US3866198A

US3866198A - Ultrasonic intrusion detection systems employing turbulence discrimination

Info

Publication number: US3866198A
Application number: US407062A
Authority: US
Inventors: Leopold Cohen
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-10-17
Filing date: 1973-10-17
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11

Abstract

There is disclosed an ultrasonic intrusion detection system which is selectively activated and deactivated according to the existence of air turbulence effects as provided by a heating and/or cooling system. The ultrasonic system is deactivated when a heating system is operated and activated when a predesired temperature is achieved resulting in the heating system being inoperative. The operation of the system therefore prevents the ultrasonic intrusion system from operating during air turbulence.

Description

United States Patent Cohen [76] Inventor: Leopold Cohen, 518 Seven Oaks Rd., Orange, NJ. 07051 [22] Filed: Oct. 17, 1973 [21] Appl. No.: 407,062

[52] 1.1.8. Cl. 340/258 A, 340/1 R, 340/276 [51] Int. Cl. G08b 13/16 [58] Field of Search 340/258 A, 258 B [56] References Cited UNITED STATES PATENTS 2,779,935 1/1957 Loudon et a1. 340/258 A 2,782,405 2/1957 Weisz et a1. 340/258 A 2,972,133 2/1961 MacDonald 340/258 A 3,111,657 11/1963 Bagno 340/258 A 3,573,817 4/1971 Akers 340/261 X [451 Feb. 11, 1975 3,629,812 12/1971 Amato 340/258 A 3,638,210 1/1972 Hankins et a1. 340/258 A 3,725,888 4/1973 Solomon... 340/258 A 3,727,216 4/1973 Antonio 340/258 A Primary Examiner-David L. Trafton Attorney, Agent, or Firm-Arthur L. Plevy [57] ABSTRACT 10 Claims, 3 Drawing Figures 551 BQ EE EEH/L'HM e I n M? :2 t l FORCED AIR A5 I I HEA'HNG THERMosTAT UHEQSONIC l 1 AND/0R CONTROL I I AIR cono- I 20 I ITIONING I I CONTROL 1 l SYSTEM INTRUDER I I DETECTOR I t 26 i INTRUSION I I BACKUP I I 'svsrsm I l 30 l L. .J

CENTRAL OFFICE TNTTHTED 1 I975 3.866.198

- SHEET 2 OF 2 HEATING SYSTEM 80 f To Q 8| ULTRASONIC '1 82 SOURCE THERMO- i STAT I. O"--- FROM ULTRASONIC SOURCE I87 1 DET. DOPPLER T I20 DET.

TO COMMUNlCATlQN LINE ULTRASONIC INTRUSION DETECTION SYSTEMS EMPLOYING TURBULENCE DISCRIMINATION BACKGROUND OF INVENTION This invention relates to intruder detection systems employing ultrasonics and more particularly to such a system incorporating structure to prevent spurious or false alarms.

The prior art is replete with a number of systems commonly designated as intrusion detector system and employed on premises to indicate the undesired presence of an intruder as a burgular. As such, the systems in general use employ many different techniques for detection of such an intruder and for notifying the proper authorities or for otherwise activating an alarm.

Thus, the prior art discusses microwave systems which employ radar-like signal frequencies, light-beam systems, and ultrasonic systems.

When one is concerned with ultrasonics, one is operating with relatively low frequencies and in the upper audio range. Conventionally, ultrasonic signals are not audible to the human ear, but are easily and economically generated at relatively large power levels. Ultrasonic detection is an extremely effective system in monitoring enclosed areas and has great advantage, in that they are relatively reliable and consistent.

Generally, the term ultrasonics (or Supersonics) refers to sound vibrations, say for example above about 18,000 Hz. In any event, researchers have attained ultrasonic frequency of the order of million Hz or cycles per second. Ultrasonic frequencies can be provided by a number of conventional techniques such as the Galton pipe, magnetostriction generators or piezoelectric devices and are used in many technical applications, which, of course, include intrusion detection schemes. In such schemes an ultrasonic transmitter produces a ultrasonic frequency which is utilized to saturate an area by propagating ultrasonic waves. The intrusion systems employ the Doppler effect to detect the presence of an intruder by monitoring movement.

