GB2258073A - Intruder detector - Google Patents

Abstract

An intruder detector comprises three microphones each connected to respective pass filters. Each of the pass filters is set to detect a predetermined frequency range for example first pass filter is set to detect high frequency sounds characteristic of breaking glass, second pass filter is set to detect low frequency vibration caused by air pressure changes and third pass filter is set to detect middle frequency sounds characteristic of vocal noises. The filters are each associated with respective timers the outputs of which are supplied to a gate and a gate array. Activation of either of the first or second microphones opens the gate for fifteen seconds during which time activation of third microphone will cause the gate array to generate an alarm signal.

Description

INTRUDER DETECTOR This invention relates to an intruder detector which reduces the proportion of error signals by use of multiple sensors.

A wide range of detectors or sensors are available to alarm premises such that unauthorised entry will be detected and, if required, signalled to a monitoring station. Such sensors include passive infra-red sensors, sound sensors, pressure sensors, positional sensors on doors or windows, air pressures sensors to detect movement in an enclosed space etc.

Each of these sensors has an individual weakness by which a false alarm signal can be generated, for example, an infrared sensor could generate an alarm signal on sensing the fall in temperature caused by a cloud obscuring the sun or an air pressure sensor could generate an alarm signal due to air pressure changes caused by the ignition of a central heating system or due to the suction effect of a wind passing an inlet into the monitored volume such as a chimney or air vent.

Other actions can also give rise to "false" alarm signals.

As the generation of an alarm signal often requires a physical inspection to determine the cause of the alarm activation it is desirable to reduce the number of "false" alarm signals and increase the accuracy of intruder detection.

In order to increase the accuracy of such detection it is known to provide units having two sensors each of different types, for example, air pressure and infra-red or sound detector and infra-red. In such units an alarm signal is generated only when both sensors are acted upon thus reducing the chance of a random or natural event triggering an alarm.

Such units can however miss detecting an actual intruder who by good fortune or design does not activate one or other of the paired sensors.

It is an object of the invention to provide an intruder detector which overcomes the disadvantages of known single or dual sensor units.

According to the present invention there is provided an intruder detector operative to detect three different intrusion events and signal an alarm condition if at least two events occur according to a predetermined sequence.

In a preferred aspect of the invention the intruder detector includes means to detect sounds having a high frequency, an air pressure change and sounds having a middle frequency range.

With such an arrangement the intruder detector will be able to detect three distinct intrusion events which may occur in different sequences to indicate an attempt to intrude into an area or volume. First the high frequency sounds would be characteristic of the cutting or breaking of a pane of glass prior to a physical intrusion. The air pressure change would be characteristic of the air pressure wave associated with the opening of a window or door and the middle frequency sounds would be those associated with physical movement inside the protected volume, talking etc.

By requiring the occurrence of at least two of the above events to cause an alarm condition the possibiltiy of error is minimised.

In order to reduce still further the chance of "false" alarm condition the intruder detector preferably includes one or more timing mechanimsms operative such that occurrence of one sensed intrusion event will actuate the timer and an alarm condition will be signalled only if a second intrusion event is detected within a predetermined time period.

In a practical embodiment of the invention an intruder detector comprises three microphones each connected to a respective pass filter arranged to detect respectively specified frequency ranges, each associated with respective timers and the outputs therefrom are passed to a gate array operative to generate an alarm signal if at least two of the microphones are activated within predetermined time periods.

In a modification of this embodiment only one microphone is provided which is connected to a pre-amplifier and thence to the three pass filters arranged in parallel.

The invention will be described further by way of example with reference to the accompanying drawings in which: Fig. 1 is a block diagram of a first embodiment of the invention; Fig. 2 is a block diagram of a second embodiment of the invention; and Fig. 3 is a block diagram of a third embodiment of the invention.

Referring firstly to Fig. 1 a first embodiment of the invention comprises three microphones 12, 14, 16, each connected to respective pass filters 18, 20, 22. The three pass filters 18, 20, 22 are each set to detect a predetermined frequency range. In the illustrated embodiment a first pass filter 18 is set to detect high frequency sounds that would be characteristic of the breaking or cutting of glass.

A second pass filter 20 is arranged to detect low frequency vibrations such as would be generated by an air pressure wave caused by opening a door or window. A third pass filter 22 is set to detect middle frequency vibration characteristic of talking, clothes rustling or other noises associated with movement of one or more persons about a room.

The first and second pass filters 18, 20 are each connected to a respective timer 24, 26 pre-set to be activated for a period of fifteen seconds. The output from each timer is supplied to a gate 28 and a gate array 30. The gate 28 is associated with a third timer 32 which receives output from the third pass filter 22. The third timer 32 is set to be activated for a period of five seconds after opening of gate 28.

The output of the third timer 32 is supplied to the gate array 30.

As a result of this arrangement if either the first or second microphone 12, 14 is activated by detection of breaking glass or a pressure change the gate 28 is opened for a period of fifteen seconds controlled by first or second timer 24, 26 respectively. If the third microphone 16 also has a positive detection in this period the gate array 30 is actuated giving rise to an alarm signal.

A second practical embodiment of the invention is illustrated in Fig. 2 in which a single microphone 40 replaces the three individual microphones of the first embodiment. A pre-amplifier 42 is provided and the output therefrom is passed to three pass filters 18, 20,22 connected in parallel.

The pass filters 18, 20, 22 are each associated in accordance with the first embodiemnt with timers 24, 26, 32, gate 28 and gate array 30. The results of an intrusion event detection by the microphone 40 are treated as described in the first embodiment.

In Fig. 3 a third embodiment of the invention is illustrated comprising a microphone 50 attached to a preamplifier 52 and a switched capacitor filter 54. The output from the filter 54 is supplied to a microcomputer 56 which thereby replaces the timers 24, 26, 32 and gates 28, 30 of the first and second embodiments. This form of construction results in a cheaper and simpler construction.

Claims (6)

CLAIMS:

1. An intruder detector operative to detect three different intrusion events and signal an alarm condition if at least two events occur according to a predetermined sequence.

2. An intruder detector as claimed in claim 1 which includes means to detect sounds having a high frequency, means to detect an air pressure change and means to detect sounds having a middle frequency range.

3. An intruder detector as claimed in claim 1 or claim 2 in which the detector includes at least one timing mechanism cooperative such that an occurrence of one sensed intrusion event will actuate the timing mechanism and an alarm condition will be signalled only if a second intrusion event is sensed within a predetermined time period.

4. An intruder detector as claimed in claims 1, 2 or 3 comprising three microphones each connected to a respective pass filter arranged to detect respectively specified frequency ranges, each associated with respective timers and wherein the outputs therefrom are passed to a gate array operative to generate an alarm signal if at least two of the microphones are actuated within predetermined time periods.

5. An intruder detector as claimed in claims 1, 2 or 3 comprising one microphone connected to a pre-amplifier the output of which is supplied to three pass filters connected in parallel, each associated with respective timers and a gate array operative to generate an alarm signal if at least two of the pass filter's thresholds are activated within predetermined time periods.

6. An intruder detector substantially as herein before described with reference to and as illustrated in the accompanying drawing.