Classifications

• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING SYSTEMS, e.g. PERSONAL CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B17/00—Fire alarms; Alarms responsive to explosion
• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING SYSTEMS, e.g. PERSONAL CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B13/00—Burglar, theft or intruder alarms
• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING SYSTEMS, e.g. PERSONAL CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B13/00—Burglar, theft or intruder alarms
  • G08B13/02—Mechanical actuation
  • G08B13/08—Mechanical actuation by opening, e.g. of door, of window, of drawer, of shutter, of curtain, of blind
• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING SYSTEMS, e.g. PERSONAL CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B19/00—Alarms responsive to two or more different undesired or abnormal conditions, e.g. burglary and fire, abnormal temperature and abnormal rate of flow
• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING SYSTEMS, e.g. PERSONAL CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B23/00—Alarms responsive to unspecified undesired or abnormal conditions

Definitions

• This invention relates to event detection and indication apparatus.
• the present invention is concerned with a system for providing an indication of the occurance of events such as the actuation of an intruder actuated switch in a burglar alarm system, the actuation of a switch associated with a sensor responsive to a predetermined situation such as a hazard condition i.e. fumes, smoke, fire.
• a hazard condition i.e. fumes, smoke, fire.
• hazard condition i.e. fumes, smoke, fire.
• an event detection and indication system including a circuit loop incorporating at least one event responsive actuatable switch, characterised in that each switch has associated therewith means for both maintaining electrical continuity of the loop following operation of an associated switch and for producing an electrical condition in the loop which is indicative of switch operation.
• Figure 1 is a circuit diagram illustrating features of the system of the invention.
• Figure 2 is a circuit diagram of a first embodiment of a circuit switch over-ride arrangement.
• Figure 3 is a circuit diagram of a second embodiment of a circuit over-ride arrangement.
• Figure 4 is a circuit diagram illustrating in. a more detailed form the system of the invention.
• Figure 5 schematically illustrates a circuit condition checking probe arrangement.
• Figure 6 is a circuit diagram of a first mode of connecting a switch unit into the system of the invention.
• Figure 7 is a circuxt diagram of an arrangement for introducing a radio frequency signal into the system.
• Figure 8 is a circuit diagram illustrating the incorporation of a trigger circuit into the system to facilitate the use of sensors with a slow response to the associated control parameter.
• Figure 9 is a circuit diagram illustrating the mode utilising a sensor arrangement using a photocell.
• Figure 10 is a circuit diagram of an ultrasonic detection arrangement for use with the system of the invention.
• FIG 11 is a circuit diagram illustrating a mode of connecting into the system a switch unit incorporating an auxiliary powered control unit.
• Figure 12 is a modified form of the circuit shown in Figure 8
• Figure 13 is a circuit diagram which illustrates a method of extending the system of the invention to incorporate auxxlrary buildings.
• 1B, 1C, 1D are serially connected to provide a loop circuit 2 whose ends 3 and 4 are respectively connected to terminals at circuit points 5 and 6 of a control circuit 7 shown within a dashed rectangle 8.
• a passive or dynamic circuit element 9A, 9B, 9C.... such as a resistance, impedance, unidirectional conductive device or other means for producing a voltage pulse on initiation into operation, is connected in parallel with the switches 1A, 1B, 1C, In the figure the elements 9A
• LEDs 9B are depicted as light emitting diodes (LED's).
• the control circuit as shown in Figure 1 includes an upper or positive voltage rail 10, and a lower or negative voltage rail 11 which is an earth rail.
• the circuit point 5 connects to the upper rail 10 by way of a constant current source 12, capable of providing a current level of typically 5 to 10 milliamps.
• the circuit point 6 connects with the earth rail 11.
• An amplifier unit 13 is connected across the voltage rails 10, 11, and a pair of transistor 14, 15 whose collectors connect with the upper rail through a relay 16, and whose emitters are connected with the earth rail 11.
• the base of the transistor 14 is connected via serially connected resistors 17, 18 and a capacitor 19 to the circuit point 5.
• a dc voltage level detector 20 serially connected with a capacitor 21 is connected between the circuit point 5 and the base of transistor 14.
• the relay 16 has a normally open contact set 22 connected between the voltage rails 10 and 11, there-being a combined circuit timer/amplifier unit 23 connected in series with the contact 22.