Basically, the Doppler effect is provided when a vibrating source of waves (such as an ultrasonic transmitter) is impinging on a moving target (such as an intruder). Generally as the source approaches the target, the frequency observed at a receiving location is higher than the frequency emitted by the source. If the source is receding, the observed frequency is lower. It is, of course understood that motion is relative and hence, the source is stationary and the observer or target moves with respect to the same.

The sensitivity of an ultrasonic Doppler system employing Doppler detection is extremely great as indicated, as well as the fact, that the system will not penetrate walls or normal barriers as the ultrasonic waves bounce off or impinge from such surfaces. Hence, one can accurately monitor a closed or walled location, such as a room, store, warehouse and so on, without fear that the waves will penetrate beyond the area as is the case in certain microwave systems.

However, it is well known that ultrasonic frequencies are randomly produced by all types and sorts of vibrating equipment as well as by harmonics of whistles and such vibrations. Thus, the prior art, fully cognizant of the utility of such systems, also understood that discrimination had to be made of such spurious sources to avoid false alarms.

Literally, there are numerous factors which can give rise to false signals. Some of these factors are relatively easy to overcome as they are related to the magnitude of the Doppler signal received and can be adjusted for by the use of limiting circuits and so on.

In any event, there are certain system noises which exist which are extremely difficult to discriminate against. These spurious signals are generated both within the ultrasonic range and which also produce Doppler frequency signals of a magnitude sufficient to trigger the ultrasonic alarm. Examples of interfering sources encompass vibrating water pipes, horns, shattering glass and so on, all of which can produce undesirous false alarms.

The prior art has implemented devices and structure to avoid these problems. Most solutions utilize bandwidth limiting or frequency selection and so on to afford discrimination. Examples of typical apparatus are evidenced by US. Pat. No. 3,513,463 entitled SOUND MONITOR INTRUDER ALARM SYSTEM issued on May 19, I970 to L. A. Stevenson, Jr., et al, US. Pat. No. 3,304,457 entitled ALARM SYSTEM issued on Feb. 14, 1967 to B. Bristol III. There are of course, many other patents too numerous to mention.

In any event, a major problem and threat to the ultrasonic intrusion system resides in the action of air turbulence. Thus, normal air turbulence as that produced by the operation of a heating fan, air conditioning unit and so on, produces air currents or frequencies which are within the range of the Doppler shift or frequency caused by an intruder. Such turb ulences as generated create false signals and are extremely difficult to discriminate against. One cannot decrease the sensitivity of the system to avoid these spurious signals as the system would become inoperative. Thus, the prior art proposed electronic solutions which systems alter time constants, employ narrow band amplifiers or ignore short interval spurious signals. Examples of systems which are concerned with the problem and propose solutions are evidenced by at least the following patents:

1. US. Pat. No. 2,794,974 entitled COMPENSA- TION FOR TURBULENCE AND OTHER EF- FECTS IN INTRUDER DETECTION SYSTEMS by S. M. Bango, et al issued on June 4, 1957.

2. U.S. Pat. No. 3,111,657 entitled COMPENSA- TION FOR TURBULENCE AND OTHER EF- FECTS IN INTRUDER DETECTION SYSTEMS by S. M. Bango, et al issued on Nov. l9, 1963, and

3. US. Pat. No. 3,638,2l0 entitled INTRUSION ALARM WITH TURBULENCE COMPENSA- TION by T. C. Hankins, et al issued on Jan. 25, 1972.

In spite of the proposed solutions, the problem still exists to the extent that many suppliers of ultrasonic intrusion systems, will not install the system in an area where a forced air heating system or an air conditioning system is utilized.

