Note: Descriptions are shown in the official language in which they were submitted.
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LATCH INTEGRATED, TAMPER RESISTANT,
ELECTRO-MAGNETIC ALARM SWITCH
Field of the Invention
The present invention relates to alarm switches used with security systems,
and
more particularly to an electro-magnetic alarm switch that is substantially
tamper-
resistant.
Background of the Invention
Figure 1 shows a prior art electro-magnetic alarm switch assembly 1. The
assembly generally includes a magnetic_switch 2 and a magnet 3 that are
separate
components and are mounted in close proximity to each other on a closed door 4
and
door frame 5 (or window and window frame). In operation, as the door 4 is
opened
and the switch 2 and magnet 3 move apart, at a predetermined distance the
effect
of the magnetic field of the magnet is no longer imposed on the switch, the
switch
reacts, and the alarm sounds. Typically, the switch 2 is normally "open,"
meaning
that the alarm is on. However, when the door 4 is closed and the magnet 3 is
in
close proximity, the magnet 3 holds the switch 2 in a closed (alarm off)
position. As
the door 4 is opened and the magnet 3 moves away from the switch 2, the switch
2 reverts to the normally open position, thus sounding the alarm. These types
of
switches can be easily circumvented simply by holding a magnet in close
proximity
to the switch 2 while the door is opened, thereby maintaining the switch 2 in
the
closed position.
Figure 2 shows another prior art electro-magnetic alarm switch assembly 6.
The assembly 6 includes a housing 7 having a first 8 and second 9 portion. The
first
portion 8 includes a switch and the second portion 9 includes a magnet. The
assembly 6 is designed to function with a slidable door latch 10, such as is
often
used on a raisable overhead door. The closed door latch 10 lies at rest
through the
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open mid-portion 11 of the "U". The switch is normally "closed" (alarm off).
When
the slidable latch 10 is in the closed door position, as shown, interaction
between the
magnet and the switch is blocked by the latch 10 (as shown by arrows A). When
the slidable latch is withdrawn, as to open the door, the magnet causes the
switch
to open, thereby sounding the alarm. A drawback to this design is that the
assembly
6 can easily be "tampered with" by inserting into open mid-portion 11 a metal
"dummy" latch before withdrawing the latch 10, thereby maintaining the block
betweeri the magnet and the switch and allowing the door to be opened while
preventing activation of the alarm.
Other prior art alarm switches are known, however many of these are not
provided.for use with a slidable door latch. Examples of prior art alarm
switches can
be found in U.S. Patent Nos. 5,673,021 to Woods and 5,668,533 to Jackson et
al.
A long felt need exists for an alai'tn switch assembly that is provided for
use
with a slidable door latch and is substantially tamper-resistant.
Summary of the Preferred Embodiments
In accordance with one embodiment of the present invention there is provided
an alarm switch assembly that includes a housing having first and second
surfaces
and an opening defined therein that extends from one surface toward the other
surface. The opening is further defined by at least four inner surfaces. A
magnetic
field generating device is disposed adjacent the opening and between the first
and
second surfaces, and a switch is disposed adjacent the opening and is opposed
to the
magnetic field generating device.
In a preferred embodiment, the opening is rectangular and preferably includes
a pair of projections that extend inwardly from opposed inner surfaces
that.define the
opening. In a particular embodiment, the switch is disposed in a switch
opening that
is at least partially defined in one projection and the magnet is disposed in
a magnet
opening that is at least partially defined in the other projection.
In another preferred embodiment, the opening includes a convergent section
and a divergent section.
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In accordance with another aspect of the present invention there is provided a
latch system that includes the alarm switch assembly described above and a
latch
received in the opening. A clearance not exceeding about .020 " is defined
between
the latch and at least one of the inner surfaces of the housing.
In accordance with yet another aspect of the present invention there is
provided
an alarm system that includes the latch system described above and an alarm.
The
lead wires electrically connect the switch to the alarm.
In accordance with another aspect of the present invention there is provided a
door assembly that includes a door, the latch slidably affixed to the door, a
door
frame having an opening defined therein at least partially surrounding the
door, and
the alarm system described above affixed to the door frame. The opening in the
door
frame and the opening in the housing cooperate to define a latch opening; the
latch
is received in the latch opening; and 8 clearance not exceeding about .020 "
is
defined between the latch and the housing.
15, In accordance with another aspect of the present invention there is
provided a
method of activating an alarm. The method includes the steps of providing the
door
assembly described above and detecting withdrawal of the latch from the latch
opening.
In accordance with yet another aspect of the present invention there is
provided
an alarm switch assembly that includes a housing having first, second and
third
portions that cooperate to define an opening. The housing also includes at
least one
blocking portion that extends inwardly from the first or the third portion. A
magnetic
field generating device is disposed adjacent the opening and between the first
and
second surfaces, and a switch is disposed adjacent the opening and opposed to
the
magnetic field generating device.
