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Publication numberUS3577136 A
Publication typeGrant
Publication dateMay 4, 1971
Filing dateAug 4, 1967
Priority dateAug 4, 1967
Publication numberUS 3577136 A, US 3577136A, US-A-3577136, US3577136 A, US3577136A
InventorsJohn E Wolf
Original AssigneeSecurity Systems Inc
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Short-range signaling system
US 3577136 A
Images(5)
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Description  (OCR text may contain errors)

United States Patent John E. Wolf Marengo, Ill.

[21 1 Appl. No. 658,491

[22] Filed Aug. 4, 1967 [45] Patented May 4, 1971 [7 3 1 Assignee Security Systems, Inc.

[72] Inventor [54] SHORT-RANGE SIGNALING SYSTEM 15 Claims, 19 Drawing Figs.

[52] US. Cl 340/280, 340/258, 331/156, 324/43, 325/477 [51] Int. Cl G08b 13/24, H03b 5/30 [50] Field of Search 340/258,

258 (B), 258 (C), 258 (D), 224, 280; 343/65, 6.5 (SS), 6.8; 325/475, 477; 324/5, 6, 41,43, 47;

[56] References Cited UNITED STATES PATENTS 2,504,719 4/1950 Neilson 33l/154X 2,774,060 12/ 1 956 Thompson 340/276X 3,172,106 3/1965 Zaleski 33l/156X 3,292,080 12/1966 Trikilis 340/258D 3,321,702 5/1967 Tuccinardi 324/43 3,349,323 10/1967 Mullen 324/41X 3,423,682 l/l969 Cauchois 325/477 Primary Examiner-Alvin H. Waring Assistant ExaminerPerry Palan An0rney-Lettvin and Gerstman ABSTRACT: A merchandise protection system in which an encapsulated, magnetically latched reed is hidden on the article to be protected against theft. Magnets placed adjacent an exitway from the protected enclosure release the reed, allowing it to thereafter vibrate at a predetermined frequency. The reed produces a magnetic disturbance which is detected by a sensitive, frequency-selective pickup device which, in turn, actuates an alarm.

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SHEET 5 OF 5 IVY AAA A l VVVVVV SHORT-RANGE SIGNALING SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to electrical signalingdevices and, more particularly, to methods and apparatus for detecting the presence of certain objects.

A variety of theft protection systems have been proposed for use in retail stores, libraries and museums. One of the most effective of these prior arrangements is described in U.S. Pat. application Ser. No. 609,494 filed on Jan. 16, I967, now abandoned, by Edward Devine, Peter Miller, and John E. Wolf. In that system, a small magnet is attached to each article to be protected. Should a thief attempt to carry an article through an exitway from the protected enclosure, a sensitive pickup device positioned adjacent the exitway detects the moving magnet to provide an alarm signal.

While being suitable for many applications, this system does sufi'er from several disadvantages. Extensive precautions, not always effective, must be taken to prevent stray magnetic fields from triggering the alarm. Moreover, innocent patrons carrying magnetized or large metallic objects through the exitway may actuate the alarm. To minimize this difficulty, the magnets which are placed on the protected articles must be larger than would otherwise be necessary, increasing the cost of the system and complicating the task of attaching the magnets to the protected articles.

SUMMARY OF THE INVENTION In a principal aspect, the present invention takes the form of a condition-responsive device for signaling over short distances by producing a magnetic disturbance and a sensitive receiving device for detecting the presence of such a disturbance.

The signal-producing device itself may take the form of an elongated, ferrous reed suspended cantilever fashion within a sealed capsule. A second ferrous member is positioned within the capsule in normally spaced relation from the free end of the reed. The capsule is subjected to a magnetic field sufficient to deflect the free end of the reed into contact with the second ferrous member. After this activating field is removed, the reed and the second member remain in contact due to the residual magnetism associated with the two ferrous members. By applying a field to the activated capsule which is opposed to the direction of residual magnetism, the reed may be released from the second member, thereafter vibrating for at least a brief period to produce a damped, magnetic signal having a frequency equal to the mechanical resonant frequency of the reed.

A suitable antenna for sensing the magnetic fiux disturbance thus produced is connected to the input of a signal detection device capable of distinguishing the signal created by the vibrating reed from the noise" produced from other sources.