It is therefore an object of the present invention to provide an intrusion detection system which offers improved discrimination against air turbulence effects and which system permits operation of ultrasonics under conditions and in locations not previously available.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENT An intrusion detection system for monitoring a predetermined area when activated, is of the type employing a selected frequency transmitter and a receiver responsive to said transmitted frequency as determined by a Doppler Shift due to the presence of an intruder in said area, the combination with said system including apparatus for operating said system in said area in the presence of a generating source of air current turbulence, which source is selectively operated according to environmental conditions of said area, and which air turbulence undesirably caused said system to falsely indicate the presence of said intruder, comprising first means responsive to the operation of said selectively operated generating source to provide a control signal indicative of said operation and second means coupled to said intrusion system and responsive to said control signal for deactivating said system during the presence of said control signal, whereby said intrusion system is monitoring said area only when air turbulence indicative of the operation of said generating source is not present.

BRIEF DESCRIPTION OF FIGURES FIG. 1 is a block diagram of an intrusion detection system according to this invention.

FIG. 2 is a block diagram partly in schematic form of the intrusion system depicted in FIG. 1.

FIG. 3 is a schematic diagram of an alternate operating embodiment of a control system according to this invention.

DETAILED DESCRIPTION OF FIGURES Referring to FIG. 1, there is shown an area which is to be protected by an intrusion system according to this invention. Area 10 may be a store area, a room or any other typical walled establishment. The area as ew" i a ly nq ud a qr q eirhsafinsansb /or air conditioning system 12. Such systems are well known and are used for heating or cooling the area 10 according to the temperature of the environment as monitored by a thermostat control 14. Before describing the operation of the system of FIG. 1 in greater detail an explanation of the difficulties to be experienced by an ultrasonic detection system due to the presence of the heating system or generating source of air currents 12 will be given.

Such systems 12 operate in conjunction with a fan or blower to circulate and move warmed or cooled air throughout the area '10.

Thus, when the heating system 12 is operating, the fan associated therewith causes the air within the area 10 to accelerate or move rather rapidly and sporadically. The emitted ultrasonic waves from the transmitter also circulate or propagate throughout the area 10. The radiations or ultrasonic waves must propagate through the turbulent air and if the air is moving, the radiations are affected exactly as if they had been reflected from a moving object such as an intruder. Therefore a change in temperature of the area 10 will cause the velocity to vary, and such a change in velocity or frequency appears as a Doppler shift.

Normal temperature changes are ery low and present no particular problem, but the operation of the fan incorporated in the heating system 12 causes rapid changes which appear as valid Doppler shifts.

The prior art decreased the sensitivity of the detection system to circumvent such effects resulting in a substantial decrease of system efficiency. Others, as described in US. Pat. No. 2,794,974 employed the frequency range associated with turbulence and discriminated against the same. This approach is also unacceptable as the heating systems 12 have progressed in operation, use more powerful fans and motors and in essence, provide spurious Doppler by products over the major portion of the ultrasonic detection systems operating spectrum. Thus, in actual practice, one cannot conveniently without a loss in efficiency, operate an ultrasonic system in the presence of such turbulence.

Accordingly, FIG. 1 depicts an ultrasonic intrusion system 15. The system 15 is typical and is well known and generally comprises an ultrasonic transducer or transmitter assembly 16 which provides a frequency in the ultrasonic range (typically from 19 to 30 KHZ). The transmitter 16 may be a piezoelectric microphone and so on and is energized by typical transistor or vacuum tube amplifying and oscillating circuitry.

The transmitter 16 conventionally saturates the area 10 with ultrasonic propogations. A receiving antenna or transponder 17 is also included in the system 15. The receiver unit 17 may be the same transponder as the transmitter 16 and the same are typically time-shared. In any event, the receiver is responsive to reflected ultrasonic transmissions and if an intruder is present, the detecting circuitry associated with the receiver is responsive to the well known Doppler shift to provide an alarm signal representative of the presence of an intruder. This signal is evidenced by module 20 designated as Intruder Detect. The presence of an alarm or a valid Doppler shift may be converted to an AC signal for transmission over a conventional communications link as a telephone circuit. For example, the operation of the intrusion system may be monitored at a remote location, such as a police station or an independentsecurity agency. Hence, if an intruder is present and a valid Doppler shift is detected via module 20, a signal is sent to the remote location to notify the authorities. If a telephone line is used this signal may include modulation on a typical telephone carrier frequency as a 2,500I-Iz signal, or the signal may be a predetermined signal, say a unmodulated signal within the telephone line bandwidth (300 Hz to 3,000 Hz). Alternatively, it is understood that the output of intruder detect 20 may be coupled directly to an alarm as a bell and so on to create an alarm condition at the area 10.

