Intrusion Detection Device Utilizing Low Frequency Sound Waves And Phase Detection Techniques

Abstract

A signal in the low audio frequency range and preferably of about 400 to 600 hertz is radiated into one or more protected areas by one or more speakers, and each speaker also acts as a transducer of reflected audio energy. A substantially amplitude independent phase sensitive detector designed to operate at the audio frequency is coupled to the source of audio signals and to each speaker to produce an output signal when a reflected signal exhibits phase shift due to even relatively slight movements of an intruder in any direction in the protected area. A low band pass circuit couples a narrow low frequency band of signals from the detector to gating and integrating circuits to provide an alarm indication in response to phase changes characteristic of the intruder movement to be detected.

Parent Case Text

The present application is a continuation-in-part of copending application Ser. No. 147,985, filed May 28, 1971.

Description

The present invention relates to an improved alarm for providing an indication of the presence of an intruder in a protected area.

Intruder detection systems utilizing ultrasonic, microwave and radio frequency energy are well known. One type of system introduces radiated energy into a protected region and detects changes in incident radiation due to the presence of an intruder in the region. A widely used system is the type which mixes the reference or radiated signal with the received signal in a diode mixer, or envelope detector, to produce an alarm condition when the frequency of the received signal differs from the frequency of the radiated signal due to the presence of an intruder. Such a system can react to changes in the amplitude of the received signal as well as to frequency shifts occurring due to the Doppler effect as the intruder moves toward or away from the signal radiated. One example of this known type of system is disclosed in U.S. Pat. No. Re 25,100.

Known systems of this type are subject to several drawbacks. One serious problem is the occurrence of false alarms due to a wide variety of extraneous causes. Systems using amplitude responsive detectors can be placed in a false alarm condition by incidental energy of a frequency similar to that radiated by the system, which incidental energy can be caused by any number of external sources. Moreover, the use of ultrasonic and higher frequencies leads to problems of false alarms caused by objects and disturbances other than the presence of an intruder to be detected. For example, small moving objects such as rodents and fast-moving objects such as fans and other equipment can reflect ultrasonic and higher frequency energy and cause false alarms. In addition, small movements such as vibrations of windows and walls due to traffic noise or other causes, and even temperature changes or air movement can trigger this type of alarm. Another difficulty is that high frequency energy is not readily confined in a region such as a room or enclosure wherein protection is desired.

Since an alarm condition may require the travel of security personnel from a remote location in order to check the protected area, it can be seen that false alarms can represent a serious expense. The problem of false alarms with known equipment is so severe that in many installations it is necessary substantially to reduce the sensitivity of the equipment. However, when sensitivity of the system is reduced to the degree necessary to eliminate a large proportion of false alarms, the system may no longer be capable of reliably detecting the presence of an intruder.

Many of these difficulties are overcome in accordance with the present invention by the use of a relatively low frequency audio signal such as a signal between 100 and 1,000 hertz and preferably between 400 and 600 hertz. This low frequency audio signal is readily reflected by conventional walls, drapes and the like and thus is readily adapted to various rooms and enclosures. In addition, the signal can be radiated in many cases with pre-existing paging or intercom speakers. In addition, small objects such as rodents are not effective reflectors of signals having long wave lengths. The audio signal is easily detected and recognized by an intruder, and thus in itself discourages intrusion.

However, detectors used in known intrusion alarms exhibit the characteristic that they provide a useful output signal only when motion in the order of magnitude of several wave lengths of the radiated signal takes place. Consequently, conventional detectors impose the requirement that at least ultrasonic frequencies, if not higher frequencies, be used because lower frequencies have longer wave lengths and would permit an intruder to move large distances without detection.

Among the important objects of the present invention are to provide an improved intrusion alarm system overcoming the many disadvantages of known ultrasound, microwave and radio frequency devices; to provide a system which is highly reliable in detecting the presence of an intruder and which avoids many causes of false alarms; to provide a system which is easily installed and maintained; and to provide a system which is easily adapted to many types of environments and which is readily combined with existing paging or intercom equipment.

