EP0062461A1

EP0062461A1 - Alarm system

Info

Publication number: EP0062461A1
Application number: EP82301593A
Authority: EP
Inventors: Bohumil Maria Jan Plichta
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-03-30
Filing date: 1982-03-26
Publication date: 1982-10-13
Also published as: IL65388A0; AU8180182A

Abstract

A security alarm system (10) has a fine trip wire (12) and a continuity monitor (14) for monitoring continuity of the trip wire. If the trip wire is broken then a trigger signal is provided by the continuity monitor and activates an alarm (16) such as a rocket or flare. The continuity monitor may monitor the continuity of the trip wire continuously, or at periodic intervals.

Description

This invention relates to a security alarm system. More specifically, it relates to a security alarm system whereby unauthorised entrance by a person into a prescribed area may be detected and an alarm signal provided.
According to the invention there is provided a security alarm system which includes an unauthorised presence detector comprising at least one thin wire locatable along at least a part of the periphery of a designated area; a continuity monitor for monitoring continuity of the wire and for providing a trigger signal when the wire is broken; and an alarm means operable by the continuity monitor.
Further according to the invention there is provided a security alarm device which includes a pair of input terminals connectable to an unauthorised presence detector wire locatable along at least a part of the periphery of a designated area; a continuity monitor for supplying an electric continuity sensing signal to the input terminals for monitoring continuity of the wire and for supplying a trigger signal when the wire is broken; a pair of output terminals connectable to an alarm means; and an alarm actuating signal supplying means responsive to the trigger signal for supplying an actuating signal to the output terminals.
Still further according to the invention there is provided a secured installation which occupies a designated area and includes at least one thin wire located along at least a part of the periphery of the designated area and breakable by an intruder, connected to a continuity monitor for monitoring continuity of the wire and for providing a trigger signal when the wire is broken; and an alarm means operable by the continuity monitor.
Even further,the invention provides a method of detecting the presence of an intruder into a designated area, in which a thin wire which is breakable by the intruder is located along at least a part of the periphery of the area; the continuity of the wire is monitored; and an alarm'signal is generated if the wire is broken.
The or each wire may be in the form of a trip wire which would be anchored on insulating spikes about 15 cm above the ground. Accidental breakage of the trip wire by animals could be avoided by erecting a fence to seal off the area covered by the wire. This fence could be lower and of lighter construction than a conventional security fence and should therefore be considerably cheaper. Instead of a protecting fence a more practical approach would be to employ hedges around the periphery of the secured area, with the or each trip wire than being woven into the hedges on a few different levels from the ground so that a wire will be broken if an attempt is made to penetrate any part of the hedges. The or each wire should be very thin (with a diameter less than 0.1 mm) and of a colour which blends well with the environment so that it is not easy to see.
The continuity monitor may be electrically operable. The alarm means may also have a sensing signal supply means for supplying a continuity sensing signal to the wire. This ensing signal could be supplied continuously or periodically. In one embodiment, a voltage level detecting means may be connected in parallel with the wire for monitoring the voltage across the wire and for providing the trigger signal when the voltage across the wire increases above a predetermined value.
Filtering means may also be provided to protect the triggering device from interference signals which could result in false triggering of the device. Surge suppressors or the like may also be provided to inhibit response to lightning flashes or the like..
The alarm means may be of any suitable type. It may for example be in the form of a rocket which is electrically fired by the voltage sensitive device referred to above. The rockets should produce both smoke and light so that they are effective during the day and at night. A noise of sufficiently high intensity should also be generated to attract the attention of security forces. Rockets of various colours could be employed to facilitate precise location of the point of intrusion. The rockets could also include a parachute or the like to lengthen the period of visibility of the like,and smoke. Instead of using rockets, or the like,floodlights and/or sirens maybe used.
A test means may also be provided to test the continuity of the wire.
The invention is further described, by way of example, with reference to the accompanying drawings in which Figs. 1 and 2 are circuit diagrams of two security alarm systems in accordance with the invention.
The security alarm system 10 shown in Figure 1 comprises a trip wire 12 which is of fine wire, a continuity monitor 14 and an electrically activated rocket 16.