• This unit 23 has an output 24 connected with the base of the transistor 15 and a second output 25 which connects with an output terminal 26 of the control circuit 7.
• An alarm device or unit 27 which can provide an audible and/or visual indication or alarm and which is shown as a bell, is connected across the terminals 4 and 26.
• This control circuit 7 is energised by a battery 28 with the polarities as shown.
• the above discussed circuit has a quiescent state in which with the battery supply connected into circuit as shown, the relay 16 is de-energised so that the timer/amplifier unit 23 is switched-off.
• the constant current source 12 maintains the current conditions within the requirements for the voltage levels of the circuit 7.
• the amplifier unit 13 can be conveniently regarded as a voltage sensing and amplifier circuit which is responsive to positive excursions of the voltage within the switching loop 2.
• the circuit values of the components of the unit 13 are chosen such that the unit 13 responds to a positive excursion of typically 1.5 volts or more sufficient to activate the timer unit 23.
• the detector unit 20 is arranged to transmit a positive voltage to operate the unit 13 if the dc level at point 5 exceeds a threshold level conveniently set at 2.5 volts less than the upper rail voltage level,
• the above described circuit operates as follows: On opening one of the switches i.e. the switch 1C the associated LED 9C is effectively connected into circuit and on so doing produces a voltage pulse in the loop 2 which corresponds to the forward voltage of the LED 9C and which is at a level of typically 1.5 volts.
• This voltage pulse is developed at the circuit point 5 and is also detected by the voltage detector 20 in such manner that if the voltage at the point 5 exceeds the predetermined threshold voltage level i.e. 2.5 volts less than positive rail voltage a positive voltage equal to that of the positive voltage supply is transmitted via capacitor 21 to the base of transistor 14.
• the amplifier unit 13 On receipt of this voltage pulse the amplifier unit 13 produces an output which actuates the relay 16 whereby contacts 22 close and allows voltage to be applied to the timer and amplifier unit 23, whose timing cycle can be selectively adjusted.
• Energisation of the unit 23 has three principal results, firstly an output voltage is produced at output 24 which switches transistor 15 on thereby to complete a 'hold' circuit for the relay 16; secondly operation of the selected timing cycle of the unit 23 is started; and thirdly a second output voltage is produced which activates the alarm device 27. Summarising it will be seen that operation of any one of the loop switches 1A, 1B, 1C , - - - automatically causes operation of the alarm device 27.
• the latter switches off, and thus breaks the relay hold condition so that the relay switches off allowing contacts 22 to open.
• the control circuit then returns to its quiescent state. This occurs even though the opened switch 1C remains open thereby permitting energisation of the LED 9C whereby the latter continues to provide a visual indication that the switch 1C is open.
• a second switch i.e. the switch 1A
• the opening thereof will lead to the production of apositive pulse as a result of the energisation of the associated LED 9A; thereby initiating the above described sequence of events within the control circuit 7 thereby to reactivate the alarm device 27 for the time period set into the timer unit 23.
• the control circuit 7 returns to the quiescent state, even though two loop switches i.e. 1C and 1A remain open and two LED's, 9C and 9A are thus illuminated.
• the opening of one or more switches in the loop does not immobilise or mute the system from operation if a further switch is opened.
• the number of loop switches which can be functionally accommodated will be related to the total number of switches / LED combinations in the loop and the voltage levels of the system.
• loop 2 is connected to the control circuit 7 through an ac coupling only i.e. via capacitors 19 and 21, so that a steady voltage condition will not produce the requisite positive voltage pulse excursion necessary to trigger the circuit.
• the switches 1A, 1B, 1C serve as intruder actuated switches and take the form of e.g. door operated switches, window operated switches, the system, whilst responding to the opening of any one of the protected doors or windows, is not immobilised following the opening of any particular switch, so that even after switch operation the system remains capable of detecting intruder actuation of any of the switches still closed in the loop.
• a security system it is clearly desirable for legalusers of a protected building to be able to open doors or windows without setting off the alarm system.
• Figure 2 illustrates an over-ride facility for a door switch and includes two additional switches 29, 30 connected in parallel with the intruder actuated switch (i.e. switch 1A) in the loop 2.
• the switch 29 is a key operated switch and the switch 30 is a push button switch.
• the switch 29 would be on the entry side of the door and the switch 30 on the opposite side of the door.
• the key switch is operated, this completes a second current path across the loop switch 1A, so that when the door is opened, thereby opening the switch 1A, circuit continuity is maintained so that the LED 9A is not energised.
