y of the Wheatstone Bridge is connected to the positive input terminal of IC1 via R11 and to the negative input terminal IC2 via R14. Note that the positive input terminals of both IC1 and IC2 are biased slightly positive by the voltages arriving via R9 and R10 respectively. With the Bridge set to a true null (points X and Y at precisely the same potential) this biasing gives the offset that must be caught up by any ascending voltage applied to the negative input terminals. With the Key Switch set to "on" this biasing differential is increased by short circuiting R8. The balance of the Bridge can be upset in only two directions, either X can go positive relative to Y, or Y can go positive relative to X. In the former case, with X going positive, the input to IC1 via R11 and R13 will cancel the differential bias and hence, as soon as a state of voltage equilibrium between the two input terminals is reached, the IC1 output will switch to near earth potential, causing diode D9 to conduct. The identical input to IC2 via resistors R12 and R14 will simply back-up the bias differential and continue to hold diode D10 in its non-conducting state.

[0022] In the latter case, with Y going positive, the input to IC1 via R11 and R13 will back-up the bias differential and continue to hold diode D9 in its non-conducting state. The identical input to IC2 via resistors R12 and R14 will cancel the differential bias and hence, as soon as a state of voltage equilibrium between the two input terminals is reached, the IC1 input will switch to a near earth potential, causing diode D10 to conduct.

[0023] Thus with the Bridge accurately balanced and points X and Y at the same potential, neither IC1 or IC2 will switch their outputs to earth, both diodes D9 and D10 being held in their non-conducting mode, even if the voltages of points X and Y are raised or lowered in unison by virtue of equal adjustments to R5 and R6. Apart from the primary mode of switching either IC1 or IC2, namely an out of balance condition between points X and Y, the circuit shown allows for a secondary mode of switching, namely the application of a positive voltage to the negative input terminals of IC1 and/or IC2, via resistors R16 and/or R17, to swamp the standing bias differential and consequently switch IC1 and/or IC2 "on" as heretofore described. Diode D14 prevents the normal positive voltage existing at the junction of R13 and R16 leaking to earth via R16 and hence upsetting the operating conditions of IC1. Diode D15 prevents the normal positive voltage existing at the junction of R14 and R17 leaking to earth via R17 and hence upsetting the operating conditions of IC2. This secondary mode of switching is used for lock or "hold on" signals from other parts of the circuit, for example, with the Key Switch set to "off" (as drawn). IC1 and/or IC2 can only switch the Light Emitting Diode D11 on and off: (R15 is a current limiting resistor), whereas with the Key Switch set to "on" IC1 and/or IC2 will energise/denergise the coil of Relay RLA, the normally open contacts of which switch the HT + rail via D14 to the "hold on" circuit R16, thus locking IC1 on, even though the original out of balance signal from points X and Y was of a momentary nature. Likewise, providing the Key Switch is "on", a short term actuation of the lid sensing switch LS1 will permanently lock ICl in the conducting mode, as will a short term positive voltage via D15 and D17 permanently lock IC2 in the conducting mode. In either case the alarm will be actuated until such time as the Key Switch is switched "off".

[0024] In the example shown a second or External Alarm can be supplied if required, the components thereof being shown on the attached circuit diagram in dotted form. The normally closed contacts A1 of relay RLA feed the coil of a further relay RLC via fuse F1 and the remote lid switch LS2. The normally closed contact Cl of relay RLC controls the external alarm, these contacts being held open by virtue of the energisation of the coil RLC. The externally fitted alarm will thus be operated by any break in the continuity of the circuit Al-Fl-LS2. The wire joining F1 and LS2 is part of a long three core cable connecting the internal Control Box with the External Alarm, the remaining two cores of this cable being the earth return lead and the HT + supply via F2 and F3. The battery Bl is normally transfered to position B2 within the External Alarm Unit, where the trickle charge circuit R23 and the discharge diode D22 are in effect duplicates of R2 and D4. Battery B2 can thus maintain operation of the entire circuit in the event of mains failure, in addition to actuating the external alarm in the event of either short circuiting or open circuiting of the interconnecting cable. Unauthorised removal of the External Alarm Unit lid will actuate the lid switch LS2 and rupture the quick-blow fuse Fl, thus irreversably closing the RLC contact C1 and sounding the External Alarm. As stated, the lead joining F2 and F3 is part of a three core cable interconnecting the internal and external units. If this cable is short circuited to earth, for example by virtue of being severed by metal faced cutters, F2 will rupture and protect the internal unit from damage and F3 will likewise rupture and protect the external unit from damage, whilst F1 will rupture and actuate the alarm as heretofore stated. In a further beneficial arrangement of this circuit a four core interconnecting cable can be used in between the internal and external units, the fourth core and earth return being part of the Bridge loop, together with a small fixed value resistor fitted within the external unit, joining terminals 1 and 2, thus increasing the tamper proof protection afforded.