This signal detection device preferably includes a first bandpass filter tuned to the frequency of reed vibration and second and third band-pass filters tuned respectively above and below the reed vibration frequency. The signals at the outputs of the three filters are rectified to produce control signals, the rectified signal corresponding to the signals at the reed vibra tion frequency having a first polarity and the other two control signals having a second, opposite polarity. The three control signals are then summed to produce an output signal. Should an impulse noise" be picked up at the antenna, signals will pass through all three band-pass filters to create a net output signal having the second polarity. The output signal will have the first polarity only when the magnitude of the signal passing through the first filter exceeds the sum of the two signals passing through the second and third filters.

The present invention may be employed to protect articles against theft. An activated reed capsule is affixed to each article to be protected. One or more magnets placed adjacent each exitway from the protected enclosure causes the reed to be released and hence creates a detectable signal whenever an attempt is made to carry the article through the exitway. Should removal of an article be authorized, the reed capsule may be purposely deactivated by applying a magnetic releasing field to the capsule at a point out of range from the signal detector. The capsule may be reactivated at any time by applying a magnetic reed latching field thereto.

BRIEF DESCRIPTION OF THE DRAWINGS In the course of the detailed description which follows, frequent reference will be made to the attached drawings in which:

FIG. 1 is a schematicized pictorial view of a merchandise protection system which utilizes the principles of the present invention;

FIGS. 2 and 2A are cross-sectional and end views, respectively of a magnetically actuated, vibrating reed capsule of the type contemplated by the invention, the capsule being shown positioned adjacent a magnet which closes the two reeds;

FIG. 3 is a cross-sectional view of the capsule of FIG. 2 shown positioned adjacent a magnet which releases the two reeds to cause reed vibration;

FIG. 4 is a graph showing an illustrative sequence of signals of the type produced by a pickup antenna;

FIG. 5 shows the waveform which appears at the output of the first channel rectifier in FIG. I in response to the presence of the signals shown in FIG. 4;

FIG. 6 shows the output waveform from the second channel rectifier;

FIG. 7 shows the output waveform from the third channel rectifier;

FIG. 8 shows the waveform applied to the input of the threshold device of FIG. 1 in response to the signal input waveform depicted in FIG. 4;

FIG. 9 illustrates the output waveform applied to the alarm in FIG. I by the threshold device;

FIG. I0 depicts the gain characteristics of the signal detection circuit shown in FIG. 1;

FIGS. 11 and 11A shows an alternative construction for the reed capsule which embodies the principles of the invention cross-sectional and end view, respectively;

FIGS. 12 and 12A illustrate another reed capsule construction according to the invention cross-sectional and end view, respectively;

FIGS. 13 and I3A show still another reed capsule embodiment capable of emitting a pair of signals at different frequencies cross-sectional and end view, respectively;

FIG. 14 is a detailed schematic drawing of a preferred signal amplifying and filtering circuit; and

FIG. 15 is a detailed schematic drawing of a preferred signal rectifying and summing circuit and an alarm control circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 of the drawings shows a signal detection unit positioned adjacent an exit 20 from a protected enclosure. Partitions 22 and 23 extend outwardly from exitway 20. A set of four magnets (which may be permanent magnets or electromagnets), one of which is indicated at 25, is positioned in partition 22 near exit 20. A similar set of magnets, one of which is indicated at 27, is housed in partition 23 at a position farther from exit 20. Those magnets which are vertically aligned are magnetically poled in the same direction such that they cooperate to produce an enhanced magnetic field. The horizontally spaced pairs of magnets are poled in difierent directions such that, when a reed capsule (to be described) is carried between the partitions, it will be subjected to magnetic fiux lines of changing orientation.

A multiple-tum coil is associated with each vertically aligned pair of magnets. (In FIG. I, single turn coils are shown for purposes of illustration.) These four coils are then connected in series-bucking" relationship; that is, all four coils are connected in series in such a way that, if each coil is subjected to the same magnetic field disturbance, no net voltage will be produced. In this way, stray magnetic field fluctuations created at a distance from the coils tend to produce cancelling signals while signals generated between the two partitions 22 and 23 tend to add constructively.

Signals picked up by the four coils hidden in partitions 22 and 23 are passed through a line transformer 29 and a shielded cable 30 to the input of a frequency selective detection circuit 34 which actuates an alarm 36 whenever the presence of an activated capsule is detected between partitions 22 and 23.