Another important object of the present invention is to provide an improved detector circuit which is substantially amplitude independent and which is constructed in novel manner to detect changes in phase of signals in the low audio range.

Another object is to provide a novel signal processing circuit for operating a polarized gate in response to input signals of either polarity.

In brief, in accordance with the present invention there is provided an intrusion alarm for detecting movement of an intruder in one or more protected areas including a speaker located within each protected area, together with an oscillator for generating low audio frequency signals preferably in the range of from 400 to 600 hertz. The audio signals are radiated into the protected areas by the speakers, and each speaker also receives reflected audio signals.

In accordance with an important feature of the invention, there is provided a phase sensitive detector coupled to each speaker and also coupled to the oscillator. The detector operates substantially independent of the amplitude of reflected signals to provide an output signal when the phase of a reflected signal varies due to movement of an intruder within an area.

The output signal from the phase sensitive detector is coupled through a low band pass device having a band width of only a few hertz, thereby achieving a high signal-to-noise ratio and limiting operation of the system to phase changes representative of those caused by an intruder desired to be detected. A gate circuit is operated by signals received from the low band pass amplifier to apply energy to an integrator having a predetermined time delay characteristic such that an alarm circuit is operated when the system indicates the presence of an intruder in the area. A signal processing circuit coupled between the low band pass device and the gate serves to operate the gate in novel manner in response to either positive or negative signals.

The invention together with the above and other objects and advantages thereof may best be understood with reference to the embodiment illustrated in the accompanying drawing.

The single FIGURE of the drawing is a schematic and diagrammatic illustration of an intrusion alarm constructed in accordance with the principles of the present invention.

Having reference now to the drawing, there is illustrated an intrusion alarm designated as a whole by the reference numeral 20 and embodying the features of the present invention. In general, the intrusion alarm 20 functions to create an alarm condition in response to detection of an intruder within a protected area. Although the present invention is described in connection with the securing of a room or other enclosure against unauthorized intrusion by a human intruder, it should be understood that principles of the present invention may be applied to many other uses and environments wherein detection of a predetermined type of motion within a given area is desired.

In general, the intrusion alarm 20 includes a speaker 22 adapted to be located in the room or enclosure or other area to be protected. An oscillator generally designated as 24 generates a low frequency audio signal, and a speaker drive amplifier generally designated as 26 operates the speaker 22 at the low audio frequency. The illustrated intrusion alarm 20 operates at a frequency of 460 hertz, although it is believed that signals within the general frequency range of from 100 to 1,000 hertz and preferably within the range of 400 to 600 hertz would be acceptable in carrying out the present invention.

One advantage of the intrusion alarm 20 is that the speaker 22 may comprise part of a pre-existing intercom system or paging system. Preferably the speaker 22 is a high efficiency horn type speaker, but alternatively a cone speaker may be used. In the latter case, it may be advisable to use a speaker baffle resonant at the frequency of operation of the oscillator 24. If desired, additional speakers 22 may be connected in parallel with the illustrated speaker and disposed in other rooms or enclosures to be protected. Up to at least six parallel speakers may be used without disturbing the operation of the device 20.

Many important advantages flow from the use of a low freuqency audio signal within the preferred frequency range of the present invention. In the first place, such a signal has a long wave length. Since the acoustic energy reflected from objects moving within the protected area depends on the size of the object relative to the wave length, the intrusion alarm 20 is not substantially affected by rodents or other small moving objects which are a cause of frequent false alarms in system known to the prior art. The long wave length, on the other hand, is ideally suited for detecting motion of a human intruder. Another advantage of a signal within the preferred frequency range is that the radiated energy is easily confined within a room or other enclosure by walls, windows, drapes and the like of conventional structure. Moreover, the use of a relatively low frequency in the audio range provides a deterrent to intrusion in the protected area and thus prevents undesirable confrontation situations between an intruder and security personnel.

