Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1983-06-29
1985-07-09
Rowland, James L.
Communications: electrical
Condition responsive indicating system
Specific condition
250573, 356439, G08B 1710
Patent
active
045285551
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a light extinction smoke detector comprising a light source emitting pulsed light and a light receiver disposed in the path of light from the source so as to receive radiation attenuated by smoke particles in the path of the light. Such smoke detectors are used e.g. as fire alarms.
BACKGROUND ART
In general known light extinction type smoke detectors produce a fire-alarm signal upon detecting the extent to which the output of a photoelectric element receiving light from a light source at the opposite side of a transmission measurement path for smoke is reduced by the smoke of a fire. Therefore, this type of smoke detector has the disadvantage that the interception or interruption of a light path extending from the light source to the photoelectric element, attributable to a shift of the light path or the like, might reduce the output of the photoelectric element and generate a false alarm. The known means for eliminating this disadvantage is described below.
On the output side of the photoelectric element which receives the light from the light source that is spaced or located across a smoke passageway or transmission measurement path, there are connected a first level discriminator which detects a gradual reduction in the output level of the photoelectric element ascribable to the smoke of the fire, and a second level discriminator which detects a rapid or sudden reduction in the output level of the photoelectric element ascribable to an interruption of the light path extending from the light source to the photoelectric element or which detects the rate of variation of such output level. The output of the first level discriminator enters one of the input terminals of a logical product circuit or AND gate through a delay circuit, and the fire-alarm signal is provided by the output terminal of the logical product circuit. The output of the second level discriminator enters the set input terminal of an R/S flip-flop circuit, the output of which enters the other input termminal of the logical product circuit through a "not" or inverter circuit and is also used as a light path interception signal.
Such detection means is unproblematical in a case where the received-light output of the photoelectric element decreases slowly due to the smoke of the fire but does not fall below the second level. However, in a case where high-density smoke initially enters the light path, the output of the photoelectric element falls below the second level and when low-density smoke subsequently enters, causing the output of the photoelectric element to assume a value somewhere between the first and second levels, the output of the second level discriminator enters the input terminal of the logical product circuit through both the R/S flip-flop circuit and the "not" circuit, to inhibit the logical product circuit in providing the fire signal and simultaneously provides the light path interception signal. In addition, when low-density smoke and high-density smoke enter the light path alternatingly, the fire-alarm signal is provided if the output of the photoelectric element is above the second level. However, when the latter output falls below the second level, the second level discriminator produces an output, which inhibits the continuous provision of the fire signal and provides the light path interception signal. It is in this case inevitable that the light path interception signal be produced without the provision of the fire signal, if the delay time of the delay circuit connected to the first level discriminator is longer than the time interval during which the smoke density changes.
DISCLOSURE OF INVENTION
The invention aims at providing a smoke detector which uses simple pulsed light or modulated pulsed light as a light source instead of continuous light in order to reduce energy consumption, and which is capable of avoiding false alarms by correctly detecting and discriminating between smoke generated by a fire and interruption of the light path extending from the light sour
REFERENCES:
patent: 4185278 (1980-01-01), Lintelmann et al.
patent: 4203100 (1980-05-01), Yamauchi et al.
Patent Abstracts of Japan, vol. 2, No. 55, Apr. 21, 1978, p. 1196E78, T. Denki.
Ishii Kenji
Yasukawa Makoto
Cerberus AG
Kleeman Werner W.
Myer Daniel
Nohmi Bosai Kogyo Co. Ltd.
Rowland James L.
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