Before describing further the detection circuit 34 schematically shown in FIG. I, it will be helpful to first consider the construction and operation of a typical signal-generating capsule of the type shown in FIGS. 2 and 3.

The capsule shown in FIG. 2 consists of a pair of elongated beams or reeds' 40 and II suspended cantilever fashion within an evacuated glass envelope 43. A small latching" magnet 45 is attached to the outside wall of the glass envelope 43 near the overlapping ends of the reeds 40 and M. When, as shown in FIG. 2, the capsule is subjected to a magnetic field which cooperates with latching magnet 45, the ends of the reeds are pulled into contact. This may be accomplished by bringing the capsule near one end of the activating" magnet 46 as shown in FIG. 2.

When activating magnet 46 is removed, the reeds M) and II are held in contact by latching magnet 45 which supplies just enough residual magnetism to hold the two reeds together.

When a capsule thus activated is subjected to a magnetic field in opposition to the residual field provided by latching magnet 45, the reeds 40 and 41 break contact and swing away from one another. Thereafter, the two reeds vibrate for a short period at their natural frequency of mechanical resonance. This vibration creates a magnetic field disturbance which may be detected at short ranges.

Detection circuit 34 shown in FIG. I includes an amplifier 47 whose input is connected through shielded cable 30 to the pickup coils in partitions 22 and 23. The output of amplifier 47 is connected to the inputs of three band-pass filters 51, 52, and 53. Filter 52 resonates at the reed vibration frequency while filters I and 53 are respectively tuned slightly higher than and slightly lower than the reed frequency. The signals appearing at the outputs of filters 511, 52 and 53 are amplified by amplifiers 56, 57 and 58 respectively. A capacitor-diode rectifier is connected to the output of each of these amplifiers, rectifiers 611 and 63 delivering positive signals while rectifier 62 delivers a negative signal. The output of the three rectifiers is connected to a summing node'point be by means of resistors 67, b8 and as. The summing node 66 supplies the input signal to a threshold device 70.

The detection circuit 34 must not only be very sensitive but must also be capable of distinguishing the signal produced by an actuated reed capsule from a variety of other signals. The illustrative waveforms depicted in FIGS. 4} through 8 of the drawings typify the operation of this circuit.

In FIG. d, an amplified waveform of the type which might appear at the output of amplifier 47 is shown. This waveform may be divided into three parts: a noise impulse A; sustained random noise I3; and a reed signal C.

The impulse A shown in FIG. 4 contains a wide band of frequencies, including the reed vibration frequency. When applied to the band-pass filters SI, 52 and 53, the impulse causes all three filters to ring" and thus produces a decaying output from all three rectifiers M, 62 and 63. Since the outputs from rectifiers 61 and 63 are positive (as seen in FIGS. 5 and '7) while only the output from rectifier 62 is negative (as seen in FIG. 6), the net signal appearing at summing node 66 in response to impulse A is positive as seen in FIG. 3.

Similarly, sustained random noise as illustrated at B in FIG. 4 produces a net positive input signal to threshold device 70.

When a signal from an actuated reed is applied to the detection unit 34, however, the threshold device 3% is triggered. Having a discrete frequency, the reed signal passes only through channel 2 filter 52 to produce a net negative signal at summing node 66 which has a sufficient amplitude to trigger threshold device 70 and actuate alarm 36.

The frequency response characteristics of detection circuit 341 are depicted in FIG. 10 of the drawings. Curves 76, 77, and 78 show the passband characteristics of filters 56, 57 and 58 respectively. The dashed line curve 79 shows the combined characteristics of the three filters; that is, the overall transfer gain between the input to amplifier 47 and the summing node 66. The horizontal dashed line 80 indicates the threshold voltage level V, at which threshold device 70 is actuated. As may be readily appreciated from FIG. III, the effective selectivity of the detection circuit may be considerably better than the selectivity of the filter 52 by itself. This selectivity may be further enhanced by increasing the gain of the channel I and channel 2, signal paths (curves 76 and 78), by reducing the gain of the channel 2 signal path (curve 77), or by lowering still further the threshold level V, at which threshold device 70 is triggered. In practice, the triggering range should have sufficient width to allow for the expected deviations in frequency between different reed capsules yet be narrow enough to reject single frequency tones which are near to but not the same as the predetermined reed vibration frequency.

