GB2108301A

GB2108301A - Intruder alarm device

Info

Publication number: GB2108301A
Application number: GB08125047A
Authority: GB
Inventors: Barrington John Leonard White
Original assignee: MUNFORD AND WHITE Ltd
Current assignee: MUNFORD AND WHITE Ltd
Priority date: 1981-08-17
Filing date: 1981-08-17
Publication date: 1983-05-11
Also published as: GB2108301B

Abstract

In an intruder alarm device using, the Doppler effect, to reduce the incidence of false alarms, a circuit (16) operates in response to the receipt of an apparent alarm signal by a receiver to stop the wave transmission by a transmitter (10) and to start a timer (20). If the apparent alarm signal terminates when the wave transmission stops, at the end of the timed period a further circuit (22) provides an alarm signal. If the apparent alarm signal continues when the wave transmission stops, indicating a spurious response e.g. due to noise or electrical interference, the timer (20) is reset and the provision of the alarm signal is prevented. <IMAGE>

Description

SPECIFICATION Intruder alarm devices This invention relates to intruder alarm circuits of the kind having a transmitter emitting a wave transmission and a receiver responsive to the recognition of a Doppler effect in a reflected wave from a moving object. Such alarm circuits may be of the ultrasonic kind or of the microwave kind.

Alarm circuits responsive to the Doppler effect are subject to false alarms in certain circumstances. For example, circuits using ultrasonic waves are subject to false alarms if noise or electrical interference takes place with a frequency content close to the fundamental transmission frequency or harmonics of the transmission frequency. The object of the present invention is to reduce the incidence of false alarms.In accordance with the invention, an intruder alarm circuit of the kind having a transmitter emitting a wave transmission and a receiver responsive to an alarm signal in the form of a Doppler effect in a wave reflected from a moving object, includes means responsive to an apparent alarm signal to stop the operation of the transmitter and means operable if the apparent alarm signal terminates when the wave transmission stops to initiate an alarm indication and operable if the apparent alarm signal continues when the transmission stops to preventthe initiation of an alarm indication. In the preferred embodiment, an apparent alarm condition in a receive signal starts a timer, as well as stopping the transmission of the wave. If the apparent alarm signal then stops, at the end of the time period the timer causes the alarm indication to be given.If the apparent alarm signal continues when the transmission stops, the timer is reset.

In one form of the circuit embodying the invention, an apparent alarm signal which continues when the transmitted wave stops is used to trigger a further circuit which reduces the performance level of the equipment to an extent such that although it will still respond to movement at short range, it is insensitive to weak spurious signals. This arrangement prevents the unit from being totally disabled by an intruder jamming it with spurious signals from a distance.

In order that the invention may be better understood, one example of a circuit embodying the invention will-now be described with reference to Figures 1 and 2 of the accompanying drawings.

In Figure ian ultrasonic transmitter 10 has a transducer 12 for transmitting an ultrasonic wave into the space to be protected. A Doppler amplifier 14 receives reflected waves and, if it recognizes a Doppler condition in the received wave its output changes the state of a circuit 16, which may be a mono-stable circuit or a bistable circuit. A change of state in circuit 16 results in sending a stop signal by path 18 to the transmitter 10 and to initiating the operation of a timer 20.

If the alarm condition resulted from true movement, and not from ultrasonic noise or electrical interference, the output from Doppler amplifier 14 will cease when the transmitter 10 switches off. In this case, operation of the timer continues and at the end of the timed period a circuit 22 is triggered and this results in an alarm indication signal 24.

If however when the wave transmission ceases the apparent alarm condition persists, as would be the case if it resulted from the spurious conditions mentioned above, then a gate circuit 26 is opened (rendered conductive) by circuit 16, passing the continuing alarm condition signal from the output of amplifier 14 to the reset terminal of timer 20. This prevents the timer from progressing to the point at which circuit 22 is triggered and consequently no alarm indication results on output 24. In an alternative arrangement, the output of gate 26 resets circuit 16, as indicated by the dotted line, and this in turn resets timer 20.

Figure 2 is a more detailed circuit diagram of a part of an ultrasonic intruder alarm incorporating the invention. Pulses from the doppler amplifier are present at the input of the circuit portion shown, oniy if movement occurs or if there is a spurious signal simulating the effect of movement. The signals from the doppler amplifier are integrated by capacitor C1 and trigger the C-MOS monostable circuit comprising integrated circuits IC1 and IC2 when the gate threshold is reached. Such triggering causes a 1 at the output of the circuit IC2 and capacitor C3 begins to charge through resistor R6. The capacitor C1 is now discharged by diode D2 and resistor R4.At the same time, the output of circuit IC1 goes low and this acts through diode D4 to prevent the signal from the crystal oscillator and buffer circuits (IC5 and ICe) from reaching the transducer driving stages IC7 and IC8.

This stops transmission.

If after transmission has stopped no signal is present at the output of the doppler amplifier, capacitor C3 will charge to the trigger point of circuit 103 to cause an alarm signal to be generated when the monostable circuit comprising circuits 103 and IC4 switches on.If after transmission has stopped there is still an output from the doppler amplifier, indicating that this output is spurious, transistor Q1, which receives the doppler receiver output at its base, will conduct and cause capacitor C3 to discharge, preventing the charge on C3 from reaching the trigger point of the circuit 103. In such a case, the monostable circuit IC1-lC2 will reset after a period determined by its time constant, governed by C2 and R3, without the triggering of the second monostable circuit IC3-lC4; in this case, no alarm signals results.

The circuit is now back at its starting point with capacitor C1 and C3 discharged and the transmitter switched on.

The invention is particularly useful in installations employing a multiplicity of transmitters and receivers as these tend to have longer leads and are more subject to electrical interference. A response from any receiver stops all transmitters.

As indicated above, if desired the recognition of a spurious signal can be used to reduce the performance level of the equipment to make it insensitive to weak spurious alarms.

It will be appreciated that the invention may also be applied to a microwave intruder alarm circuit.

Claims

1. An intruder alarm circuit of the kind having a transmitter emitting a wave transmission and a receiver responsive to an alarm signal in the form of a Doppler effect in a wave reflected from a moving object, the receiver including means responsive to an apparent alarm signal to stop the operation of the transmitter and means operable if the apparent alarm signal terminates when the wave transmission stops to initiate an alarm indication and operable if the apparent alarm signal continues when the transmission stops to prevent the initiation of an alarm indication.

2. An intruder alarm circuit in accordance with claim 1, including a timer which is started by the receipt of an apparent alarm signal, a circuit operative at the end of a period determined by the timer to initiate an alarm indication, and a circuit for resetting the timer if the apparent alarm signal continues when the transmission stops.

3. An intruder alarm circuit in accordance with claim 2, further including means for restarting the transmitter if the apparent alarm signal continues when the transmission stops.

4. An intruder alarm circuit in accordance with claim 3, in which the means responsive to the apparent alarm signal to stop the operation of the transmitter includes a first monostable circuit and the means for initiating an alarm indication includes a second monostable circuit and a timing circuit through which the output of the first monostable circuit is coupled to the input of the second monostable circuit.

5. An intruder alarm circuit in accordance with claim 4, wherein the said timing circuit through which the monostable circuits are coupled includes a capacitor which is discharged, if the apparent alarm signal continues after the transmission is stopped, by a transistor connected across the capacitor.

6. An intruder alarm circuit in accordance with any one of the preceding claims, further including means whereby an apparent alarm signal which continues when the transmitted wave stops is used to trigger means for reducing the sensitivity of the equipment to receive signals.

7. An intruder alarm circuit, substantially as herein described with reference to the accompanying drawings.