INTRUSION DETECTION SYSTEM RESPONSIVE TO INTERRUPTION OF A TRANSMITTED BEAM

• US 3,704,461 A
• Filed: 03/25/1970
• Issued: 11/28/1972
• Est. Priority Date: 03/25/1970
• Status: Expired due to Term

• Current Assignee: Donald E. Slater, Edward A. Rose, Richard H. Krohn
• Original Assignee: Donald E. Slater, Edward A. Rose, Richard H. Krohn
• Inventors: Rose, Edward A., Krohn, Richard H., Slater, Donald E.
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Curtis, Marshall M.
• Application Number:
  US05/022,183
• Time in Patent Office: 979 Days
• Field of Search: 340/258A,258B,258,258D 250/83.3IR,199,221 325/364,419,329,346
• US Class Current: 340/556
• CPC Class Codes: G08B 13/183 by interruption of a radiation beam or barrier

Abstract

An infrared intruder detection system operable under severe weather conditions including fog, rain and snow over great distances exceeding 500 feet in the severest weather comprises a pulsed infrared transmitter and a self-contained spaced apart self-synchronized infrared receiver. Upon interruption of the transmitted beam for a predetermined period, the receiver generates an alarm in response to the remaining ever present signals due to ambient light and thermal instrument noise. When the beam is not interrupted, a local synchronous oscillator will track the incoming signal pulses. In the presence of an intruder, the pulsed beam is no longer received and the oscillator is unable to track the resulting amplified noise signal which arises from ambient light and thermal instrument noise. This results in the generation of an error signal at the output of the demodulator to which the oscillator is coupled. By the inclusion of appropriate bandpass amplifiers, filters and integrating circuits set to respond to predetermined thresholds coupled to the output of the demodulator, the receiver is able to discriminate between an intruder and such momentary interrupts as are caused by birds, falling leaves and blowing bits of paper which otherwise might cause false alarms.

Claims

1. An intrusion detection system comprising, a transmitter for radiating a modulated beam of radiation, a receiver, spaced apart from said transmitter for receiving said modulated beam, said receiver including:

a detector adapted to detect said beam, a circuit responsive to said detector for deriving waves with a substantially uniform duty cycle in response to said modulated beam being received and of random rapidly varying duty cycle in response to said beam being interrupted, means connected to receive the output of said circuit for tracking the nearly uniform duty cycle waves derived from said circuit and not tracking the rapidly varying random duty cycle of said waves, said tracking means including means for deriving a further signal having a first amplitude range indicative of the duty cycle being substantially uniform and a second amplitude range, different from the first amplitude range, indicative of the duty cycle rapidly varying, and means responsive only to the second amplitude range of the further signal for deriving an indication of the beam being interrupted by an intrusion.

2. The system of claim 1 wherein said tracking means includes a relatively high-Q synchronous oscillator responsive to said waves derived by said circuit.

3. The system of claim 2 wherein said tracking means further includes demodulator means responsive to oscillations derived by the oscillator and the waves derived by said circuit for deriving another signal having an amplitude dependent on the phase difference of said oscillations and waves.

4. The system of claim 3 wherein said tracking means includes means responsive to the demodulator means for integrating the another signal to selectively derive the first and second amplitude ranges of the further signal.

5. The system of claim 1 wherein said circuit includes means for deriving said waves as rectangular waves having leading and trailing edges dependent upon the amplitude of the received beam.

6. The system of claim 5 wherein said tracking means includes means for deriving a signal having an amplitude that is a function of the signal to noise ratio of the received beam such that the signal amplitude has a relatively low value in response to a high signal to noise ratio of the received beam and a relAtively high value in response to a low signal to noise ratio of the received beam.

7. The system of claim 1 wherein said tracking means includes means for deriving a signal having an amplitude that is a function of the signal to noise ratio of the received beam such that the signal amplitude has a relatively low value in response to a high signal to noise ratio of the received beam and a relatively high value in response to a low signal to noise ratio of the received beam.

8. The system of claim 7 wherein said tracking means includes means for integrating the signal having an amplitude that is a function of the signal to noise ratio.