FIGS. II, I2 and I3 of the drawings show alternative reed capsule constructions. The reed capsules shown in FIGS. II, I2 and 13 may be latched and actuated in the same manner as discussed in connection with the capsule shown in FIGS. 2 and 3 of the drawings.

In FIG. II, a single elongated reed 81 is suspended cantilever fashion within a glass capsule 82 and is shown held in latched position against a permanent magnet 83 imbedded in capsule 82.

The capsule shown in FIG. I2 likewise includes a single reed 85 mounted for vibration in a capsule 86. The reed 85 is, however, constructed of a remanent magnetic material such that it provides its own residual latching magnetism to hold its free end in contact with a fixed iron slug 87 imbedded in envelope 86.

FIG. I3 shows still another reed capsule embodying the principles of the invention. A long reed 87 and a shorter reed 88, both of which may be remanently magnetic, extend toward one another from opposite ends of a glass envelope 89. Being of different lengths, the reeds 87 and 88 vibrate at different frequencies to produce a pair of detectable signals. Two separate detection circuits (each of which preferably takes the form of the detection unit 34 discussed in conjunction with FIG. I) may then be employed to detect these signals. The outputs of these two circuits may then be connected to actuate an alarm when, and only when, signals are present at both reed frequencies.

FIG. Id of the drawings is a detailed'schematic drawing of a transistor preamplifier and passband filter circuit which may be employed to instrument the invention. Except where otherwise noted, the circuit shown in FIG. 1 employs conventional transistor circuitry and need not be discussed in detail.

The signal from the pickup coils is applied to input 101 and is amplified by the four cascaded transistors I02, I03, I04 and 1105. Transistors I03 and 105 are connected as emitter-followers for buffering. The series combination of a capacitor 1% and a switch I07 is connected between the base and collector of input transistor I02. When switch I07 is closed, additional high-frequency negative feedback is applied to the base of transistor I02 to provide reduced high-frequency gain in high noise areas.

The series combination of capacitor I09 and inductor III is connected between the emitter of transistor I05 and ground. The base of a transistor III is connected to the junction of capacitor I09 and inductor III which acts as the first stage of the first channel filter. The filtered first channel signal is further amplified by transistors III, II2 and 113 and again filtered by the series combination of capacitor I15 and inductor I lb. The first channel output terminal I20 is connected to the junction of capacitor II5 and inductor H6.

The signal appearing at the emitter of transistor I05 is amplified by the cascaded transistors I22 and I23. The series combination of capacitor I26 and inductor 127 (the second channel filter) is connected from the emitter of transistor I23 to ground. Similarly, the series combination of capacitor 128 and inductor 129 (the third channel filter) connected between the emitter of transistor 123 and ground. The second channel output terminal 130 is connected to the junction of capacitors 126 and 127 while the third channel output terminal 135 is connected to the junction of capacitor 128 and inductor 129.

The circuitry shown in the detailed schematic of FIG. amplifies and rectifies the three channel signals developed by the circuit of FIG. 14. Signals appearing in the second channel (including the signal produced by the vibrating reed) are applied to the transistor 140 which is connected in an emitterfollower configuration. The series combination of a capacitor 141 and a diode 143 is connected between the emitter of transistor 140 and ground. Capacitor 144 is connected in seties with diode 145 between the anode and cathode of diode 143. Diodes 143 and 145 in conjunction with capacitors 141 and 144, act as a voltage doubler producing a negative, rectified signal across capacitor 144, the magnitude of which varies with the envelope of the signal passing through the second channel filter.

Similar amplifiers and voltage doublets are connected to receive the first and third channel signals. These voltage uoublers produce rectified voltages across capacitors 148 and 149 which are proportional to the envelopes 'of the first and third channel signals respectively.

The three rectified signals are combined on the summing conductor 150. A resistor 151 connects the capacitor 144 to conductor 150 while resistors 152 and 153 connect conductor 150 to capacitors 148 and 149 respectively.

The series combination of a diode 160 and a capacitor 161 is connected between summing conductor 150 and ground. So long as conductor 150 is more positive than the voltage across capacitor 161, diode 160 remains in a blocking condition. Capacitor 161 thus charges to a low-positive potential through the series combination of resistors 164 and 165. Resistor 164 normally supplies a forward biasing current to the base of transistor 170, maintaining it in a normally conductive condition. Likewise, transistor 171, which is capacitively coupled to the output of transistor 170, is also normally conductive.

