US3893022A - Sealed envelope examining device for detecting the presence of metal - Google Patents

Abstract

A device for detecting the presence of metal in sealed flat envelopes, whereby the presence of ''''letter bombs'''' and similar destructive devices can be sensed and an alarm device actuated. The device includes a base of generally rectangular configuration containing a detecting circuit including a pair of coils, one of said coils being mounted within said base, and the other said coil supported on a bridge-like structure extending transversely above said base to define an interstice through which envelopes pass. The presence of metal changes the electromagnetic field existing between the coils, resulting in a change in impedance sufficient to actuate the alarm.

Description

United States Patent 1 91 Kulik et al.

[ SEALED ENVELOPE EXAMINING DEVICE FOR DETECTING THE PRESENCE OF METAL [76] lnventors: Herbert S. Kulik; Jacob Schachter,

both of 100 Ricefield Ln,

Hauppauge, NY.

[22] Filed: Feb. 28, 1974 [2]] Appl. No.: 446,698

[52] US. Cl. 324/41 [51 Int. Cl G01r 33/00 [58] Field of Search 324/41 [56] References Cited UNITED STATES PATENTS 2,966,582 l2/l960 Wachtel 324/41 OTHER PUBLICATIONS Greenmeyer', P., Electronic Metal Detector, Electronic Industries & Electronic Instrumentation, Nov.

14 1 July 1,1975

1947, pp. 9, 10 & 13.

Primary ExaminerRobert J. Corcoran Attorney, Agent, or FirmCharles E. Temko [57] ABSTRACT A device for detecting the presence of metal in sealed flat envelopes, whereby the presence of letter bombs and similar destructive devices can be sensed and an alarm device actuated. The device includes a base of generally rectangular configuration containing a detecting circuit including a pair of coils, one of said coils being mounted within said base, and the other said coil supported on a bridge-like structure extending transversely above said base to define an interstice through which envelopes pass. The presence of metal changes the electromagnetic field existing between the coils, resulting in a change in impedance sufficient to actuate the alarm.

2 Claims, 7 Drawing Figures ENTFUJUL] sugar FlG.2

FIG.4

SEALED ENVELOPE EXAMINING DEVICE FOR DETECTING THE PRESENCE OF METAL BACKGROUND OF THE INVENTION Metal detecting devices are per se known in the art, and have been used extensively in the detection of land mines and similar buried metallic objects. More recently, devices have been used at air terminals for the examination of passenger baggage and freight. Such devices generate a relatively wide electro-magnetic field, and are not optimized for detecting relatively small metallic articles or devices used in the construction of socalled letter bombs," neither are they physically convenient to use for this function. They may also be unsafe to use because of excessive field power.

With wide use of such destructive devices sent through postal channels, there has arisen a need for a simple device through which letters and envelopes may be passed at high speed for the purpose of metallic detection. Many letter bombs are formed from flattened cartridge cases in conjunction with spring wire, and the total metallic content of a single device is relatively small. Other types are electrically operated and use metallic parts such as batteries, wires, foil, switches and detonators.

BRIEF DESCRIPTION OF THE PRESENT INVENTION Briefly stated, the invention contemplates the provision of a letter box size detector comprising a generally rectangular base enclosing a detector circuit and a first coil forming a part thereof. A transversely extending bridge is supported by said base and encloses a second coil forming a part of the detector circuit, the bridge defining with the base an interstice between which letters and envelopes are serially passed. The letters are placed upon a sliding drawer and move horizontally beneath the bridge, or in the case ofa large number ofletters, the base may be tilted to form an inclined plane, and the letters allowed to fall under the action of gravity through the interstice. To maintain privacy, a pivotally mounted cover selectively overlies the drawer, and such position, the cover locks the drawer against withdrawal.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, to which reference will be made in the specification, similar reference characters have been employed to designate corresponding parts throughout the several views.

FIG. 1 is a side elevational view of an embodiment of the invention.

FIG. 2 is a first end elevational view thereof.

FIG. 3 is a second end elevational view thereof.

FIG. 4 is a top plan view thereof.

FIG. 5 is a second top plan view with cover and drawer elements in opened condition.

FIG. 6 is a view in perspective of the embodiment with cover and drawer elements removed, and the base supported at an angle wherein an upper surface thereof forms an inclined plane.

FIG. 7 is a schematic wiring diagram of the detector circuit forming a component of the embodiment.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT In accordance with the invention, the device, generally indicated by reference character 10 includes a base element 11 of generally rectangular configuration and comprising a lower wall I2, side walls I3 and I4, end walls 15 and I6 and an upper wall 17. Extensions I8 on the side walls 13 and 14 support a hollow bridge 19 supporting in concealed relation a first coil l9 (see FIG. 4) immediately above a second coil not shown) disposed in the base element I], the bridge 19 forming with the upper surface of the upper wall 17 a transversely extending space 21 through which letters or envelopes being examined may pass.