Returning now to a general description of the intrusion alarm 20, in order to obtain a sensitive, fast and reliable output signal when motion of an intruder takes place in a protected area, there is provided a novel phase sensitive detector generally designated as 28. The detector 28 is constructed to operate at the low audio frequency of the intrusion alarm 20 and provides an output signal due to phase shift of the reflected signal received by speaker 22 while being substantially independent of amplitude variations in the reflected signal. Since any movement of an intruder within a protected area will produce phase shifts, even if the movement is along a line equidistant from the speaker 22, the detector 28 provides a reliable indication of movement within the room. Since the detector 28 is not sensitive to amplitude variation of the reflected signal, the intrusion alarm 20 is not subject to false alarms due to incident noises from extraneous sources to the extent of many prior art devices.

The phase sensitive detector 28 is coupled to the speaker 22 so that it receives a composite signal made up of the vector sum of the radiated signal and the reflected signal received by the speaker 22. In addition the detector 28 is also connected to a reference amplifier generally designated as 30 so that it receives a signal from the oscillator 24 unaffected by the reflected signal received by the speaker 22. The phase sensitive detector 28 is highly sensitive and is capable of providing an output signal due to phase shift without the necessity of the intruder moving through a distance corresponding to multiple wave lengths of the generated signal.

In accordance with the present invention, the output signal from the phase sensitive detector 28 is coupled through an impedance matching amplifier generally designated as 32 to a low band pass device generally designated as 34. Device 34 comprises an amplifier having a band pass characteristic such that it couples signals in about a five hertz range with sharp cutoff characteristics for signals above the pass band. As a result, the device 34 filters out signals provided by the detector 28 which are not characteristic of phase changes produced by motion of an intruder desired to be detected. Thus, false alarms due to such factors as traffic or thunderstorm induced vibrations and the like are largely avoided.

Connected to receive signals from the low band pass device 34 is a gate circuit generally designated by the reference numeral 36 serving to apply energy to an integrator generally designated as 38 during such time as the signals are coupled through the low band pass device 34. The integrator 38 is selected to have a time constant such that it operates a relay control circuit generally designated as 40 in response to a predetermined accumulation of energy which could result from a substantial number of small motions or alternatively from a single large motion. An alarm circuit 42 is operated by the relay control circuit 40 when an alarm condition is initiated.

Proceeding now to a more detailed description of the intrusion alarm 20, the oscillator 24 may comprise any type of signal generator capable of producing signals in the preferred frequency range. As illustrated, the oscillator 24 includes a transistor 44 with its emitter electrode coupled to ground through a resistor 46 and with its collector electrode coupled to a tank circuit including resistors 48, 50 and 52 and capacitors 54, 56, and 58. The tank circuit is coupled to the base electrode of transistor 44 by a capacitor 60, and operating potential for the oscillator 24 is coupled to the collector electrode through a bias resistor 62, and to the base electrode through a voltage divider circuit including resistors 64 and 66.

One advantage of the intrusion alarm 20 of the present invention is that the frequency of operation of the oscillator 24 does not require special stabilization. Minor relatively slow changes such as might be caused by temperature variations and the like do not result in false alarms. However, it is desirable to isolate operation of the oscillator 24 from the remainder of the intrusion alarm 20. In the illustrated arrangement, this function is carried out by the drive amplifier 26 and the reference amplifier 30.

Oscillating output signals of the oscillator 24 appearing at the collector electrode of the transistor 44 are coupled to the drive amplifier 26 through a resistor 68 and a coupling capacitor 70, and to the reference amplifier 30 through a resistor 72 and a coupling capacitor 74. Amplifier 26 includes a transistor 76 with its base electrode biased to an operating potential by a voltage divider circuit including resistors 78 and 80. The emitter electrode of transistor 76 is coupled to ground through a resistor 82 and a parallel-connected capacitor 84. The output circuit of the transistor 76 includes the speaker 22 coupled between the collector electrode and a source of positive potential.

The reference amplifier 30 may be similar in construction and includes a transistor 86 having a base electrode biased to an operating potential by a voltage divider including resistors 88 and 90 and an emitter electrode coupled to ground by a resistor 92. The collector electrode of transistor 86 is coupled to a positive source of operating potential by means of a capacitor 94.