[0025] The twin delay circuits are fed from the single change-over contacts Bl, which, in the normally closed position, feed the HT + line direct to R19, D16 and R21 as soon as the Key Switch is set to "on". This action initiates the first time delay, namely the slow charging of C5 via the high value resistor R19. The HT + voltage develops across R21, which, together with D16, serves to quickly discharge C5 in the event of either the Key Switch being turned "off" and/or the relay contacts B1 changing over. The critical time delay, which is adjustable or pre-set by changing the values of R19/C5, is the time necessary (typically 45 seconds) to charge C5 up to a potential sufficient to ensure that, upon closing the switch SW3, it will discharge via D18 sufficient energy into the coil of relay RLB for the contacts of that relay to change-over and initiate the latching or hold on voltage via D19. If the door switch SW3 is closed BEFORE the expiration of the time period referred to above, C5 still discharges via D18 and the coil of RLB,but at a peak current rate insufficient to effect change-over of the contacts B1, the relatively low resistance of this coil thereafter keeping the charge potential of C5 near zero volts, until such time as the door contacts SW3 are finally open-circuited by virtue of the closing of the relative exit/re-entry door, the time delay countdown thereupon recommencing from zero. If, at any time AFTER the expiration of the time period referred to above, irrespective of whether it be seconds or weeks, the door switch SW3 is actuated, the relay RLB contacts B1 will change over and latch on, thus initiating the second time delay, namely the charging of C6 via R20, and, at the same time, sounding the internal alarm at a low volume level by virtue of the voltage drop across R18-D20. Alternatively a separate low level buzzer may be connected directly between the normally-open contact of RLB and the earth rail. This low level audible warning is to give notification of an impending alarm condition to a key holder, who may thereupon Key Switch the system "off^u, thus cancelling the impending alarm. The removal of the HT + voltage from the normally open contacts of RLB allows C6 to be quickly discharged via D17 and the coil of RLB. If the system is NOT key switched "off" C6 will continue to slowly charge via R20, the resulting potential ultimately overspilling via the zener diode Z2 and thereafter upsetting the balance of IC2 via D15 and R17, thus irreversibly triggering actuation of the alarm. This second time delay (typically 30 to 45 seconds) is adjustable and/or pre-set by changing the values of R20, R22, C6 and Z2.

[0026] Note also that the two normally fixed resistors forming the top end of the bridge (R3 and R4 on the accompanying circuit diagram) may be, either individually or together, protection loops or zones, either instead of or in addition to the loop or loops heretofore referred to (forming R5 of the exemplary circuit).

[0027] In an embodiment of the invention (not illustrated) a 24-hour anti-tamper, or fault detection, facility is provided by connecting a current or voltage sensitive device such as a relay coil or a zener diode into the top leg of that side of the bridge circuit which feeds the protective loop; for example, in the embodiment illustrated resistor R3 could be replaced by a relay coil. This device is set to operate at a predetermined threshold value which is greater than the fluctuations occurring during normal operations, these variations being insufficient to trigger the device. A cut or break in the circuit, even with the alarm key-switched "off", will cause a change which is greater than said threshold value, causing the relay or diode to operate and actuate an alarm, which may be the internal alarm for the system, or a separate alarm.

Claims

1. An alarm system comprising an electrical resistance bridge circuit (R3, D5; R5; R6; R4, D6) in which a first arm (R5) of the bridge is comprised of one or more resistive detector units, each detector unit having more than one state and having a resistance which varies in dependence upon its state, and in which a second arm (R6) of the bridge comprises a variable resistance (R6) operable to balance the bridge and hence the potential between two balance points (X, Y) of the bridge, means (IC₁, IC₂) operable to produce an output signal in response to a potential difference between said balance points (X, Y), and alarm indicating means (Dll, RLA, RLC) responsive to said output signal, characterised in that said output signal producing means (IC_I, IC₂) is arranged to produce an output signal only if the potential difference between said balance points (X, Y) is greater than a preset value, and in that adjusting means (R7, Key SW) operable to vary said preset value are provided whereby the sensitivity of the bridge circuit can be altered.