When a reed frequency signal passes through the second channel filter (capacitor 126 and inductor 127 in FlG. 14), a negative-going pulse appears at summing conductor 150, turning transistors 170 and 171 off. This negative pulse is large enough to override any positive noise signal contributed from the first and third channels.

As an additional precaution against noise signals, means are employed for lowering the collector voltage appearing at the collector of transistor 171 during and immediately following high-noise conditions. The collector of a transistor 178 is connected to the junction of capacitors 180 and 181 which are connected to ground between the positive voltage supply conductor 103 and ground. A pair of resistors 186 and 187 connect the positive supply conductor 183 to the collector of transistor 171, the junction of these resistors being connected to the junction of capacitors 180 and 181. The emitter of transistor 178 is grounded and its base is connected to the summing conductor 150 through a resistor 190. During highnoise conditions, conductor 150 is highly positive, turning transistor 178 on and discharging capacitor 181, thus lowering the collector voltage of the switching transistor 171. After such high noise ceases, the capacitor 181 recharges at a slow rate such that the condition of reduced collector voltage lingers for a brief period.

1f transistor 171 is switched off suddenly in response to a reed frequency signal, its collector voltage rises abruptly, transmitting a positive-going pulse to the base of transistor 192, turning that transistor on, and applying a positive gate tum-on pulse to the gate of the silicon-controlled rectifier 196. Current then flows from the positive supply terminal 197, through a closed power switch 198, a relay coil 199, a normally on transistor 200, and the transconductive path of controlled rectifier 196. The current through relay coil 199 closes contacts 202 and 203.

The contacts 202 thus close a remote circuit (of any sort) which may be connected across terminals 204 and 205. The closure of contact 203 causes the capacitor 210 to begin to discharge through resistor 212, reducing the base voltage-applied to transistor 200. Eventually, transistor 200 is turned off, opening relay contacts 202 and 203, and causing capacitor 210 to be recharged through resistor 215. (Sufficient relay coil current flows during this period to maintain controlled rectifier 196 in conduction even though transistor 192 has again turned off.) The relay coil 199 is therefor pulsed. The series combination of resistor 218 and a switch 219 is connected in parallel with resistor 215. By closing switch 219, transistor 200 remains on at all times to disable the pulsing circult.

A local alarm circuit is also connected in series with relay contact 203. This circuit comprises a signal light 220 which is connected in parallel with the series combination of a switch 222 and an audible alarm 224. By opening switch 222, the audible alarm 224 may be disabled.

It is to be understood that the arrangements which have been described are merely illustrative applications of the principles of the invention. Numerous other arrangements may be devised which nevertheless embody the invention.

lclaim:

l. The method of short range signaling comprising, in combination, the steps of magnetically latching a vibratory ferrous member in a deflected position against a second ferrous member,

applying an externally created magnetic field to said members to release said vibratory member such that it thereafter freely vibrates at a predetermined frequency to create a magnetic disturbance at a predetermined frequency, and

sensing the presence of said disturbance at a position remote from said vibrating member.

2. The method as set forth in claim 1 including the steps of sensing the presence of noise disturbances at frequencies other than said predetermined frequency and comparing said noise disturbances with said disturbances at said predetermined frequency.

3. A short range signaling apparatus comprising, in combination,

an elongated ferrous signaling member supported in cantilever fashion for vibratory motion within a capsule,

a second ferrous member mounted in said capsule in spaced relation from the free end of said elongated member, means for deflecting the free end of said elongated signaling member into contact with said second member,

a source of residual magnetism associated with said members for holding said elongated member in contact with said second member,

a source of magnetic initiating field opposed to said residual field for releasing said elongated member from said second member such that the elongated member thereafter freely vibrates to produce a magnetic signaling disturbance into the air surrounding said signaling member having a predetermined frequency, and

means remote from said capsule for detecting said magnetic signaling disturbance having said predetermined frequency.

4. A device as set forth in claim 3 wherein said source of residual magnetism comprises a magnet attached to said capsule.

5. A device as set forth in claim 3 wherein said source of residual magnetism comprises the remanent magnetism of at least one of said members.