When the audio signals are radiated into the protected area such as a room or enclosure by the speaker 22, the signals are reflected throughout the protected area in a complex pattern of paths, and a part of the radiated energy is reflected back to the speaker 22. Once a steady state condition is reached, a resultant reflected signal of relatively constant amplitude and phase is received at the speaker 22. The speaker 22 serves as a transducer for the reflected signal, and as a result there is produced at the collector electrode of transistor 76 of the drive amplifier 26 a composite signal. This composite signal includes a large component corresponding to the signal produced by oscillator 24 and radiated by speaker 22. The composite signal also includes a smaller component corresponding to the reflected audio signal received by the speaker 22. The vector sum of these two signals is the composite signal which appears at the collector electrode of transistor 76.

One feature of the invention resides in the fact that the intrusion alarm 20 is not effectively responsive to amplitude variations in the reflected signal. To bring about this result, the speaker drive amplifier is driven substantially to saturation in operation and thus functions relative to the detector 28 as a limiter stage. Consequently, although signals appearing at the collector electrode of the speaker drive amplifier transistor 76 vary freely in phase when movement occurs in the protected area, nevertheless changes in amplitude of the reflected signal have only a negligible effect on the composite signal.

In order to provide an output signal indicative of movement in the protected area, the phase sensitive detector 28 is provided. Detector 28 includes a transformer 100, and reference signals unaffected by signals reflected to the speaker 22 are applied to the detector by a transformer primary winding 102 coupled between a source of positive potential and the collector electrode of the reference amplifier transistor 86. The composite signal including the reflected signal component is applied to a transformer secondary winding 104 through a centertap 106 and a coupling capacitor 108. In order to operate the detector 28 at optimum sensitivity, the signals from amplifiers 26 and 30 are applied to the detector 28 roughly in quadrature, or roughly 90.degree. out of phase. This is accomplished by designing the components of the amplifiers 26 and 30 in accordance with known practice so that one exhibits roughly a 90 degree phase shift relative to the other.

Detector switching devices in the form of two diode rectifiers 110 and 112 are coupled between the ends of secondary winding 104 and a pair of resistors 114 and 116. A capacitor 118 cooperates with resistors 114 and 116 to filter out any low frequency audio oscillations remaining at the detector output, while capacitors 120 and 122 provide differentiating and smoothing of the output signal.

In operation of the detector 28, alternating signals applied to the transformer primary winding 102 by reference oscillator 30 tend to render diodes 110 and 112 simultaneously periodically conductive in periods when the oscillating reference signal is positive. The alternating composite signal applied to centertap 106 of the secondary winding 104 tends to render the diodes 110 and 120 alternately conductive as the alternating signal varies between positive and negative peaks, with resistor 124 providing a current path for flow through the diodes.

In steady state operation with no movement in the protected area, a constant DC signal appears at the output of the detector 28. When movement occurs, the phase of the reflected signal changes and causes a change in the phase of the composite signal coupled from the amplifier 26. If the movement is toward or away from speaker 28, the Doppler effect causes a frequency change detected by detector 28 simply as a phase shift. However, the device is not dependent on the Doppler effect. If movement equidistant to the speaker 22 occurs, no frequency shift occurs, but a phase shift results as the total path of travel of the signal reflected to speaker 22 is altered. This type of phase shift is also detected by detector 28.

A shift in phase of the composite signal relative to the reference signal alters the periods of conductivity of the diodes 110 and 112 relative to the wave forms of the applied signals and thus produces a shift or fluctuation in the detector output signal. As described below, if such fluctuations are of a frequency characteristic of the movement of an intruder desired to be detected and if they are of sufficient magnitude and duration, then an alarm indication is produced by the intruder alarm 20.

Although similar in some respects to high frequency devices such as, for example, FM discriminators, radar phase detectors and the like, the detector 28 has been designed in novel fashion to operate at a low audio frequency to produce an output signal useful in connection with the alarm 20. The composite signal is coupled through the capacitor 108 rather than through a transformer winding or the like, and capacitor 108 is balanced, or unpolarized, in order to prevent distortion of the signal. In addition, resistor 124 is used in place of the inductance used commonly at higher frequencies. Since a quantitatively accurate, balanced signal is not required, the single capacitor 118 is used instead of two discrete capacitors, and ground is applied directly to the cathode of the diode 112.