• the push button switch 30 is depressed, once again to complete a still further electric circuit in parallel with the switch 1A, and the key switch is opened i.e. the key removed (at this point the switch 1A will still be open). The door is then closed, thereby closing the switch 1A, and the push button switch released.
• This resistance is of the kind which has a control 32 loaded by a spring 33 which acts so as to maintain the resistance in circuit at a maximum value; and which control has to be turned against the spring loading to obtain a minimum value (i.e. zero) resistance value.
• the resistance value of resistor 31 is set to zero, and the window opened.
• the resistance control 32 is released on window opening.
• the release of the control allows the resistance level to return to the initial setting in which the resistance value in the circuit is at a maximum.
• the rate of change of the resistance value introduced into the circuit loop is not sufficient to prevent attenuation of the positive pulse arising from the opening of the window loop switch 1D, to such an extent as to prevent operation of the control circuit.
• the window can be closed at any time without initiation of the alarm since, as mentioned above, the negative going pulse arising from closure of a loopswitch does not affect the operational condition of the control circuit.
• a circuit over-ride or 'cheat' resistance into the loop as a means of bypassing the effect of opening one or more of the loop switches.
• Such a 'cheat' resistance can be regarded as a variable resistance which is introduced into the loop at minimum value, i.e. zero, and is gradually increased in value.
• the voltage excursion produced is attenuated by the capacitor 19 sufficiently to ensure that the cheat resistance itself exhausts most of the loop voltage.
• the voltage detector will sample the d.c. voltage of the point 5 and if it exceeds the power supply voltage less 2.5 volts the output of the voltage detector will switch from zero volts to the full power supply voltage, this voltage excursion triggering the alarm by way of the capacitor 21 and the amplifier 14 thereby initiating operation of the timed alarm system.
• the voltage detector will trigger. if on the other hand, the magnitude of the cheat resistance is relatively low the control circuit will operate normally, thus there is no value of 'cheat' resistance which can cheat the systems.
• the loop switches have been shown as simple mechanically operated intruder actuated switches.
• the loop can be provided with various forms of non-intruder actuated switches such as electronic switches which are responsive to the outputs of sensors which in turn are responsive to predetermined external conditons or situations such as hazard conditions. information transfer from a remote position to the control circuit.
• sensors can be responsive to a wide variety of hazards, such as, overheating; fire; smoke or other fumes; the presence of foreign bodies within pre-determined regions relative to a sensor, sounds etc.
• the loop can be provided with 'panic' or personal attack alarm actuating switches.
• the sensor devices for the purposes of this Application can be conveniently regarded as comprising two main forms, those which are energised principally from supply sources external to the loop, and those which are energised exclusively from the currents following through the loop.
• Figure 4 illustrates a loop 2 which is provided with sensors and associated electronic switches, and a more complex control circuit 7 which is able to accommodate the introduction of the sensor actuated switches into the loop and which includes a loop switch condition check facility, and a general monitoring facility.
• a block 35 having an associated LED 36 For convenience in the loop of Figure 4 those sensors which are indicative of external conditions and which are loop or external battery energised are generally indicated by a block 35 having an associated LED 36.
• the panic or personal attack switch arrangements are represented by a normally closed push button switch 37.
• Such switches may have an associated serially connected impedance 37a such as a LED connected in parallel therewith.
• Those sensors which incorporate some form of audio signal or tone generator facility are conveniently shown as a block 38 having an associated LED 39 or diode 78 (Fig.7) It will be appreciated that by suitable selection of the alarm indications provided by the system on operation of the intruder actuated switches; the hazard responsive sensors; and other switches such as the personal attack switches, the nature of the alarm indication given would immediately indicate the reason for the alarm signal being given.
• actuation of an intruder switch could be represented by the indication provided by the alarm 27 only; indication of the actuation of a hazard responsive sensor could be effected by operation of the alarm 27 accompanied by emission of a loud distinctive tone from the speaker 68; and indication of a non-hazardous situation could be provided by the emission of a distinctive tone only from the loud speaker.
• the loop should be capable of being used for information communication purposes, i.e. capable of transmitting or receiving audio signals, transformer coupled microphone/receivers or the like can be included in the loop. This facility is represented by a loudspeaker 40 and associated transformer 41.