It will be understood that the device is not limited to activation by a
magnetic
field generating device. Any field, beam or the like that is blocked by the
latch and
can be sensed when the latch is removed is within the scope of the present
invention.
For example, a light beam emitting device and appropriate receptor can be
used. A
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jet of air emitting device and appropriate receptor can be used. Sound waves,
such
as ultrasonic waves produced by a magnetostrictive element can be used.
Other objects, features and advantages of the present invention will become
apparent to those skilled in the art from the following detailed description.
It is to be
understood, however, that the detailed description and specific examples,
while
indicating preferred embodiments of the present invention, are given by way of
illustration and not limitation. Many changes and modifications within the
scope of
the present invention may be made without departing from the spirit thereof,
and the
invention includes all such modifications.
Brief Description of the Drawings
The invention may be more readily understood by referring to the accompanying
drawings in which
FIG. 1 is a view of a door and a door frame having a prior art 2-piece efectro-
magnetic alarm switch device affixed thereto.
FIG. 2 is a front elevation of a prior art 1 piece, non-enclosed electro-
magnetic
alarm switch assembly having a slidable door latch engaged therewith.
FIG. 3 is an exploded perspective view of a switch assembly in accordance with
a first embodiment of the present invention, a door frame and a striker plate.
FIG. 4 is a sectional rear elevation of the switch assembly of FIG. 3 taken
along
line 4--4 of FIG. 3 showing the alarm switch and magnet disposed in the
housing.
FIG. 5 is a perspective view of an overhead door having a slidable door latch
and the switch assembly of FIG. 3 affixed to the door frame.
FIG. 6 is a front elevation of the switch assembly of FIG. 3 having a slidable
door latch inserted therein.
FIG. 7 is a sectional side elevation of a door assembly including the switch
assembly of FIG. 3, showing the latch slot and the latch in the open position.
FIG. 8 is a top sectional plan view of the switch assembly of FIG. 3 taken
along
line 8--8 of FIG. 3:- -The-magnet and switch are omitted.
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FIG. 9 is a top sectional plan view of the a switch assembly with projections
having rounded edges in accordance with an alternative embodiment of the
present
invention.
FIG. 10 is a front elevation of the housing in accordance with an alternative
embodiment of the invention.
Like numerals refer to like parts throughout the several views of the
drawings.
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Detailed Description of the Preferred Embodiments
Referring to FIGS. 3-8, a preferred embodiment of an electro-magnetic
alarm switch assembly 12 for use with a slidable door latch 10 is shown.
Switch
assembly 12 generally includes a housing 14, an alarm switch 16 and a magnet
18.
It will be appreciated that terms such as "front" and "rear" used hereinbelow
are merely for ease of description and refer to the orientation of the
components as
shown in the Figures. It should be understood that any orientation of switch
assembly 12 described herein is within the scope of the present invention.
For exemplary purposes only, described hereinbelow is a preferred embodiment
wherein switch assembly 12 is provided for use with a slidable door latch 10
(being
slidable between an open and a closed position) of an overhead door 19 and is
mounted to a door frame 20 using a"striker plate 22 and screws 24. It will be
understood, however, that the type of door, the door frame, striker plate or
hardware
for mounting switch assembly 12 are not limitations on the present invention.
For
example, switch assembly 12 can be used with a hinged door.
Housing 14 is preferably comprised of a plastic such as nylon or a phenolic or
other synthetic resin. Housing 14 can be comprised of any non-magnetic
material
that does not interfere with the magnetic effect exerted on alarm switch 16.
Housing 14 has a front surface 14a and a rear surface 14b. A slot 25 is
defined
through housing 14 from front surface 14a to rear surface 14b. It will be
understood
that the term slot is not a limitation on the present invention and that slot
25 can be
any opening. As shown in the Figures, slot 25 is substantially rectangular,
however
it can be circular, ellipsoidal or other shape. In a preferred embodiment,
slot 25 is
divided into three sections, a front section 25a, middle section 25b and rear
section
25c. Middle section 25b is defined by inner surfaces 26b that are
substantially
parallel. As illustrated in Figure 8, front section 25a and rear section 25c
are defined
by surfaces 26a, 26c, respectively, that slope away from middle section 25b
and
toward the front 14a and rear surfaces 14b of housing 14, respectively.
Preferably,
from front surface 14a to rear surface 14b, section 25a is convergent, section
25b
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is a throat section and section 25c is divergent. It will be understood that
sections
25a, b and c can be defined by surfaces that are planar, concave or convex.
Surfaces 26a, 26b and 26c define a pair of projections 27. The projections 27
extend into slot 25. In an alternative embodiment, the projections 27 can also
have
rounded corners, as shown in F)G. 9.