Output signals from the phase sensitive detector 28 are coupled through the impedance matching amplifier 32 to the low band pass device 34. Amplifier 32 includes a transistor 130 connected in emitter-follower configuration serving the functions of providing isolation between the low band pass device 34 and the phase sensitive detector 28 and of establishing the signal for driving the device 34 at the desired level. Transistor 130 is biased to the desired operating condition by bias resistors 132, 134, 136 and 138 coupled as illustrated between the transistor electrodes, a source of positive reference potential, and ground.

In order to limit operation of the intrusion alarm 20 to situations wherein the motion detected is characteristic of the type made by an intruder desired to be protected against, the low band pass device 34 is designed to pass a narrow band of frequencies. In the illustrated arrangement, the device 34 passes a band of frequencies having a low end at the approximate level of a few tenths of a hertz and an upper end at the level of approximately 5 or 6 hertz. As a result, for example, if extremely rapid movements are detected within the protected area, such as might be made by a fan or vibrations or the like but which are not characteristic of a human intruder, the resulting signals are not coupled through the low band pass device 34.

At this point it should be noted that an additional important feature of the use of a relatively low audio frequency for the intrusion alarm 20 is that it permits operation with a very narrow band pass such as the approximately five hertz band pass achieved with the device 34. With ultrasonic, microwave or radio frequency devices, the minimum band pass obtainable is in the area of several hundred hertz. As a result, the signal-to-noise ratio obtained with the present invention far exceeds results which have been obtainable in the past.

Referring more specifically now to the construction of the device 34, in the illustrated arrangement the low band pass device 34 is an active filter including an integrated circuit operational amplifier 140. However, it should be understood that it would be possible to use a passive filter device preferably having sharp drop-off characteristics, although it is believed that better results can be obtained with the illustrated active filter circuit. In the illustrated arrangement, operational amplifier 140 comprises a General Electric model PA230 integrated circuit. If desired, any similar device such as a type LM741 integrated circuit manufactured by Fairchild Camera and Instrument Corp. and by Texas Instruments Incorporated may be substituted.

Signals coupled from the impedance matching amplifier 32 are received at the input terminals 12 and 10 of amplifier 140, and output signals appear at output terminal 7. Stabilizing capacitors 142, 144 and 146 are provided in the conventional manner while the amplifier is protected against excessive input signals by diodes 148 and 150. The amplifier 140 is energized by coupling of terminal 3 to a source of operating voltage.

In order to limit the pass band of amplifier 140, there is provided a feedback circuit including one branch composed of a resistor 152 and a parallel branch including a resistor 154 and a capacitor 156 coupled between output terminal 7 and input terminal 10. Input terminal 10 is also coupled to ground by a circuit including a capacitor 158 and a gain control variable resistor 160. Additional output filtering is provided by a resistor 162 and capacitor 164 coupled between output terminal 7 and ground. The values of the components of the feedback and gain control circuits are chosen in order to limit the pass band of amplifier 140 to the desired level. As will be readily understood by those skilled in the art, high frequency cut-off is essentially established by the value of capacitor 156, while low frequency cut-off is substantially established by the value of capacitor 158.

Output signals from output terminal 7 of the amplifier 140 are received at an input terminal 166 of the gate circuit 36. In general, the gate circuit 36 includes a gate transistor 168 operated by a negative gate signal provided by either of two branches of a novel signal processing circuit generally designated as 170 coupled between the gate circuit input terminal 166 and the gate transistor 168.

More specifically, the gate transistor 168 is normally maintained in a nonconductive condition by means of a positive voltage applied to its base electrode through a bias resistor 172. When output signals are received from the low band pass device 34, the gate transistor 168 is placed in a conductive condition.