6. A device as set forth in claim 3 wherein said second member comprises a second elongated member mounted cantilever fashion within said capsule and having a free end overlapping and spaced from said first-named elongated member.

7. A device as set forth in claim 6 wherein both of said members are adapted to vibrate at substantially said predetermined frequency.

8. A device as set forth in claim 3 wherein said means for deflecting said elongated member comprises an external source of a magnetic field cooperative with said residual field for creating a magnetic attraction between said members sufficient to deflect said elongated member into contact with said second member.

9. A combination as set forth in claim 3 wherein said means for detecting magnetic disturbance having a predetermined frequency comprises, in combination,

an antenna or pickup coils,

an amplifier coupled to said antenna,

a first band-pass filter coupled to the output of said amplifier and tuned to a frequency lower than said predetermined frequency,

a second band-pass filter coupled to the output of said amplifier and tuned to said predetermined frequency,

a third band-pass filter connected to the output of said amplifier and tuned to a frequency higher than said predetermined frequency,

first, second and third rectifying means coupled to the outputs of said first, second and third filters respectively, said first and third rectifying means producing control signals of one polarity and said second rectifying means producing a control signal of the opposite polarity, and

means coupled to the outputs of said filters for summing said control signals to produce a sum signal the polarity of which indicates the presence of said magnetic disturbances.

110. The combination as set forth in claim 9 wherein said antenna is positioned adjacent an exitway from a protected enclosure, wherein said source of a magnetic initiating field is positioned adjacent said exitway, and wherein an alarm device is coupled to said summing means such that it is actuated in response to the presence of a sum signal having said opposite polarity.

11. The method of short range signaling comprising,- in combination, the steps of initiating thevibration of a vibratory ferrous signaling member having a predetermined mechanical resonant frequency such that said member thereafter freely vibrates at said frequency to radiate a magnetic signal at said predetermined frequency into the air surrounding said member, and detecting the presence of said magnetic signal at a position remote from said vibratory member.

12. The method as set forth in claim ll including the steps of sensing the presence of noise disturbance at frequencies other than said predetermined frequency and comparing said noise disturbances with said signal at. said predetermined frequency.

13. A short range signaling apparatus comprising, in combination,

a ferrous signaling member mounted for vibratory movement within a housing, said member having a predetermined mechanically vibratory resonant frequency,

means for initiating the vibration said ferrous signaling member such that said member thereafter freely vibrates at said resonant frequency to radiate a magnetic signal into the air surrounding said member at said' predetermined frequency, and

means for detecting the presence of said magnetic signal at a position remote from said ferrous member.

14. The apparatus as set forth in claim 13 wherein said means for detecting the presence of said magnetic signal comprises, in combination,

an antenna,

an amplifier coupled to said antenna,

a first band-pass filter coupled to the output of said amplifier and tuned to said predetermined frequency,

a third band-pass filter connected to the output of said amplifier and tuned to a frequency higher than said predetermined frequency,

first, second and third rectifying means coupled to the outputs of said first, second, and third filters respectively, said first and third rectifying means producing control signals of the opposite polarity, and

means coupled to the outputs of said filters for summing said control signals to produce a sum signal the polarity of which indicates the presence of said magnetic disturbance.

115. The apparatus as set forth in claim 14 wherein said antenna is positioned adjacent an exitway from a protected enclosure, wherein said means for initiating vibration is a source of magnetic initiating field positioned adjacent said exitway, and wherein an alarm device is coupled to said summing means such that it is actuated in response to the presence of a sum signal having said opposite polarity.

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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3713133 *Feb 16, 1971Jan 23, 1973R NathansRf and sonic systems for preventing shoplifting of goods and unauthorized removal of capsules affixed thereto for protecting goods
US3755803 *Mar 31, 1970Aug 28, 1973Unisearch LtdElectronic surveillance system
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US3863142 *Sep 3, 1971Jan 28, 1975Us NavyMagnetic fluxmeter
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Classifications
U.S. Classification340/572.1, 340/572.4, 340/572.8, 331/156
International ClassificationG08B13/24
Cooperative ClassificationG08B13/2437, G08B13/2474, G08B13/2471, G08B13/2442, G08B13/2477, G08B13/2434
European ClassificationG08B13/24B7A1, G08B13/24B7A2, G08B13/24B3H, G08B13/24B3M2, G08B13/24B7A3, G08B13/24B3M