Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1988-06-17
1990-03-27
Crosland, Donnie L.
Communications: electrical
Condition responsive indicating system
Specific condition
340565, 340566, 340511, 340683, 340429, G08B 1300
Patent
active
049124552
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to alarm systems. It is a development of that described in European Pat. No. 0044725 and is primarily concerned with the local detector units which are distributed over the zone to be protected and wired back to a central control panel.
BACKGROUND OF THE INVENTION
It is important that such units be small and unobtrusive and highly reliable. It is also desirable that there should be certain operational options available, which could be factory-set or made field selectable.
One particular problem is premature triggering of an alarm. With a delicately set detector sensitive to vibrations, for example, a single shake such as may be occasioned by a passing lorry rattling a window may set it off. If an intruder was attempting entry, the disturbance would be more prolonged and repeated. It is therefore desirable to distinguish between the two types of disturbance.
It is also useful to know, when investigating a disturbance to know which of a group of detectors was excited first, but without a complex wiring arrangement back to the central control.
While the invention is designed primarily to be used in conjunction with a vibration sensitive element, there is no reason why its principles should not be applied to the processing of a disturbance signal generated by other means, such as interference of a light beam or contact with a pressure pad.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a detector for an alarm system in which an intruder generated disturbance signal is transmitted by the detector as an alarm signal to a remote station, wherein the detector has means for suppressing the transmission on a first disturbance by an intruder and for allowing such transmission on a subsequent disturbance.
Means may be provided for gating the disturbance signal through when it attains a predetermined level, the suppressing means then being arranged normally to close the gate but open at a predetermined time after an initial disturbance signal.
In the preferred form, the suppressing means includes a clock, a level detector to which the disturbance signals are applied and means normally holding the level detector in a state such that its output closes the gate but which is nullified a set time after the clock has registered the disturbance signal. Thus, further disturbance signals after attaining a set predetermined level open the gate.
Preferably, the disturbance signal will be formed into a pulse train before application to the gate and the clock. A counter will then receive the gated pulse train and provide an alarm signal trigger, but only after a given number of pulses. This will suppress noise. The clock will be arranged to reset the counter after said set time.
There are selectable means for blocking the alarm signal trigger, the counter being arranged to deliver an alternative alarm signal trigger after a greater given number of pulses. By selecting this feature, a substantially delayed alarm may be generated.
Preferably, means for generating the alarm signal from the disturbance signal will remain activated unless reset, even when the disturbance signal has ceased. However, there may be a selectable reset facility for the alarm generating means which will use the clock output after said set time. Thus, the alarm signal will automatically be cut off at that point.
Conveniently, there will also be facility for resetting the alarm generating means by remote control. This may be adapted to respond to any change of state on a remote control line, but advantageously it will incorporate a delay whereby transient signals are suppressed.
Another selectable resetting facility for the alarm generating means is provided by means responsive to the supply or restoration of power to the detector. The latter may be arranged, as is conventional, to trigger the alarm if the power is cut off and it is convenient that as soon as power is restored no special measures need be taken to shut down the alarm.
Preferably, the suppressing mean
REFERENCES:
patent: 3133276 (1964-05-01), Miller et al.
patent: 3733598 (1973-05-01), Kato
patent: 3909826 (1975-09-01), Schildmeier et al.
patent: 4333093 (1982-06-01), Raber
Crosland Donnie L.
Weyrad (Electronics) Limited
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