2. An alarm circuit as claimed in Claim 1, characterised in that said adjusting means (R7, Key SW) include first switch means (Key SW) arranged such that operation of said first switch means varies said preset value.

3. An alarm circuit as claimed in Claim 2, wherein second switch means (Key SW) are arranged to selectively connect said output signal to said alarm indicating means (RLA, RLC), characterised in that operation of said second switch means (Key SW) is arranged to operate said first switch means.

4. An alarm circuit as claimed in Claim 3, characterised in that each of said first and second switch means is a two position switch, said first and second switch means being ganged together for simultaneous operation.

5. An alarm circuit as claimed in Claim 3 or 4, wherein said second switch means are key operated.

6. An alarm circuit as claimed in any preceding claim, characterised in that said output signal producing means (IC₁, IC₂) are connected to the balance points (X, Y) of the bridge by way of a resistive biassing circuit (R7, R8, R9, R10), and in that said adjusting means are arranged to adjust the resistance of said biassing circuit.

7. An alarm circuit as claimed in Claim 6, characterised in that said output signal producing means (IC₁, IC₂) comprises a first comparator having first and second inputs and an output, the potential at one of the balance points (Y; X) of the bridge being coupled to said first input, and the potential at the other of the balance points (X; Y) of the bridge being coupled to said second input by way of said biassing means, said comparator being arranged to produce said output signal at its output when the potentials at its first and second inputs are substantially equal.

8. An alarm circuit as claimed in Claim 7, characterised in that said output signal producing means (IC₁, IC₂) further comprises a second comparator having first and second inputs and an output, the potential at said one balance point (Y: X) of the bridge being coupled to the first input of the second comparator by way of said biassing means, and the potential at said other balance point (X; Y) being coupled to the second input of the second comparator, said second comparator being arranged to produce said output signal at its output when the potentials at its first and second inputs are substantially equal.

9. An alarm circuit as claimed in Claim 7 or 8, characterised in that the output of the or each comparator is connected to an indicator device (D11).

10. An alarm circuit as claimed in Claim 9, wherein said indicator device is a light emitting diode (Dll),

11. An alarm circuit as claimed in any preceding claim, characterised in that a first delay circuit (C6, R20) is connected to an alarm actuable by said alarm indicating means (RLA) and is operable to connect said alarm to a power source (Bl) by way of switch means (B1) a predetermined time after a control switch (SW3) has been actuated.

12. An alarm circuit as claimed in Claim 11, characterised in that a second delay circuit (C5, R19) is operable to actuate said switch means (Bl) upon actuation of said control switch (SW3).

13. An alarm circuit as claimed in any preceding claim, characterised in that a third arm (R3; R4) of the bridge includes a current-sensitive device operable to produce an output signal if the current flowing through that arm of the bridge rises or falls above or below values indicative of a discontinuity in said first arm (R5) of the bridge.

14. An alarm circuit as claimed in any preceding claim, characterised in that said bridge circuit includes a device operable to produce an output signal in response to a potential difference at said balance points (X,Y) indicative of a discontinuity in said first arm (R5) of the bridge.

15. An alarm circuit as claimed in any preceding claim, characterised in that said first arm (R5) of the bridge comprises a plurality of separate loops each incorporating one or more resistive detector units, and switch means are provided operable to connect one or more of said loops in series and to disconnect one or more of said loops from said first arm of the bridge.

16. An alarm circuit as claimed in any preceding claim, characterised in that each resistive detector unit comprises an additional resistor wired in series with the contacts thereof.

17. An alarm circuit as claimed in any preceding claim, characterised in that said resistance bridge circuit is a D.C. bridge circuit.

18. An alarm circuit as claimed in any preceding claim, characterised in that connection means are provided operable to connect said first arm of the bridge to the earth conductor of a mains circuit whereby said earth conductor may be utilised as part of said first arm.

Drawing