The novel signal processing circuit 170 functions to supply a negative gate signal necessary to render the gate transistor 168 conductive regardless of whether the signal appearing at the gate input terminal 166 is positive or negative. Thus, when a negative signal is received at terminal 166, the signal is coupled through a capacitor 174, resistor 176 and diode 178 to the base electrode of gate transistor 168. Positive-going signals are blocked by diode 178 and are coupled to the source of positive potential through a diode 180 and a load resistor 182. The capacitor 174 and the diodes 178 and 180 establish a so-called "voltage doubling" circuit, and capacitor 174 is charged when the input signal is positive and this charge adds to negative signals when the input signal is negative.

When a positive signal is received at the input terminal 166, it is coupled through a capacitor 184, resistor 186 and diode 188 to the base electrode of an inverter transistor 190. Transistor 190 is normally maintained in a nonconductive condition, but when biased into conduction by a positive signal at the input terminal 166 the transistor 190 establishes a low resistance connection to ground through its collector-emitter circuit and a resistor 192 in order to place the gate transistor 168 in a conductive condition. During the receipt of a negative signal at the input terminal 166, the signal is coupled to ground through a diode 194 and load resistor 196, and is blocked by diode 188 from the base electrode of the transistor 190. Diodes 188 and 194 and capacitor 184 establish a voltage doubler circuit so that during receipt of negative signals a charge is imposed upon capacitor 184, which charge adds to a subsequent positive-going signal. The values of the components of the branch including transistor 190 are preferably chosen to compensate for the gain of transistor 190 to the end that the gate transistor 168 is operated in identical fashion by either negative or positive signals. Thus, the signal processing circuit 170 functions in a manner analogous to that of a full-wave rectifier but eliminates the need for a coupling transformer or other large and expensive components.

During those periods of time in which the gate transistor 168 is rendered conductive, current flows through a resistor 198 to the integrator 38 comprised of a capacitor 200 and resistor 202. Signals operating the gate transistor 168 as a result of detected motion characteristic of the type of intruder sought to be protected against are time-integrated by the integrator 38. The value of the capacitor 200 in relation to the value of the resistor 198 is chosen so that after a predetermined time of operation of the gate transistor 168, a predetermined charge level is established on the capacitor 200 in order to initiate an alarm condition. The resistor 202 is selected to provide a discharge path such that the level of charge on the capacitor 200 is dissipated in inactive periods to make the device immune to trivial widely spaced disturbances.

When the integrator 38 is charged to a predetermined level indicating an alarm condition, the relay control circuit 40 is operated in order to operate the alarm circuit 42. More specifically, a positive voltage of predetermined magnitude appearing across the capacitor 200 operates an emitter-follower transistor 204 having its emitter coupled to ground through a resistor 206 and coupled to the base electrode of an inverter transistor 208 through a resistor 210. The emitter-follower transistor 204 isolates the integrator 38 from operation of the relay control circuit 40.

To provide fail-safe operation, the alarm circuit includes a normally open set of relay contacts 212 held in closed condition during operation of the intrusion alarm 20 when no alarm condition is experienced by steady state energization of a relay winding 214. More specifically, a relay control transistor 216 is normally maintained in a conductive condition by a positive operating voltage applied to its collector electrode through a resistor 218 and the winding 214 and by a positive control voltage applied to its base electrode through a resistor 220 and a resistor 222. When an alarm condition takes place, the inverter transistor 208 is rendered conductive by operation of the emitter-follower transistor 204 with the result that the base electrode of the relay control transistor 216 is coupled to ground by the inverter transistor 208. As a result, the relay control transistor 216 is biased to a nonconductive condition and current flow through the relay winding 214 is discontinued. At this time, relay contacts 212 of the alarm circuit 42 move to their normally open condition and as a result an alarm condition is established in the alarm circuit 42. It will be understood that the alarm circuit 42 may comprise any conventional alarm equipment such as a telephone-coupled remote indicator, a visible alarm, an audible alarm, or any other desired structure.

In order to provide a visual indication of the operation of the relay control circuit 40 for initial setup of the intrusion alarm 20 or the like, there is provided an indicator light 224 controlled by a light amplifier transistor 226. Normally the transistor 226 is maintained in a nonconductive condition by connection of its base electrode through a resistor 228 to the collector electrode of the normally conductive transistor 216. When an alarm condition takes place, the transistor 226 is biased to a conductive condition wherein the transistor 216 becomes nonconductive, and current for energization of the indicator light 224 flows through a resistor 230 and the output electrodes of the transistor 226.

