Photoelectric smoke sensor

• US 4,317,113 A
• Filed: 08/11/1980
• Issued: 02/23/1982
• Est. Priority Date: 08/24/1979
• Status: Expired due to Term

• Current Assignee: Hochiki Corporation
• Original Assignee: Hochiki Corporation
• Inventors: Honma, Hiroshi
• Primary Examiner(s): Caldwell, Sr., John W.
• Assistant Examiner(s): Myer, Daniel
• Application Number:
  US06/177,250
• Time in Patent Office: 561 Days
• Field of Search: 340/628, 340/630, 250/573, 250/574, 250/575, 250/576, 250/577, 250/564, 250/565, 356/438, 356/439
• US Class Current: 340/630
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G08B 17/103 using a light emitting and receiving device

Abstract

In a photoelectric smoke sensor which produces a fire alarm signal when an amount of attenuation of received light exceeds a predetermined level due to smoke flowing into a space between light emitting and light receiving devices, the level of a receiving light signal from the light receiving device is converted by a converting level control signal. An initial value of the receiving light signal is converted into a digital quantity which is stored in a memory circuit. The digital content of the memory circuit is then converted into an analog quantity. The analog quantity is then compared with the level-converted receiving signal to produce a fire alarm signal. The analog quantity and the level-converted receiving signal are compared with each other at a predetermined period to correct the converting level control signal when there is a difference between the analog quantity and the level-converted receiving signal as a result of the comparison. With such a scheme, a change of an amount of received light due to dust or dirt of an optical system of the smoke sensor is automatically corrected. When the level correction reaches a limit, an alarm is issued for inspection and maintenance of the sensor.

Claims

1. A photoelectric smoke sensor for producing a fire alarm signal in response to light generated by a light emitting device and received by a light receiving device spaced from the light emitting device, the fire alarm signal being generated when an amount of attenuation of the received light exceeds a predetermined level due to smoke flowing into the space between the light emitting device and the light receiving device, the light receiving device producing a receiving light signal, having a level, in response to the received light, said photoelectric smoke sensor comprising:

a signal level converting circuit for varying the level of the receiving light signal produced by the light receiving device in response to a converting level control signal to produce a converted level receiving light signal;

a memory circuit, coupled to said signal level converting circuit, for storing and holding in digital form an initial value of the converted level receiving light signal;

a D/A converting circuit, coupled to said memory circuit, for converting the digital form of the initial value of the converted level receiving light signal into an analog output signal;

a first comparing circuit, coupled to said D/A converting circuit and said signal level converting circuit, for comparing the analog output signal with the converted level receiving light signal to produce the fire alarm signal when there is a level difference between the analog output signal and the converted level receiving light signal;

a second comparing circuit, coupled to said D/A converting circuit and said signal level converting circuit, for comparing the analog output signal with the converted level receiving light signal at a predetermined period and for producing a comparison output signal when there is a level difference between the analog output signal and the converted level receiving light signal; and

a converting level correcting circuit, coupled to said second comparing circuit, for producing the converting level control signal in response to the comparison output signal.

2. A photoelectric smoke sensor as claimed in claim 1, further comprising means, coupled to said converting level correcting circuit, for producing an alarm signal for maintenance and inspection when the converting level control signal reaches a limit.

3. A photoelectric smoke sensor as claimed in claim 1, wherein said signal level converting circuit includes a variable gain amplifier for receiving the receiving light signal, the gain of said variable gain amplifier being varied by the converting level control signal.

4. A photoelectric smoke sensor as claimed in claim 1, wherein said signal level converting circuit includes a variable attenuator for attenuating the receiving light signal and controllable analog switches for varying said variable attenuator, said analog switches being controlled in response to the converting level control signal.

5. A photoelectric smoke sensor for producing a fire alarm signal in response to light generated by a light emitting device and received by a light receiving device spaced from the light emitting device, the fire alarm signal being generated when an amount of attenuation of the received light exceeds a predetermined level due to smoke flowing into the space between the light emitting device and the light receiving device, the light receiving device producing a receiving light signal, having a level, in response to the received light, said photoelectric smoke sensor comprising:

a signal level converting circuit for varying the level of the receiving light signal produced by the light receiving device in response to a converting level control signal to produce a converted level receiving light signal;

a first memory circuit, coupled to said signal level converting circuit, for storing an initial value of the converted level receiving light signal;

a second memory circuit, coupled to said signal level converting circuit, for storing periodically the converted level receiving light signal at a first predetermined period;

a comparing circuit, coupled to said first memory circuit and said second memory circuit, for comparing the stored initial value of the converted level receiving light signal with the periodically stored converted level receiving light signal to produce a comparison output signal when there is a level difference between the stored initial value of the converted level receiving light signal and the periodically stored converted level receiving light signal;

a converting level correcting circuit, coupled to said comparing circuit, for producing the converting level control signal in response to the comparison output signal; and

a clock circuit for generating a clock signal, said converting level correcting circuit being controlled in response to the clock signal to produce a corrected converting level control signal at a second predetermined period longer than the first predetermined period.

6. A photoelectric smoke sensor as claimed in claim 5, further comprising means, coupled to said converting level correcting circuit, for producing an alarm signal for maintenance and inspection when the converting level control signal reaches a limit.

7. A photoelectric smoke sensor as claimed in claim 5, wherein said signal level converting circuit includes a variable gain amplifier for receiving the receiving light signal, the gain of said variable gain amplifier being varied by the converting level control signal.

8. A photoelectric smoke sensor as claimed in claim 5, wherein said signal level converting circuit includes a variable attenuator for attenuating the receiving light signal and controllable analog switches for varying said variable attenuator, said analog switches being controlled in response to the converting level control signal.