Typically a telephone is used for remote monitoring and as such a normal mode signal is sent when there is no intrusion, to ascertain that the area 10 is in fact being monitored and to determine that thesystem is operating. By way of example only, assume that module 20 provides a steady 1,000Hz signal via the telephone unit, which may be a modem and thence via the telephone channel or lines 24 to a central office or remote location. During a valid Doppler shift, the detection unit 20 may convert this to 2,000 Hz or may provide a modulation on the l,000 I-Iz carrier. This condition is detected at the remote location and serves to notify the authorities. Such techniques are typical and well known and the prior art is replete with many examples of typical systems.

Also included in FIG. 1 is a control circuit 26 which has an input coupled to the thermostat 14 and an output coupled to the ultrasonic intrusion system 15. Another output of the control circuit 26 is coupled to a separate input of the Intruder Detection Module 20. There is also included a system designated as a Intrusion Back-up System 30. The Intrusion Back-up system 30 has an input coupled to control circuit 26 and an output coupled to the intrusion detector 20. Specifically, the details of operation of the apparatus of FIG. 1 can be best described as a dual-mode operation and will be discussed as such.

MODE 1 HEATING SYSTEM IS ON AND UL- TRASONIC SYSTEM IS OFF It has been determined that reliability cannot be achieved in the presence of turbulence as generated by the heating and cooling system operation. Thus by monitoring the thermostat 14, one can easily ascertain that the heating system 12 is operating. The control circuit which may be a switch activated by the thermostat serves to deactivate the ultrasonic system. Deactivation of this system can be accomplished in a number of ways, as by removing bias from the transmitter unit 16 and or the receiver, blocking a signal and so on. During this condition there can be no interference due to turbulence as the ultrasonic system ceases operation. The control circuit 26 simultaneously serves to generate the normal signal via the intruder detect module 20, so that the central office does not receive an intruder alarm. In the above example, this control circuit would therefore provide a 111MHz signal when the ultrasonic system 15 is deactivated.

The control circuit 26, during this condition may then simultaneously activate the Intrusion Back-up system 30. System 30 is also a conventional type such as a light beam security system and operates only when the ultrasonic system 15 is deactivated.

The system 30 is immune to turbulence, but may possess other undesirable features as compared to the overall more reliable ultrasonic system. Hence, while the system 15 is off the system 30 provides back-up support and if a light beam is interrupted or otherwise, the back-up system 30 provides a alarm signal via module 20 notifying the authorities of the intrusion, as above indicated.

The philosophy of operation and utility of the abovedescribed system is as follows:

1. There can never be turbulence interference as the ultrasonic system 15 is off when the heating system IS on.

2. It is recognized that a heating or cooling system, in the majority of applications does not operate continuously. This is especially true in temperate climates and further valid when one considers that the major concern of intrusion occurs in the late hours or in the evening when the temperature is more or less stabilized. Thus, the heating or cooling system is on for relatively short intervals thereby assuring that the ultrasonic system is monitoring the area 10, at highest efficiency during all times the heating system is off. In any event, it is recognized that certain conditions of weather might cause the heating system to operate continuously, thus eliminating ultrasonic protection for extended periods. This condition is rare in temperate climates, as is easily ascertained.

3. The system still provides intrusion detection when the ultrasonic system is off via system 30, which because of the system type is immune to spurious conditions as would effect the ultrasonic system. MODE II THE HEATING SYSTEM IS OFF AND THE ULTRASONIC SYSTEM IS ON As indicated above when the temperature of the environment of area is within the desired range the heating system 10 is not operating as determined by typical thermostat 14 operation. Thus the control circuit 25 activates the ultrasonic system 15 and disables or deactivates system 30, this action results in a normal ultrasonic operative mode and the area 10 is so secured.