In the illustrated intrusion alarm 20, the component values and other specific descriptions set forth below were found to provide excellent results. However, it should be understood that this information is provided for completeness of disclosure and should not be taken to limit the scope of the invention;

Component Description Resistor 46 68 ohms 48 6,200 ohms 50 6,200 ohms 52 6,200 ohms 62 3,300 ohms 64 110,000 ohms 66 30,000 ohms 68 220,000 ohms 72 120,000 ohms 78 100,000 ohms 80 22,000 ohms 82 62 ohms 88 100,000 ohms 90 15,000 ohms 92 82 ohms 114 1,000,000 ohms 116 1,000,000 ohms 124 15,000 ohms 132 1,500 ohms 134 15,000 ohms 136 680,000 ohms 138 820,000 ohms 152 1,000,000 ohms 154 10,000 ohms 162 47,000 ohms 172 120,000 ohms 176 15,000 ohms 182 120,000 ohms 186 18,000 ohms 192 18,000 ohms 196 47,000 ohms 198 47,000 ohms 202 270,000 ohms 206 18,000 ohms 210 18,000 ohms 218 620 ohms 220 18,000 ohms 222 27,000 ohms 228 220,000 ohms 230 510 ohms Capacitor 54 0.047 microfarad 56 0.047 microfarad 58 0.047 microfarad 60 0.047 microfarad 70 0.001 microfarad 74 0.047 microfarad 84 270 microfarads 94 0.33 microfarad 108 10 microfarads (unpolarized) 118 1 microfarad 120 25 microfarads 122 0.47 microfarad 142 82 picofarads 144 22 microfarads 146 22 microfarads 156 0.047 microfarad 158 270 microfarads 164 15 microfarads 174 40 microfarads 184 40 microfarads 200 15 microfarads Variable resistor 160 220 ohms - 2,720 ohms Transistor Type 44 2N5306 76 HS5308 86 HS5308 130 2N2924 168 2N3638A 190 2N2924 204 2N2924 208 2N5306 216 2N5306 226 2N5306 Diode 110 1N4001 112 1N4001 148 1N4001 150 1N4001 178 1N4001 180 1N4001 188 1N4001 194 1N4001

the power supply for the intrusion alarm 20 may be conventional in structure. In many installations it may be desirable to operate the alarm on battery power when conventional sources are interrupted. In order to prevent false alarms due to switching transients or other alterations in the supply voltage, it is preferred that a regulated power supply be used for the alarm 20 with the exception of the relay control circuit. In the illustrated device, the source of positive potential was 15 volts supplied from a regulated power supply, and the amplifier 140 was supplied with 12 volts DC from a regulated power supply.

Although the present invention hass been described with reference to the details of the illustrated embodiment, it should be understood that other modifications and embodiments will be apparent to those skilled in the art. As noted above, many advantages are achieved with the use of an audio frequency for the alarm 20. However, inaudible sonic frequencies, i.e., ultrasonic frequencies, may be a requirement in environments where silent operation is desirable. Features of the present invention are applicable to systems using any sonic frequency, whether in the audible range or in the ultrasonic range. The details of the illustrated embodiment are not intended to limit the scope of the present invention as set forth in the following claims.

