GB2263771A - Intruder alarms - Google Patents

Abstract

An intruder alarm comprises a sensor responsive to air disturbances, a filter R2, C1 for removing the components of the signal from the sensor produced in response to high frequency air disturbances, an amplifier U1A for the output signal from the filter, a comparator U2A-D producing an output in response to a signal from the amplifier in excess of a level adjusted continually U1B-D as variations in the ambient conditions occur, and trigger circuitry operating a sounder in response to an output signal from the comparator. By having a continually adjustable threshold level that takes account of ambient conditions, the sounder is not liable to spurious actuation. <IMAGE>

Description

TITLE: Intruder Alarms DESCRIPTION; Technical Field The invention relates to intruder alarms, and in particular to alarms actuated by air disturbances resulting in pressure perturbations.

Background Art There are alarms that have sensors responsive to air disturbances caused by an intruder, for example, opening a door or window of an alarmed premises. GB 2119984 discloses an alarm that has a pick-up for producing a signal in response to a pressure variation caused by an intruder. The signal is amplified, filtered and compared to a threshold level. A sounder is activated if the filtered signal exceeds the threshold level. The threshold level is fixed once having been adjusted and must be reset to take account of any variation in the ambient conditions of the alarmed premises, for example, an increase in background noise. If the threshold level is not adjusted in accordance with the variation in conditions, the sounder may be liable to spurious activation.

The Invention The invention provides an intruder alarm comprising a sensor responsive to air disturbances, a filter for removing the components of the signal from the sensor produced in response to high frequency air disturbances, an amplifier for the output signal from the filter, a comparator producing an output in response to a signal from the amplifier in excess of a level adjusted continually as variations in the ambient conditions occur, and trigger circuitry operating a sounder in response to an output signal from the comparator. By having a continually adjustable threshold level that takes account of ambient conditions, the sounder is not liable to spurious actuation.

The Drawings Figure 1 is a diagram of circuitry for an alarm according te the invention; and Figure 2 is a diagram of the power supply circuitry for an alarm according to the invention.

Best Mode With reference to Figure 1, circuitry for an intruder alarm indicated generally at 1 has a microphone MIC 1 that is sensitive to pressure perturbations caused by air disturbances which could be the result of, for example, draughts or sounds. Opening a door or window to a room, particularly a pivotting door or window, causes a temporary change to the volume occupied by the air in the rcom. The change in the volume produces an air dnsturbance, and in response the microphone MIC 1 produces an alternating current signal.

A variable resistor VR2 coupled across the microphone MIC 1 determines the sensitivity of the alarm circuitry by dictating what proportion of the alternating voltage across the microphone is sampled. The sampled voltage is applied through a low pass filter, consisting of a resistor R2 and capacitor C1, to an operational amplifier U1A. Values for the resistor R2 and capacitor C1 are chosen so as to block frequency components in the sampled voltage above approximately 6-10Hz, which would be produced in response to spurious air disturbances. The amplifier U1A amplifies the sampled voltage from approximately 200 mV up to 500 mV.

The output from the amplifier U1A is fed to a comparator which consists of the bank of operational amplifiers U2A-D configured in a conventional manner using reference voltages. The comparator has upper and lower threshold levels, as determined by the reference voltages, and two outputs; a set output which changes state when a threshold level is exceeded by the output from amplifier UlA, and a reset input which changes state when the output from amplifier U1A returns from above to below the threshold level. The threshold levels are set according to the outputs of three operational amplifiers U1B-D. Together, the amplifiers U1B-D continually monitor the output from the amplifier U1A. The amplifiers UlB-D, and the associated components, determine a level that is representative of the ambient conditions sensed by the microphone MIC 1.The level is the mean voltage of the amplified output from the microphone MIC 1 produced in response to ambient conditions. The amplifiers U1B-D also maintain threshold levels at predetermined voltages above and below the mean level. As the ambient conditions are continually changing, for exampe because of variations in the background noise, the threshold levels are also continually changing. There is in effect a "window" that shifts up and down about a mean level, and the alarm will not be triggered if the output from the amplifier U1A remains within the confines of the window.

The set and reset outputs from the comparator control the operation of a set/eset latch consisting of gates U3A and U3B. An output from the latch U3A, U3B is in turn connected to the set input of a set/reset latch consisting of gates U4A and U4B. When the set output from the comparator becomes active, this is indicative of an alarm trigger condition in response to an intruder.