• the switch checking facility includes a multiposition switch 42 having a selector arm 43 and switched contacts 44, 45, 46 47, 48 and 49.
• the arm 43 is connected to the circuit point 5.
• the contact 44 is used as a switch OFF contact.
• the contacts 45 to 48 are used for the loop condition checking facility and the contact 49 may be regarded as a spare; whose use will be considered hereinafter.
• a cascade of serially connected LED's 50, 51, 52 and 53 is connected to the emitter of a transistor 54 whose collector is connected to the earth rail 11, and whose base is connected via a potentiometer 55 to the upper rail 10.
• the switch contact 45 is connected via the LED 53 to the transistor 54 ; the contact 46 is connected through LED's 53 and 52 to the transistor 54; the contact 47 is connected through the LED's 51, 53 and 52 to the transistor 54, whilst the contact 48 is connected through all four LED's 50-53 to the transistor 54.
• transistor 54 is considered not to be included in the circuit), and that al of the loop switches are closed. under these conditions there is for the purposes of the present discussion essentially no potential difference between the circuit points 5 and 6. Since it is not desired to sound the alarm during the operating of the check facility the junction 60 of resistor 17 and transistor 14 is connected by a switch 61 to the earth line, thereby to inhibit operation of amplifier unit 13.
• the switch 61 as shown has a plurality of switching positions which will be considered hereinafter.
• the number of loop switches open determines the number of the LED's 50 to 53 which can be maintained in an illuminated condition.
• This probe 69 can include a test prod 69A having an integral LED 69B in series therewith and which connects by way of a lead 69C with a polarised mains plug 69D which is such that when inserted in any convenient mains socket connects the test prod by way of the LED to mains earth.
• the prod 69A is now placed in electrical connection with a switch located as near as possible to the mid-point of the loop. If the prod LED 69B illuminates the open circuit condition will be located between the switch being tested and the lower circuit point 6. (note point 6 is always at earth potential).
• the prod 69A is then engaged with a switch at the mid-point of this part of the loop, once again plugging the probe plug 69D into the nearest convenient mains socket, the switch being positioned adjacent the loop part mid-point. If the probe LED 69B illuminates the process is repeated. If the LED 69B does not illuminate this indicates that the fault is located between the last two switches to be tested.
• a flashing LED is easier to see than a LED with steady illumination.
• an astable circuit 62 is provided and is switchable into circuit by a switch 63.
• the switch 63 serves to connect either the source 12 or the astable circuit 62 to the circuit point 5.
• This switch 63 is ganged with the switch 61 so that when the control circuit operation is inhibited by the switch 61 the astable circuit 63 is connected into circuit so that the flashing LED facility is automatically switched in for monitoring purposes.
• a communication facility can be provided in the loop 2, and this implies complementary facilities in the control circuit 7 for coupling with those in the loop.
• the loop is closed by electrically connecting the circuit points 5 and 6.
• a transmitter/receiver unit 64 which includes a transformer 65 and series resistor 66 connected between contact 49 and the lower voltage rail 11, the transformer connects through an amplifier 67 to a microphone/loudspeaker facility 68.
• the transmitter/ receiver facility at the control unit can be readily arranged to reproduce these tones at the loudspeaker 68. Also, in view of the transmitter/receiver facility 64 the system can clearly be utilised to transmit signals to the loop for operating devices responsive thereto and for the relay of messages etc., from the control unit 7 to the loop 2.
• the senor is a pressure pad switch 70.
• the switch 70 is normally open-, and is connected across the base-emitter circuit of a transistor 71 whose emitter-collector circuit is connected across serially connected diode 72 and LED 73 which latter corresponds to the LED 36 in Figure 4.
• a resistor 74 is in the base-emitter circuit of the transistor.
• This circuit operates as follows:
• the series connected diode 72 and LED 73 effectively act as a voltage supply equivalent to the combined forward voltage of the diode 72 and LED 73, i.e. 1.5 + 0.8 volts.
• This voltage provides most of the loop current to the circuit consisting of transistor 70, base-collector resistor 67 and the open contacts 69 of a pressure mat 68.
• the full base current provided by resistor 67 is amplified such that the typical loop current available, say 7ma, pulls down the collector-emitter voltage to .3v for a germanium (Ge) transistor (.7v for a silicon (Si) transistor).
• the rate of voltage excursion is dictated principally by the switching time of the pressure mat contacts 69 so the signal attenuation through capacitor 11 is negligible.