FIG. 2 depicts certain aspects of FIG. 1 in greater detail.

A typical thermostat 40 is shown and numerous examples are available commercially. Generally thermostat 40 responds to temperature and when the temperature falls below the desired setting, a switch is closed to activate the heating or cooling apparatus, which condition determines the inception of an air turbulence condition. Shown coupled to thermostat 40 is a switch 41. Switch 41 is normally included within the thermostat 14. In any event, when the heating system is not operating, switch 41 is open as shown, as soon as the thermostat indicates a need for operation of the heating system, switch 41 is closed, as shown in the dashed line position.

It is, of course, understood that this description is by way of example only and the switch can operate in any other manner to achieve the desired results. In any event, closure of switch 41 places a ground potential on the input of a gate as a transistor or diode logic circuit 43, while simultaneously energizing a source of infrared energy, depicted as a lamp assembly 44. Such infrared sources as 44 are numerous, as light emitting diodes, lamps and so on and many examples of light beam apparatus are well known and available. While FIG. 2 shows one such source 44, it is understood that switch 41 can activate a plurality of such sources.

The radiation emanating from source 44 is directed about the area to be protected by suitable lens systems and so on and caused to impinge upon a number of strategically placed photo detectors as 45, 46 and 47. Each detector as 45, 46 and 47 is illuminated by a source as 44; if an intruder caused the interruption of the radiation by placing his person between the beam and the detector, an alarm indication is detected according to a predesired interruption via detecting

modules

48, 49 and 50. The outputs of these detectors are coupled to a common OR gate 53 Thus, if any one or more beams is interrupted, this indication is transmitted via gate 53 and thence via gate 54 as an alarm. It

is noted that gate 54 is enabled only when switch 41 is closed. Thus, during ultrasonic operation one cannot receive or generate an alarm from the back-up light beam system.

When switch 41 is closed, lead 60 disables the ultrasonic transmitter 61. As indicated, there are numerous techniques for accomplishing this.

It is also seen that lead 60 is also directed to a NOR- MAL CONDITION SIGNAL module 63. This module may be a conventional transistor oscillator and provides the exact normal signal to the communications line via OR gate 64, during back-up intrusion operation. Thus, closure of switch 41 turns on module 63 to thereby maintain a valid no intrusion signal to the communications line or to prevent an alarm unit from sounding. As soon as the heating unit is turned off via thermostat 40, switch 41 opens as shown. The back-up light system is deactivated by turning off the infrared source 44 and blocking gate 54 :as this condition appears as a beam interruption. Since gate 54 is deactivated and also oscillator 63, there can be no spurious alarm signal. In this mode (switch 41 opened), the ultrasonic system is operating. Thus transmitter 61 is propogating ultrasonic signals which are monitored for Doppler shifts by receiver and detector module 70. If an intrusion exists, an alarm signal is provided conventionally, transmitted to module 71 and thence via gate 64 to central control or to an alarm unit.

Thus, FIG. 2 depicts the above described modes, namely, when turbulence is occuring the ultrasonic system is inactive and hence one does not fear false Doppler signals. In any event, a back-up system is implemented and operating. In regard to central control, all

conditions appear normal, exee r'ir an intrudei' is within the area to be protected and he disrupts a light beam, an alarm is sent. When the heating system is off, the back-up system is inactivated and the ultrasonic system operates to provide protection. It is understood that under most normal conditions, this will be the case and ultrasonic protection will afford the majority of operating time protection.

In any event, it is also clear that due to the random operation of the thermostatically controlled heating system as further controlling both the ultrasonic system ndin 2% .r923m maasaebiexzseasstaiwte q beyond the complete discrimination of air turbulence.

Clearly one of the prime disadvantages of a light beam security system is that an intruder can monitor the premises with the use of special glasses and accurately determine, prior to entry, where the beams are positioned. In the above system this is extremely difficult as the beams are activated only during the time the ultrasonic system is off.

Furthermore, the beams are off when the ultrasonic system is on, and even if an intruder was successful in locating and thence avoiding the beams, he would eventually be detected during the ultrasonic mode.