Slidably mounted between the side walls 13 and I4 is a drawer element 22, the element 22 including an outer end wall 23, a bottom wall 24 and an inner end wall 25. The longitudinal edges of the bottom wall 24 ride in guiding grooves 26 (see FIGS. 5 and 6) so that the lower surface thereof is free of contact with the upper surface of the wall 17 during movement.

Supported by hinge means 27 and 28 is a pivotally mounted cover element 29 including a curved end wall 30, a planar main wall 31 and a first transverse member 32 having detent means 33 at each end resiliently engageable in corresponding recesses 34 in the side walls 13 and 14. A second transverse member 35 includes a surface 36 adapted to engage an inwardly disposed wall of the inner end wall 25, and maintain it in the position shown in FIG. 4.

The device may be operated in either of two manners, depending upon the quantity of letters and envelopes to be examined. In one method, the cover element 29 is swung open as shown in FIG. 5, and one or more letters are placed on the upper surface of the wall 24. After activating the detector circuit, the drawer is withdrawn as shown in FIG. 5, causing the envelopes to pass beneath the bridge 19, the presence of metal being thereby detected.

Where a relatively large number of pieces of mail are to be examined, the drawer element 22 and cover element 29 are removed, and the base element II is propped using a selectively engageable wire frame member 38 to the position shown in FIG. 6. Pieces of mail are merely dropped on the inclined upper surface of the upper wall I8 to fall under the action of gravity beneath the bridge I9 effecting a similar result.

Referring to FIG. 7 in the drawings, there is illustrated a transistorized circuit suitable for use in the disclosed embodiment, it being understood by those skilled in the art that other circuits of well known type may be substituted where desired.

Referring to FIG. 7, transistor 42 is an oscillator with a tuned circuit consisting of coil 43 and a 0.0 l 5 capacitor 44 operating at a frequency of approximately 20 KHz. Feedback is provided by coil 45.

When a metallic object is inserted between coil 43 and coil 45, the feedback is disturbed and the level of the oscillator decreases. The sensitivity of the circuit is adjusted by means of a 5K reheostat 46.

The output of the oscillator 42 is amplified and detected by transistor 47. The level at the collector of transistor 47 is compared with the level of the 5K reheostat 46 setting. When the anode of transistor 48 rises 0.3 volt above the potentiometer setting, transistor 48 conducts and triggers a silicon controlled rectifier 49 which, when conducting, causes the alarm 50 to sound.

Integrated circuit 51 is a voltage regulator which provides a stable voltage. The power supply 52 is a battery of two 9 volt rechargeable batteries. When the switch S4 is in the on position, a light emitting diode 55 illuminates. The power supply 52 is recharged by interconnection with a 117 volt ac power source. A diode 56 rectifies the current to dc through a 4.7K resistor 57.

It will be observed that during operation. on the occurrence of a detecting action as a metallic object passes between the coils. the electromagnetic field existing therebetween collapses, thereby minimizing the possibility of an electrical force discharging an explosive device.

We wish it to be understood that we do not consider the invention limited to the precise details of structure shown and set forth in this specification, for obvious modifications will occur to those skilled in the art to which the invention pertains.

We claim:

A sealed envelope examining device comprising: a base element forming a housing, a metal-sensitive oscil lating detector circuit at least partially disposed within said housing. said circuit including first and second coils developing an electromagnetic field therebetween and an alarm device; said base element including a planar upper wall and a pair of side walls on either side thereof, a bridge structure supported by portions of said walls and extending over said upper wall to define an interstice therebetween for the passage of an envelope therethrough, one of said coils being supported by said bridge structure, the other of said coils being supported by said base element on opposite sides of said interstice; a slideable drawer element supported by said base element and passing beneath said bridge structure, and a pivotally mounted cover element selectively overlying said base element and having means maintaining said drawer element against movement when said cover element is in overlying position.

2. Structure in accordance with claim 1, including means for mounting said upper wall of said base element at an angle relative to horizontal, whereby envelopes placed upon said upper wall may slide under the force of gravity through said interstice.

Claims (2)

1. A sealed envelope examining device comprising: a base element forming a housing, a metal-sensitive oscillating detector circuit at least partially disposed within said housing, said circuit including first and second coils developing an electromagnetic field therebetween and an alarm device; said base element including a planar upper wall and a pair of side walls on either side thereof, a bridge structure supported by portions of said walls and extending over said upper wall to define an interstice therebetween for the passage of an envelope therethrough, one of said coils being supported by said bridge structure, the other of said coils being supported by said base element on opposite sides of said interstice; a slideable drawer element supported by said base element and passing beneath said bridge structure, and a pivotally mounted cover element selectively overlying said base element and having means maintaining said drawer element against movement when said cover element is in overlying position.

2. Structure in accordance with claim 1, including means for mounting said upper wall of said base element at an angle relative to horizontal, whereby envelopes placed upon said upper wall may slide under the force of gravity through said interstice.

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