• PNP transistors can be substituted for NPN transistors and vice-versa so long as conventional polarity connections are observed. This will be the case for all future circuits described.
• That part of the above discussed circuit comprising the LED 73, the diode 72, and the transistor amplifier 71 can conveniently be regarded as a sensor circuit connection module since the above discussed mode of producing a loop triggering voltage pulse can be utilised with a variety of different sensors.
• the module can be regarded as providing a means, which (whilst in its quiescent state) is able to restrict or divert loop current to an extent which prevents operation of the associated loop LED and which (in its activated state) is able to divert the loop current back through the associated loop LED to produce the positive triggering pulse in the loop.
• Figure 7 is a diagram of a circuit which produces an audio or radio frequency output in the loop and can be regarded as being a particular form of the block 38 of Figure 4.
• the circuit of Figure 7 includes upper voltage rail 77 and a lower voltage rail 76 which are connected in parallel with a diode 78 included in the loop 2.
• the collector of a transistor 79 connects through a resistor 80 to the voltage, rail 77, the emitter thereof being connected to the voltage rail 76.
• a transistor 81 connects through a resistor 82 to the voltage rail 77, whilst the emitter thereof is connected to the voltage rail 76.
• the collector of transistor 81 is connected to the base of transistor 79.
• a pair of serially connected resistors 83, 84 is connected across the voltage rails 76 and 77. The junction of these resistors is connected to the base of transistor 81.
• a positive feed back path including a capacitor 85 connects the collector of transistor 79 with the junction between the resistances 83 and 84.
• the above described circuit is essentially an oscillatory circuit in which in its quiescent state the transistor 81 is switched OFF and the transistor 79 is switched ON, by way of loop voltage derived via resistor 82.
• the circuit is set into oscillation by producing an inbalance between the resistors 83 and 84 which causes the transistor 81 to switch ON: whereupon the transistor 79 is switched OFF.
• the circuit is set into oscillation producing, for example, an audio frequency output across the diode 78.
• the average voltage in the rail and across the diode 78 remains substantially at the quiescent value, whilst the frequency of this voltage is principally set by the circuit values of the resistor 83 and the capacitor 85.
• the diode 78 serves to restrict the modulation from exceeding 0.8 volts in the event that a fault develops in the remainder of the circuit of Figure 7.
• This alternating voltage will be detected at the control circuit by the transmitter/receiver unit 64, is amplified by amplifier 67 and can be heard through speaker 68.
• an independently powered receiver unit can be plugged into the loop.
• the above mentioned resistance inbalance between resistors 83, 84 can be produced by making either or both of these resistors 83, 84 responsive to an external condition to be monitored. That is the actual detectors rely upon a change of resistance value to provide an output. In other words the detector variable resistor is used as the resistor 83 or 84 according as to whether the detector response increases or decreases the value of the resistance.
• the switch has contacts 86, 87, 88, 89 and 90. Also the switch has a contact arm 91 which is connected to the lower voltage rail - that is the arm 91 is earthed.
• the contact arm of the switch 63 is ganged with the switch 61.
• the contact arm of the switch is ganged with the arm 91, the ganging being such that when the arm 91 co-operates with contact 86, the switch 63 connects the astable circuit 62 to the circuit point 5 and disconnects the source 12 from the circuit point 5, and that at all other settings of the arm 91 the switch 63 connects the source 12 to the point 5 and disconnects the astable circuit 62.
• the switch contact 86 is connected to circuit point 60 so that relay operation and timer circuit operation are inhibited during that part of the checking operation during which the switch arm 91 co-operates with the contact 86.
• the switch contact 87 is connected to the junction between the timer unit 23 and the base of transistor 15.
• a push button switch 92 is connected between the circuit point 5 and the switch 63 so that depression of the push button switch 92 allows the timer unit 23 to be energised for a time period equating to the time constant of the capacitor 19 and resistances 17, 18.
• a diode 92A is connected across the switch 92.
• the switch contact 88 is connected by way of serially connected LED 93 and zenerdiode 94 to the positive end of the battery 28.
• the switch contact 89 is connected to the circuit point 60A.
• the switch contact 90 defines the normal or OFF position for the switch arm 91.
• the switch 61 is employed as follows:-
• the positive side of the battery 28 is connected through the LED 93 and zener diode 94, and the switch 61 to the lower voltage rail 11. If the battery is providing an adequate potential the LED 93 is caused to illuminate. Thus the third switch position enables a battery checking facility.