One of the outputs from the latch U3A, U3B is connected through a fast charge/slow discharge resistor, capacitor and diode network R24, C15 and D5. The network holds a high voltage at a first input to a gate U5A for three seconds when the output from the gate U3A changes from a low to a high state in response to the trigger condition.

The second input to the gate U5A is held at a high voltage after an initial charge time by a capacitor C28.

The output from the gate U5A changes to a low state for the three seconds that the first input to gate U5A is held.

A free running oscillator, consisting primarily of gate U5B, is enabled by the three second output from the gate U5A and by placing the alarm in test mode. The output from the gate USB drives a transistor TR2 that controls the alarm sounder SND1. Enabling the gate U5B causes the sounder to produce a test bleep of three seconds duration. The test bleep can be used to recognise a trigger condition and so to set the sensitivity of the circuitry by varying VR2.

The output from gate U4B is enabled only when the voltage at the second input to gate U4B is high, as determined by the voltage across a capacitor C14 connected to the second input to gate U4B. The slow charge/fast discharge resistor, capacitor and diode network R25, C14 and D8 serves as an exit delay. It takes thirty seconds after arming the alarm before the voltage across capacitor C14 is sufficient to enable the latch U4A, U4B to change state in response to a trigger condition. The alarmed premises can be vacated during the thirty second exit period.

A capacitor C13 at the output from the gate U4A prevents the latch U4A, U4B from resetting, once set, for a period of 5 mins, so as to hold the triggered alarm on.

The output from gate U4B is connected to a fast charge/slow discharge resistor, capacitor and diode network R29, R39, ClS and D9 which determines the input to an inverting gate U4C. When there is a trigger condition, and the output from gate U4B changes state, it is thirty seconds before the input to the inverting gate U4C reaches a level that causes its output to change state. The purpose of this time delay is so as to provide the legitimate entrant to an alarmed premises the opportunity to disarm the alarm before the sounder is activated. This is termed the delayed mode of operation.

The time delay of the delayed mode of operation can be overriden by placing the alarm in what is termed the instant mode of operation. If the premises is to be alarmed whilst occupied, for instance whilst the occupier is in bed, an entry delay is not necessary but the occupier would expect the alarm to react immediately to an intruder.

The output from gate U4B is also connected to a series of gates U4D, U5D and U5C. The first input to gate U5D is held high when the alarm is placed in instant mode. A change in state of the output of gate U4B has the effect through gates U4D and U5D of causing the output from U5C to go high, which switches a transistor TR1.

The transistor TR1 controls the operation of a relay RLA 1 which in turn controls a CMOS 556 dual timer U5 set-up to produce a swept frequency output. The timer U5 operates the transistor TR2 to control the sounder SND1 to produce a swept frequency audible alarm signal of about 106-108 dB.

The transistor TR1 can also be controlled by a remote sensing device, such as a P1R as shown in Figure 1.

Alternatively the transistor can be remotely controlled using a modem as shown in Figure 1. Also shown in Figure 1 are external sounder ports.

Figure 2 shows the power supply circuitry required to supply a regulated 12V, 500mA signal from the 280v, 50Hz mains. Figure 2 also shows how the supply is routed when the alarm is in each of the various modes described with reference to Figure 1. The encircled letters can be matched up to those encircled letters on Figure 1, and identify the application of supply voltage when the alarm is in the various modes of operation. Also shown in Figure 2 are the switch positions required to route the supply voltage.

Claims (5)

1. An intruder alarm comprising: a sensor responsive to air disturbances to generate a sensor signal; filter for removing from the sensor signal components generated in response to high frequency air disturbances; an amplifier for amplifying the output signal from the filter; a comparator for producing an output signal when the output signal from the amplifier exceeds a threshold level, the threshold level being adjusted continuously in response to variations in the ambient conditions; and trigger circuitry operating a sounder in response t= an output signal from the comparator.

2. An intruder alarm in accordance with claim 1 or airn 2, the alarm being switchable between alternative modes of operation; including a delayed mode, wherein a delay is introduced between triggering the alarm and operating the sounder, and an instant mode, wherein the sounder is operated as soon as the alarm is triggered.

3. An intruder alarm according to claim 2, further being switchable to a test mode of operation, in which the duration of operation of the sounder is limited approxiately three seconds, for use in determining the sensitivity of the alarm.

4. An intruder alarm according to claim 1 or claim 2, further including a latch circuit to maintain operation of the sounder for a predetermined period after the alarm has been triggered.

5. An intruder alarm substantially as described herein with reference to the drawings.