FIG. 3 is included to show the diversity of the system and to depict an alternate mode of implementing operation, it being understood that numerous alternatives are so available to one skilled in the art.

Briefly, a thermostat 80 monitors the environment v and as desired, operates the heating or cooling system.

When such operation is not afforded, switch 81 closes inactivating the ultrasonic system via gate or amplifier 82. A plurality of infrared or other

light sources

83, 84 and 85 are energized with switch 81 closure. Thus, a plurality of beams of radiation are propagated to photodetectors 86 strategically positioned about the area. If an intruder interrupts a beam, the photodetector 86 detects interruption and via a detector as 87 or 88 energizes a common gate 89. Gate 89 then activates module 90 designated as a Doppler modulator. The function of 90 being to provide the normal signal immediately upon switch 81 closure via activation of the modulator 90 by gate 91. Thus the modulator 90 with an oscillator 95, prior to activation of gate 89 provides the normal signal to the alarm via oscillator 95 as coupled to an alarm gate 100. As soon as gate 89 indicates a beam interruption modulator 90 causes oscillator 95 to be modulated with a Doppler shift, thus gate 100 provides the exact signal to the communications control as would be provided via lead during an intruder detection signal as provided by the ultrasonic system.

It is, of course understood that the thermostat control can be alternate to the normal control and the user can have the option of keeping the temperature at a mini- 'a'cc'ordingto environmental conditions of said area,"

and which air turbulence can undesireably cause said intrusion system to falsely indicate the presence of said intruder, comprising:

a. first means including a thermostat responsive to the operation of said selectively operated generating source to provide a control signal indicative of said operation and therefore of said air turbulence,

b. second means coupled to said intrusion system and responsive to said control signal for deactivating said system during the presence of said control signal, whereby said intrusion system is monitoring said area only when said selectively operated source is not operating as determined by said environmental condition.

2. The apparatus accordng to claim 1 wherein said intrusion detection system is of the ultrasonic type.

3. The apparatus according to claim 1 wherein said generating source of air current is a forced air heating or cooling means.

4. An ultrasonic intrusion detection system of the type employing an ultrasonic transmitter for propagating an ultrasonic signal within an area to be protected and a receiver responsive to a change in said transmitted signal evidenced by a Doppler shift caused by the movement of an intruder within said area, in combination therewith apparatus for selectively energizing said system during the absence of propagating air currents which undesirably produce false Doppler shifts, comprising:

a. a thermostat to provide a control signal indicative of the presence of propagating air currents, and

b. selectively operated switching means coupled to said ultrasonic system and operative in a first mode to deactivate said system in response to the generation of said control signal and in a second mode to activate said system during the absence of said air currents as monitored by said monitoring means.

5. The apparatus according to claim 4 further comprising:

a. a back-up intrusion system capable of being selectively activated and deactivated and operative when activated to protect said area by means uneffected by said air currents, and

b. means coupling said back-up intrusion system to said selectively operated switching means to activate said system only during said first mode, to thereby assure said area is continuously protected.

6. The apparatus according to claim 4 where said propagating air currents are generated by a forced air heating, air conditioning unit employing a fan causing 65 said propagating air currents to circulate.

7. The apparatus according to claim 6 wherein said monitoring means comprises a thermostat responsive to the temperature within said area as determined by the operation of said forced air heating or air conditioning unit.

8. An intrusion detection system, comprising:

a. an ultrasonic intrusion detection system employing b. a source of circulating air currents operative in a first mode to alter the temperature of said area by circulating forced air currents and in a second stand-by mode determined by the absence of said air currents, said air currents characterized by possessing a velocity indicative of Doppler shifts,

. monitoring means including a thermostat responsive to the operation of said source in said first mode to provide a control signal indicative of such operation,

. control means responsive to said control signal and coupled to said intrusion system to selectively deactivate said intrusion system only during the presence of said control signal whereby said intrusion system is operative only during the absence of said interfering air currents.

