Smoke detector including ambient temperature compensation

• US 5,530,433 A
• Filed: 03/29/1994
• Issued: 06/25/1996
• Est. Priority Date: 03/31/1993
• Status: Expired due to Term

• Current Assignee: Nohmi Bosai Limited (Secom Company Limited)
• Original Assignee: Nohmi Bosai Limited (Secom Company Limited)
• Inventors: Morita, Toshikazu
• Primary Examiner(s): Swarthout, Brent A.
• Assistant Examiner(s): LIEU, JULIE BICHNGOC
• Application Number:
  US08/219,488
• Time in Patent Office: 819 Days
• Field of Search: 340/628, 340/630, 340/584, 250/573, 250/574, 374/100, 374/178
• US Class Current: 340/630
• CPC Class Codes: G08B 29/185 Signal analysis techniques for reducing or preventing false alarms or for enhancing the reliability of the system
• CPC Class Codes: G08B 29/24 Self-calibration, e.g. compensating for environmental drift or ageing of components

Abstract

A smoke type fire detector accurately detects a smoke density even when an internal temperature thereof changes. An internal temperature detecting unit detects an ambient temperature at a light emitting element and a light receiving element. A correction coefficient having a value associated with the ambient temperature detected by the temperature detecting unit is used to correct an output level of the light receiving element.

Claims

1. A smoke type fire detector comprising:

a light emitting element;

a light receiving element which receives scattered light emitted from said light emitting element and scattered by smoke particles;

an amplifier for amplifying the output level of said light receiving element;

a memory having predetermined temperature correction coefficients stored therein;

a temperature detecting means for detecting an ambient temperature at said light emitting element and said light receiving element; and

,

a microcomputer coupled to said light emitting element and to said amplifier and to said memory and to said temperature detecting means, said microcomputer including (a) means for accessing said memory and retrieving a temperature correction coefficient corresponding to said ambient temperature detected by said temperature detecting means, (b) means for calculating a temperature compensated output value or a temperature compensated reference value by applying said temperature correction coefficient to a respective one of an output level of said amplifier or a given reference level, and (c) means for detecting a smoke density by comparing the temperature compensated reference value or the temperature compensate output value with a respective one of the output level of said amplifier or the given reference level;

wherein said amplifier includes semiconductor elements, and wherein said temperature correction coefficients are for correcting an overall temperature characteristic of the combination of said light emitting and light receiving elements and said amplifier.

2. A smoke type fire detector according to claim 1, wherein said temperature detecting means includes at least one diode and utilizes a temperature characteristic of said diode relative to a voltage across said diode.

3. A smoke type fire detector according to claim 1, wherein said temperature detecting means includes a transistor and utilizes a temperature characteristic of said transistor relative to a base-emitter voltage of said transistor.

4. A smoke type fire detector according to one of claims 1, 2 or 3, wherein said microcomputer includes means for storing an initial output of said temperature detecting means as a reference temperature in said memory, and performs temperature compensation using an ambient temperature calculated by computing a deviation, which is calculated using a subsequent output of said temperature detecting means and said initial output, and said stored reference temperature.

5. A smoke type fire detector according to claim 1, wherein said temperature detecting means includes a power supply switch controlled by said microcomputer.