Photoelectric intrusion detector

• US 4,942,385 A
• Filed: 12/22/1988
• Issued: 07/17/1990
• Est. Priority Date: 12/24/1987
• Status: Expired due to Term

• Current Assignee: Hochiki Corporation
• Original Assignee: Hochiki Corporation
• Inventors: Takada, Koichi, Mitsuse, Motoharu, Kobayashi, Shinji, Watanabe, Kazuo
• Primary Examiner(s): Swann, III, Glen R.
• Application Number:
  US07/288,691
• Time in Patent Office: 572 Days
• Field of Search: 340/556, 340/557, 340/555, 340/567, 340/566, 250/340, 250/395, 250/371, 250/221
• US Class Current: 340/556
• CPC Class Codes: G08B 13/183 by interruption of a radiation beam or barrier
• CPC Class Codes: G08B 29/24 Self-calibration, e.g. compensating for environmental drift or ageing of components

Abstract

A photoelectric intrusion detector for detecting the interruption by an intruder of a monitoring beam of optical radiation such as an infrared radiation includes circuitry for preventing the generation of a false alarm when the optical beam is attenuated during its propagation through space due to fog or the like. This false alarm preventing circuitry is designed so that when the attenuation of a beam of optical radiation during its propagation through space is increased due to the occurrence of fog in cloudy weather, the occurrence of a false alarm due to the attenuation of the pulsed light in cloudy weather is prevented by decreasing a comparator reference value to follow the decrease in the level of a light receiving signal, correcting the gain of AGC amplification to maintain constant the level of a received light signal or correcting the level of a received light signal which follows a received light initial value.

Claims

1. A photoelectric intrusion detector comprising:

light sensitive means for receiving a pulsed light projected from a light emitting unit at a place remote from said light emitting unit to generate a corresponding electric signal;

first comparison means for comparing said electric signal with a reference voltage to remove a noise component lower than said reference voltage from said electric signal;

circuit means for receiving an output from said first comparison means to generate an output signal having a DC level corresponding thereto;

second comparison means for comparing the DC level signal generated by said circuit means with a predetermined threshold value to generate an output when said DC level is lower than said threshsold value;

alarm signal generating means for generating an alarm output when the output of said second comparison means lasts over a predetermined storage time; and

reference voltage generating means for producing a voltage output varying to follow variations of the output from said light sensitive means with a predetermined delay time exceeding the storage time of said alarm signal generating means so as to apply said voltage output to said first comparison means as said reference voltage.

2. A photoelectric intrusion detector comprising:

light sensitive means for receiving a pulsed light projected from a light emitting means at a place remote from said light emitting means to generate a corresponding electric signal;

circuit means for receiving the electric signal generated by said light sensitive means to generate an output having a DC level corresponding thereto;

comparison means for comparing the DC level signal generated by said circuit means with a reference voltage to generate an output when said DC level is lower than said reference voltage;

alarm signal generating means for generating an alarm output when the output of said comparison means lasts over a predetermined storage time; and

reference voltage generating means for producing a voltage output varying to follow variations of the output from said light sensitive means with a predetermined delay time exceeding the storage time of said alarm signal generating means so as to apply said voltage output to said comparison means as said reference voltage.

3. A photoelectric intrusion detector comprising:

light sensitive means for receiving a pulsed light projected from a light emitting unit at a place remote from said light emitting unit to generate a corresponding electric signal;

automatic gain control amplifying circuit for amplifying the electric signal generated by said light sensitive means with a gain determined by an AGC control voltage;

circuit means for receiving the amplified output from said automatic gain control amplifying means to generate an output signal having a DC level corresponding thereto;

comparison means for comparing the DC level signal generated by said circuit means with a reference voltage to generate an output when said DC level is lower than said reference voltage;

alarm signal generating means for generating an alarm output when the output of said comparison means lasts over a predetermined storage time; and

delay circuit means for supplying the DC level signal from said circuit means as said AGC control voltage to said automatic gain control amplifying means with predetermined delay time exceeding the storage time of said alarm signal generating means, whereby an automatic gain control is performed such that the input to said comparison means is maintained at a constant level against variations in the received light signal level of said light sensitive means with a predetermined time delay within an automatic gain control range when the received light signal level of said light sensitive means is higher than a predetermined level.

4. A photoelectric intrusion detector comprising:

light sensitive means for receiving a pulsed light projected from a light emitting unit at a place remote from said light emitting unit to generate a corresponding electric signal;

circuit means for receiving the electric signal generated by said light sensitive means to generate an output signal having a DC level corresponding thereto;

memory means for storing as an initial value a DC level signal generated by said circuit means when said detector is connected to a power source;

correction factor alteration means for comparing a value of the DC level signal generated by said circuit means with the initial value stored in said memory means at predetermined intervals to generate a correction factor output varying in correspondence to a difference therebetween;

operation means for performing an operation on the correction factor output from said correction factor alteration means and DC level signal generated by said circuit means to correct said DC level signal to follow said stored initial value with a predetermined time delay;

comparison means for comparing the corrected DC level signal generated from said operator means with a predetermined reference voltage to generate an output when the DC level of said corrected DC level signal is lower than said reference voltage; and

alarm signal generating means for generating an alarm output when the output of said comparison means lasts over a predetermined storage time shorter than said predetermined interval.