• Schmitt trigger action A typical electronic switch of this variety is illustrated in Fig. 8. Once again LED 73 and diode 72 provide the power supply across rails 96 and 97. NTC thermistor 100 divides the voltage across the rails with resistor 99 which is adjusted so that Ge transistor 98 is non-conducting. This allows a resistor
• the circuit will function without capacitor 101 but its inclusion gives a quicker switching action particularly if resistance 100 and 99 are high when switching takes place.
• a Si transistor can be substituted for Ge transistor 103 in which case the quiescent rail voltage is .7v and the trigger signal is 1.6 volts (2.3 - .7) volts.
• the quiescent and triggered conditions are shown below the various active devices. Resistance 106 is reduced so that Ge transistor 107 is conducting sufficiently to switch off the Si transistor 108 allowing resistor 109 to bias the Si transistor 110 on. If the light beam is interrupted, LDR photocell resistance increases fairly quickly resulting in transistor 107 switching off, the transistor 108 switching on, the transistor 110 switching off and the major part of the current being diverted through the LED 73 and the diode 72, resulting in an initiation of the control circuit 7 to produce a timed alarm. Once again positive feedback takes place and is enhanced by capacitor 111 particularly at low quiescent light levels when the resistance of the LDR is high and the response sluggish. In Figure 9 the voltage rails are indicated at 112 and 113.
• circuits employing operational amplifiers and CMOS integrated circuits have been used successfully but as the minimum working voltage then required is 3v as compared with voltages of .3 and .7v for transistors the loop voltage available cannot support the same number of electronic switches.
• the sensors used to far have been resistive types used in dc coupled circuits. Detectors employing ac signals may also be used, transitioning from ac to dc within the circuitry. This includes audio detectors tuned to the sound of breaking glass and ultrasonic detectors. A loop powered ultrasonic detector is shown in Fig. 10 and will now be described. An independently powered ultrasonic transmitter
• the signal from the receiver is amplified first through the common emitter stage formed conventionally by Ge transistor 115, base resistor 116 and collector resistor 117.
• the amplified signal is passed by capacitor 118 to a second comraon emitter amplifier consisting of Ge transistor 119, base resistor 120 and collector resistor 121.
• the further amplified signal is passed by capacitor 122 to a third common emitter amplifier consisting of a Ge transistor 123, base resistors 124 and 125 and a collector resistor 126 acting in conjunction with a capacitor 127 to maintain the base of Si transistor 128 at virtually zero volts and therefore non-conducting.
• Fig. 11 illustrates an economical solution requiring much less hardware than the self contained unit.
• One of the terminals of the independent battery 135, in this case the negative terminal, is connected to the loop.
• the battery powers the first stage only of the smoke alarm 136 the output of which is fed to a loop powered amplifier/Schmitt trigger 137 similar to one of those contained in Figs. 8 or 9.
• the trigger 137 is connected across LED 138 and diode 139 provided in the loop 2.
• the battery 135 never has to provide more than a few micro-amps and its life can be guaranteed, 137 provides the interfacing and is powered by the loop and a more strident alarm than that provided by a self cntained unit is powered from the high current supply 28 ( Figures 1 and 4) of the PMLAS.
• the donor system this configuration of feeding an independently powered sensor to a loop powered amplifier will be called “the donor system”.
• the donor system Mention will also be made of the fact that the donor system can be worked in reverse since a healthy trigger signal is available from across the loop LED and diode.
• the trigger signal can be extracted by sensing the light from the LED 73.
• Transformer coupling into the loop is recommended for high signal transfer, isolation of the microphone and/or loudspeaker from the dc current in the loop and the low resistance afforded by the secondary winding gives a lower dc voltage drop thus conserving loop voltage.
• a transformer coupled receiver 68, 67 and 65 (it could be a transmitter receiver if so desired) has been placed in the control unit 7 and is powered conveniently direct from the power supply 28. It will receive and amplify any audio signal in the loop and the sound will be emitted from loudspeaker 68.
• Figure 7 discloses a first form of a tone generator, represented by 38 (Fig.4) which can be substituted where necessary for the electronic switches.