The continuity monitor 14 comprises a three volt battery 17 which is preferably of the lithium type, a storage capacitor 18, an SCR 20 which has a sensitive gate which is triggered by currents of the order of 20_P A, and terminals 22 to which the trip wire 12 is connected. Between the terminals 22 there is a surge suppressor 24 (such as a varistor). The terminals 22 are also connected to the gate and cathode of the SCR 20 via an integrating network formed by a resistor 26 and capacitor 28. This network minimises the effect of interference signals. Further terminals 30 are provided to which the rocket 16 is connected.
It will be appreciated that, whilst the trip wire 12 is integral, there will be a low voltage between the terminals 22 and accordingly the gate of the SCR 20 will be by-passed. The current drain in this mode of operation is also minimal, allowing for long battery life. However, if the trip wire 12 is broken, then the SCR 20 will be triggered, thereby causing the capacitor 18 to discharge via a suitable initiator of the rocket 16, thereby activating the rocket.
The embodiment of security alarm system 40 in accordance with the invention shown in Fig. 2 is similar to the system 10 of Figure 1 and similar components have the same reference number.
Thus, the system 40 has a continuity monitor 14 having input terminals 22 and output terminals 30. A fine trip wire 12 is connected to the input terminals 22 and an electrically operable flare 42 is connected to the output terminals 30.
The continuity monitor 14 has four inverting Schmitt triggers 44 to 50. The Schmitt trigger 44 is used together with a capacitor 52, feedback resistors 54 and 56 and a feedback diode 58 to form a pulse generator which supplies pulses of 20 m sec. duration every four seconds. Thus, when the output of the Schmitt trigger 44 is low the capacitor 52 is discharged slowly until the low input trigger level of the Schmitt trigger is reached. The Schmitt trigger 44 then changes state, its output going high. The capacitor 52 is then charged rapidly via resistor 56 and diode 58. When the high input trigger level is reached the Schmitt trigger 44 then changes state, its output going low.
The voltage pulses from the Schmitt trigger 44 are supplied to the output terminals 22 via a voltage dividing network of resistors 60 and 62. These resistors 60 and 62 have suitable values so that, if the resistance of the trip wire loop 12 is lower than two Kohm, the voltage across a capacitor 64 does not reach the high trigger input level of Schmitt trigger 48. When the trip wire is broken, and the Schmitt trigger 44 supplies a sensing pulse, the voltage across the capacitor 64 will increase above the high trigger level of the Schmitt trigger 48 and it will change state, going low. The resistor 62 and capacitor 64 also perform a filtering function. A further filter is provided by means of a further series resistor 68 and a parallel capacitor 70 connected to the input of the Schmitt trigger 48.
The Schmitt trigger 46 together with a feedback diode 72, constitutes a latching device, which ensures that the output of the Schmitt trigger 48 remains low. The output of the Schmitt trigger 46 is also connected to the input of the Schmitt trigger 44 by means of a further diode 74. It will be appreciated, that the output of the latching trigger 46 is held high in a latching mode, which also latches the trigger 44, keeping its output low.
When the output of the Schmitt trigger 48 goes low, this signal is differentiated by a series capacitor 76, the differentiated signal then being supplied to the Schmitt trigger 50. The differentiating network formed by the capacitor 76 and a resistor 78 has a suitable time constant to provide the trigger 50 with a low pulse of about 10 m sec. duration. When the trigger 50 is supplied with this low pulse it supplies a high pulse to a drive transistor 80 which is connected between one of the output terminals 30 and earth. The storage capacitor 18 is connected between the other output terminal 30 and earth. Thus, when the transistor 80 is energised, the capacitor 18 is discharged through the flare 42, actuating it and supplying an alarm signal..
In order to ensure that there is minimal current drain from the battery 17 once the flare 42 has been actuated, the continuity monitor 14 is supplied with power via the flare 42. In the event that a fuse head of the flare 42 fuses into a short circuit state, there is still minimal battery drain due to the latching effect of the trigger 46. It will be appreciated that, if the latching trigger 46 and the diode 72 and 74 are omitted, and power is supplied to the continuity monitor 14 after it has triggered the flare 42, then a series of pulses will appear at the output terminals 30 which could be used to provide an audible alarm.
A light-emitting diode 82 is provided in series with the battery 17. This diode 82 is used to monitor the battery output current and provides additional information regarding the battery state and continuity of both the input and output circuits. Thus, with a trip wire connected to the input terminals 22, the output terminals 30 are shorted with a length of wire. If the diode 82 does not flash, the battery voltage is too low and the battery must be replaced.
If the diode flashes periodically, every 4 seconds, then the trip loop wire 12 is open circuit and must be replaced. The flare 42 can also be tested for continuity by shorting the capacitor 18 by connecting a wire between earth and the appropriate output terminal 30. If the diode 82 then flashes,this is an indication that the flare circuit 42 is in order.
By this means, a simple yet effective system is provided to detect the presence of unauthorised intruders into a restricted area, and to provide a clearly visible and audible alarm signal. Further, the system is reusable, thereby minimising costs.