9. The system according to claim 8 further compris- 10 ing:

a. a communications channel coupled to said intrusion system and having an input end responsive to the generator of said Doppler shift to provide a predetermined signal at an output end of said channel for indicting the presence of said intruder,

b. means coupling said intrusion detection system to said input end of said channel, said means including means for providing a normal operating signal at said input end when said Doppler shift is not generated,

c. a selectively operated back-uip intrusion system of the type unaffected by the propagation of said air currents and responsive to said control signal to be selectively operated during said first mode and to be deactivated during the absence of said control signal, and

d. means coupling said back-up intrusion system to said input end of said communications channel during said first mode.

10. The apparatus according to claim 9 wherein said back-up system is of the radiation beam generating type for providing a beam of energy about said area, the interruption of said beam being indicative of the presence of an intruder.

Claims

1. In an intrusion detection system for monitoring a predetermined area, said system, when activated is of the type employing a selected frequency transmitter and a receiver responsive to said transmitted frequency as determined by a Doppler shift due to the presence of an intruder in said area, the combination therewith including apparatus for operating said system in said area in the presence of a generating source of air current turbulence, which source is selectively operated according to environmental conditions of said area, and which air turbulence can undesireably cause said intrusion system to falsely indicate the presence of said intruder, comprising: a. first means including a thermostat responsive to the operation of said selectively operated generating source to provide a control signal indicative of said operation and therefore of said air turbulence, b. second means coupled to said intrusion system and responsive to said control signal for deactivating said system during the presence of said control signal, whereby said intrusion system is monitoring said area only when said selectively operated source is not operating as determined by said environmental condition.

4. An ultrasonic intrusion detection system of the type employing an ultrasonic transmitter for propagating an ultrasonic signal within an area to be protected and a receiver responsive to a change in said transmitted signal evidenced by a Doppler shift caused by the movement of an intruder within said area, in combination therewith apparatus for selectively energizing said system during the absence of propagating air currents which undesirably produce false Doppler shifts, comprising: a. a thermostat to provide a control signal indicative of the presence of propagating air currents, and b. selectively operated switching means coupled to said ultrasonic system and operative in a first mode to deactivate said system in response to the generation of said control signal and in a second mode to activate said system during the absence of said air currents as monitored by said monitoring means.

5. The apparatus according to claim 4 further comprising: a. a back-up intrusion system capable of being selectively activated and deactivated and operative when activated to protect said area by means uneffected by said air currents, and b. means coupling said back-up intrusion system to said selectively operated switching means to activate said system only during said first mode, to thereby assure said area is continuously protected.

6. The apparatus according to clAim 4 where said propagating air currents are generated by a forced air heating, air conditioning unit employing a fan causing said propagating air currents to circulate.

8. An intrusion detection system, comprising: a. an ultrasonic intrusion detection system employing a selectively operated transmitter, receiver apparatus for respectively transmitting an ultrasonic frequency and receiving and detecting a Doppler shift indicative of the presence of an intruder in a predetermined area to be protected, b. a source of circulating air currents operative in a first mode to alter the temperature of said area by circulating forced air currents and in a second stand-by mode determined by the absence of said air currents, said air currents characterized by possessing a velocity indicative of Doppler shifts, c. monitoring means including a thermostat responsive to the operation of said source in said first mode to provide a control signal indicative of such operation, d. control means responsive to said control signal and coupled to said intrusion system to selectively deactivate said intrusion system only during the presence of said control signal whereby said intrusion system is operative only during the absence of said interfering air currents.

9. The system according to claim 8 further comprising: a. a communications channel coupled to said intrusion system and having an input end responsive to the generator of said Doppler shift to provide a predetermined signal at an output end of said channel for indicting the presence of said intruder, b. means coupling said intrusion detection system to said input end of said channel, said means including means for providing a normal operating signal at said input end when said Doppler shift is not generated, c. a selectively operated back-up intrusion system of the type unaffected by the propagation of said air currents and responsive to said control signal to be selectively operated during said first mode and to be deactivated during the absence of said control signal, and d. means coupling said back-up intrusion system to said input end of said communications channel during said first mode.