Count discriminating fire detection system

• US 4,151,522 A
• Filed: 06/15/1977
• Issued: 04/24/1979
• Est. Priority Date: 06/17/1976
• Status: Expired due to Term

• Current Assignee: Hochiki Corporation
• Original Assignee: Hochiki Corporation
• Inventors: Yamauchi, Yukio
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US05/806,881
• Time in Patent Office: 678 Days
• Field of Search: 340/227 R, 340/228 R, 340/237 S, 340/309.1, 340/584, 340/587, 340/593, 340/595, 340/628, 340/629, 340/630
• US Class Current: 340/587
• CPC Class Codes: G08B 29/185 Signal analysis techniques for reducing or preventing false alarms or for enhancing the reliability of the system

Abstract

A fire detector senses the change in a physical parameter indicative of a fire such as smoke, heat or the like and produces detection pulses in synchronism with an oscillator circuit when the change in the physical parameter exceeds a predetermined amount. A counting means counts the detection pulses and produces an output which triggers an alarm circuit when a predetermined number of consecutive detection pulses are counted. The count of the counting means is reset by a reset means whenever the detection pulses are nonconsecutive, that is, whenever an oscillator pulse is received without receipt of a corresponding detection pulse. By this means intermittent changes in the physical parameter which indicate some false alarm signal rather than a real fire do not produce the required number of consecutive detection pulses and do not set off the alarm circuit. On the other hand when a real fire occurs the circuit always counts detection pulses from the beginning of the detection of a fire because the counting means is reset each time a series of consecutive detection pulses less than the predetermined number ends.

Claims

1. A count discriminating fire detection system comprising, in combination;

an oscillator circuit for generating oscillating pulses at a predetermined frequency;

a fire detection means connected to said oscillator circuit for detecting a change of more than a predetermined amount in a physical parameter indicative of a fire for producing detection pulses synchronous with said oscillating pulses;

a counting means having an input terminal connected to said fire detection means and a reset terminal, for counting said detection pulses for producing an output upon counting a predetermined number said detection pulses and for resetting said counting upon application of a signal to said reset terminal;

a reset means having a first input terminal connected to said fire detection means, a second input terminal connected to said oscillator circuit and an output terminal connected to said reset terminal of said counting means, for applying a signal to said reset terminal of said counting means upon receiving an oscillator pulse without receiving a detection pulse; and

an alarm circuit connected to said counting means for producing a fire alarm when said counting means produces an output.

2. A fire detection system according to claim 1, wherein said fire detection means comprises:

a fire sensor for detecting a change of more than a predetermined amount in a physical parameter indicative of a fire for producing a d.c. output; and

an AND gate having a first input connected to said oscillator circuit and a second input connected to said fire sensor for converting said d.c. output of said fire sensor into detection pulses.

3. A fire detection system according to claim 1, wherein said reset means comprises an exclusive-OR gate and wherein said counting means comprises a resettable counter.

4. A fire detection system according to claim 3 wherein said reset means further comprises a delay circuit connected to the output of said exclusive-OR gate for absorbing an output of said exclusive-OR gate due to small time differences between said oscillating pulses and said detection pulses.

5. A fire detection system according to claim 3 wherein said resettable counter comprises a fixed voltage source and a shift register having a data terminal connected to said fixed voltage source and a clock input terminal connected to the output of said fire detection means.