Claims

1. A security alarm system which comprises an unauthorised presence detector (12), a monitor (14) for the detector, and alarm means (16,42) operable by the monitor, characterised in that the unauthorised presence detector (12) comprises at least one thin wire locatable along at least a part of the periphery of a designated area; and in that the monitor (14) is a continuity monitor for monitoring continuity of the wire and for providing a trigger signal when the wire is broken; the alarm means (16,42) being operable by the continuity monitor.

2. A security alarm system as claimed in claim 1, in which the wire (12) is electrically conductive and the continuity monitor (14) is electrically operable.

3. A security alarm system as claimed in claim 2, which includes a sensing signal supply means (17,44) for supplying a continuity sensing signal to the wire (12).

4. A security alarm system as claimed in claim 3, in which the sensing signal supply means (17) is adapted to supply a continuous signal.

5. A security alarm system as claimed in claim 3, in which the sensing signal supply means (44) is adapted to supply periodically a sensing signal.

6. A security alarm means as claimed in any of claims 2 to 5, in which the continuity monitor (14) includes a voltage level detecting means (20, 48) for monitoring the voltage across the wire (12) and for providing the trigger signal when the voltage across the wire increases above a predetermined value.

7. A security ararm system as claimed in any of claims 2 to 6, which includes a filtering means (26, 28, 62, 64) connected across the wire (12) for filtering out interference signals.

8. A security alarm system as claimed in any of claims 2 to 7, which includes a surge suppressor (24) for suppressing transient signals.

9. A security alarm system as claimed in any of claims 2 to 8, which includes a test means (82) for testing the continuity of the wire (12).

10. A security alarm system as claimed in any of claims 1 to 9, in which the alarm means (16,42) is adapted to supply a visual and audible alarm signal.

11. A security alarm device which is to be used in conjunction with a detector (12) and an alarm means (16,42), characterised in that the device includes a pair of input terminals (22) connectable to an unauthorised presence detector wire (12) locatable along at least a part of the periphery of a designated area; a continuity monitor (14) for supplying an electric continuity sensing signal to the input terminals for monitoring continuity of the wire (12) and for supplying a trigger signal when the wire is broken; a pair of output terminals (30) connectable to the alarm means (16, 42); and an alarm actuating signal supplying means (50,80) responsive to the trigger signal for supplying an actuating signal to the output terminals (30).

12. A secured installation which includes a detector (12), a monitor (14) and an alarm means (16,42), characterized in that the installation occupies a designated area and in that the detector comprises at least one thin wire (12) located along at least a part of the periphery of the designated area and breakable by an intruder, the monitor is a continuity monitor (14) adapted to monitoring continuity of the wire and for providing a trigger signal when the wire is broken; and the alarm means (16,42) is operable by the continuity monitor.

13. A method of detecting the presence of an intruder into a designated area, characterised in that a thin wire which is breakable by the intruder is located along at least a part of the periphery of the area; the continuity of the wire is monitored and an alarm signal is generated if the wire is broken.