• resistor 83 is too high or resistor 84 is too low the Ge transistor 81 is non-conducting and resistor 82 biases the Ge transistor 79 on, resulting in a rail voltage potential 77-76 across Si diode 78 of .3v. If resistor 83 is reduced sufficiently or resistor 84 increased sufficiently (either resistor can be a resistive type detector e.g. thermistor LDR in which case te other is made adjustable) transistor 81 switches on and oscillation of the rail voltage takes place due to positive feedback through capacitor 85. The average dc voltage of the rail still remains at .3v and the frequency is principally dictated by the value of resistor 83 and capacitor 85.
• the tone produced is amplified by amplifier 67 and can be heard through speaker 68 in the control unit 7 or any independently powered receiver amplifier plugged into the loop.
• the Si diode 78 is included both to avoid the voltage rail 77-76 exceeding .8v should there by a malfunctioning of the transistorised "circuit and also to limit the depth of square wave modulation.
• this type of detector can be usefully employed for a host of non-hazardous warnings e.g. freezer temperature, greenhouse temperature, sensing water leaks, soil humidity detector for house plants, child with a bad wetting problem, overboiling fluid, rain detector, steam detector and door chimes to name some.
• These detectors can be plugged into the loop at any time and being powered by the loop and using the control box amplifier/speaker are economical in hardware and cost.
• the resistor 84 is deleted and several resistors are substituted for resistor 83 (one per note) which are connected in sequence, by a spring loaded rotatable switch, to rail 77.
• a short tune is emitted from speaker 68 and when the knob is released it returns to its quiescent position such that rails 77 and 76 are shorted together thus conserving loop voltage.
• a second door chime plays a different tune for identification of front/rear door.
• the second tone generator to be described is illustrated in Fig. 12 and may be recognised as the addition of a transistorised astable circuitry, contained within the dashed lines 141-142, to the circuit illustrated in Fig.8.
• diode 72 is substituted with an appropriate value resistance, or both diodes 71 and 72 are deleted if visual indication is not required, should the circuit now be triggered the rail voltage 96-97 excursion would take it above that of the voltage detector 20 threshold. Such a circuit would be useful for a hazard warning such as a fire alarm since both the alarm 27 will be triggered and a distinctive tone will be emitted at all times immediately alerting anyone to the nature of the alarm.
• Fig. 12 has been used in conjunction with the transistorised switch of Fig.8. It could also be used in conjunction with any of the transistor switches described, or in conjunction with a mechanical switch such as a fire wire, or thermostat of the bi-metal strip variety, both conventionally employed as fire detectors. If used with such a switch the offsetting voltage diode 147 will, not be required since a mechanical switch, when closed, will short the rails 96-97, completely preventing any possibility of spurious oscillation, which might otherwise occur with the quiescent voltage of .3v or .7v present in the transistorised switch, if it were not for the diode 147.
• Example of hazard would be fire and smoke.
• FIG. 13 a loop of wire 148, across a LED placed in the loop 2 at a convenient pick-off point situated in the main premises, can be taken off to the buildings concerned and several switches, represented by switches 149 and 150 placed in series. If any of these switches are opened a 1.5v trigger voltage is generated in the loop circuit 2 and a timed alarm results.
• the advantage of this arrangement is that even if the intruder found some way of opening one of these switches without the alarm sounding the security of the main premises is not prejudiced since only 1.5v across LED 151 is lost in the main loop circuit 2.
• this principle allows the owner to have as many 'lines of defence' as is required extending outside the main premises.
• these audilliary loops can include light trip wires, fusible plugs, fire wires or indeed any switch arrangement limited only the imagination of the owner.
• a more elaborate system could take advantage of placing a different frequency oscillator across every switch in the loop using these distinctive square wave tones generated at source (above aural threshold if necessary) transmitting them along the loop to a computer in the control box for processing according to time, to establish they are significant tones and not spurious interferences, then decoding according to frequency and/or average voltage, then passed through a logic unit for display perhaps consisting of an alarm and a VDU together with a loud repeating voice recorded message indicating the nature of the hazard and its location.
• system may be applied to 'paging' arrangements, for example, in hospitals, factories or the like, by imposing on the system selective frequencies, which are raised at source.

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• Business, Economics & Management (AREA)
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• Burglar Alarm Systems (AREA)
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• 1979
  • 1979-11-30 WO PCT/GB1979/000204 patent/WO1980001214A1/en not_active Ceased
• 1980
  • 1980-06-17 EP EP19790901575 patent/EP0020602A1/de not_active Withdrawn

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