Claims

What is claimed and desired to be protected by Letters Patent of the United States is:

1. An intrusion alarm for detecting the movements of an intruder in a protectd area comprising:

signal generating means for generating an electrical signal having a frequency falling within the low audio frequency range;

transducer means for converting electrical energy into sound energy waves in the air and for simultaneously converting airborne sound energy waves into electrical energy;

phase detection means for developing an output signal whose magnitude fluctuates in response to changes in the phase relationship of two incoming signals;

first circuit means for conveying the signal generated by said signal generating means to said transducer means;

second circuit means for conveying a signal from said transducer means to said phase detection means;

third circuit means for conveying a signal generated by said signal generating means directly to said phase detection means;

amplification and filtering means connected to said phase detection means for amplifying only those sub-audio frequency output signals supplied by said phase detection means having frequencies falling within a frequency band of less than four octaves the center frequency of which band is adjusted to encompass the primary output signal frequencies generated by said phase detection means when a human moves adjacent said transducer means;

rectification and integration means connected to the output of said amplification and filtering means for integrating the strength and duration of signal perturbations which appear at the output of said amplification and filtering means; and

means coupled to said rectification and integration means for producing an alarm indication in response to the achievement of a predetermined integral value by said integration means.

2. An intrusion alarm in accordance with claim 1 wherein said transducer means comprises at least one loudspeaker having a single pair of electrical terminals.

3. An intrusion alarm in accordance with claim 1 wherein the electrical signal generated by the generating means has a frequency between 100 and 1,000 Hertz.

4. An intrusion alarm in accordance with claim 1 wherein the electrical signal generated by the generating means has a frequency between 400 and 600 Hertz.

5. An intrusion alarm in accordance with claim 1 wherein said first and second circuit means include limiter means for limiting the amplitude fluctuations in signals conveyed between said signal generating means and said phase detection means over said first and second circuit means, whereby said phase detector means is rendered relatively insensitive to signal amplitude fluctuations.

6. An intrusion alarm in accordance with claim 1 wherein the first and second circuit means, the transducer means, and the third circuit means have phase shift characteristics chosen to cause signals flowing from said signal generating means over said first and second circuit means to said phase detection means to be approximately in quadrature with signals generated by said signal generating means and flowing over said third circuit means to said phase detection means.

7. An intrusion alarm in accordance with claim 1 wherein the first circuit means includes saturated signal amplification means which limit the amplitude of electrical signals supplied to said transducer means.

8. An intrusion alarm in accordance with claim 1 wherein said rectification and integration means includes at its input full wave rectification means for supplying signal perturbations of either polarity at the output of said amplification and filtering means to said integration means as perturbations of the same polarity so that either positive or negative perturbations may be summed by said integration means.

9. An intrusion alarm in accordance with claim 1 wherein said amplification and filtering means comprises an active filter having a frequency bandwidth of approximately 3 octaves.

10. An intrusion alarm comprising in combination:

an oscillator operating at a frequency between about 400 and about 600 Hertz;

a speaker;

a first amplifier coupling said oscillator to said speaker;

a phase sensitive detector including a transformer having a primary winding and a tapped secondary winding;

a second amplifier coupling said oscillator to said primary winding;

means including said first amplifier coupling both said speaker and said oscillator to the secondary winding tap;

a sub-audio frequency band pass filter having a bandwidth in the neighborhood of three octaves and a center frequency of between 1 and 5 Hertz coupled to the output of said phase sensitive detector; and

alarm means coupled to the output of said band pass filter.

11. The alarm of claim 10 further comprising full-wave rectification and integrating means coupled between said band pass filter and said alarm means.

12. An intrusion alarm for detecting the movements of an intruder in a protected area comprising:

means including a tone generator and a transducer for generating a low audio frequency standing wave in the protected area;

means for generating an electrical signal whose amplitude changes represent phase changes in said standing wave;

means for filtering said electrical signal to eliminate frequency components lying outside of a predetermined sub-audio pass band;

electrical signal integration means for integrating incoming electrical signals;

a first circuit path including a unidirectional conductive device oriented in a first direction over which the filtered electrical signal is conveyed to said integration means;

a second circuit path including a unidirectional conductive device oriented in a second direction and also including inversion means for inverting the polarity of signals flowing over said second path over which the filtered electrical signal is conveyed to said integration means;

and alarm means coupled to said integration means for sounding an alarm when said integration means reaches a predetermined integral state in response to signals flowing over said circuit paths.

13. An intrusion alarm in accordance with claim 12 wherein each of said first and second circuit paths includes voltage doubling rectification means, of which the unidirectional conductive device forms a part, for rectifying signals so as to produce increased rectified signal output from each path.