In a preferred embodiment, housing 14 includes a magnet chamber 28 defined
therein. Preferably, magnet chamber 28 extends from the top surface 14c of
housing
14 and runs substantially parallel to slot 25, as shown in Figure 4. Housing
14 also
has switch chamber 29 defined therein. Preferably, switch chamber 29 extends
from
the top surface 14c of housing 14 and runs substantially parallel to slot 25
and
magnet chamber 28. Magnet chamber 28 and switch channel 29 are shown in the
Figures.as being cylindrical (i.e., having a circular transverse cross-
section). However,
they can also have a square, rectangular or other shaped transverse cross-
section.
It will be understood that the term chamber is not a limitation on the present
invention and that magnet chamber 28 and switch chamber 29 can be any opening.
Magnet chamber 28 and switch chamber 29 are defined on opposite sides of
slot 25, as shown in FIG. 4. In a preferred embodiment, the magnet and switch
chambers 28 and 29 are each defined at least partially in one of the
projections 27,
as illustrated in FIG. 8. Magnet 18 is preferably secured in magnet chamber 28
using
glue, resin or similar adhesive. However, magnet 18 can alternatively be
disposed in
magnet chamber 28 without being secured therein. It will be understood that
any
magnet field generating device is within the scope of the present invention.
Switch
chamber 29 has alarm switch 16 secured therein in a similar manner.
Slot 25 is preferably dimensioned such that slidable door latch 10 fits
therein
with a relatively small amount of clearance on each side. It will be
understood that
the dimensions of slot 25 will be dependent on the size of the slidable door
latch with
which the switch assembly will be used. It will be further understood that the
clearance between slidable door latch 10 and inner surface 26b is such that a
"dummy" latch cannot be easily inserted therein. As used herein a "dummy"
latch
is an object that can be inserted between slidable door latch 10 and inner
surface 26b
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such that when latch 10 is withdrawn the object blocks the magnetic field
produced
by magnet 18 and prevents same from affecting alarm switch 16. Preferably, the
clearance between latch 10 and housing 14 is no more than .020". Therefore, a
"dummy" latch inserted therein must be thinner than .020". Such a "dummy"
latch
is not readily available.
Alarm switch 16 can be any electro-magnetic switch known in the art, such as
a reed-type switch. It will be appreciated by those skilled in the art that
the switch
elements of a reed switch change state when the reed switch is exposed to a
magnetic field, as is necessary for operation of the present invention. Alarm
switch
16 can be configured to be normally closed or normally open. Preferably, alarm
switch 16 is normally in a closed position. Closed position is used herein to
mean
that when the alarm switch is not affected by a magnet the switch is held
closed,
such that the alarm does not sound. Alarm switch 16 has a pair of lead wires
30 that
extend therefrom to a conventional alarm 32 or other means for indicating that
the
switch has been turned on (described below), thereby electrically connecting
alarm
switch 16 to alarm 32. !n a preferred embodiment, the wires 30 extend out of
switch chamber 29. In an alternative embodiment the wires 30 can extend
through
an aperture defined in housing 14. In a preferred embodiment, the alarm 32 is
configured such that if any of the wires 30 are cut alarm 32 is activated.
Alarm 32
is conventional and can include, for example, an alarm control assembly and
alarm
bell or the like.
Housing 14 also has defined therein at least one, and preferably a pair of
holes
34 for receiving a screw 24 or other fastener means. To affix switch assembly
12
to door frame 20 the screws 24 each are slipped through holes 22a in striker
plate
22, holes 20a in door frame 20 and threaded into the holes 34. In an
alternative
embodiment, threaded fasteners can extend from switch assembly 12 and be
engaged with door frame 20. Switch assembly 12 can also be secured to door
frame
20 using an adhesive, clamps, other threaded fasteners or the like. It will be
understood that the method of securing switch assembly 12 to door frame 20 is
not
a limitation on the present invention.
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When switch assembly 12 is secured to door frame 20 an opening 22b in striker
plate 22, an opening 20b in door frame 20 and opening 25 in housing 14 are
substantially aligned, thereby defining a latch slot 42. Slidable door latch
10, which
is secured to door 19, can be extended through latch slot 42.
In operation, as mentioned above, alarm switch 16 is normally closed. When
latch 10 is in its open position (not being fully inserted into latch slot 42
as shown
in FIG. 7) the close proximity of magnet 18 to alarm switch 16 biases alarm
switch
16 to the open position. When latch 10 is in its closed position, thus being
fully
inserted in latch slot 42 (including opening 25) the magnetic field produced
by
magnet 18 is blocked by the metal slidable door latch 10, thereby maintaining
alarm
switch 16 in the "closed" position. When latch 10 is withdrawn to its open
position
(the direction of which is shown by arrow A2 in FIG. 5), magnet 18 again
biases
alarm switch 16 to the open position thereby sending a signal through wire 30
and
sounding alarm 32.
tt' will be understood that the sloped inner surfaces 26a, '26c provide for
increased sensitivity of the assembly 12 as compared to a switch assembly
without
the sloped surfaces. The stoped surfaces 26a, 26b reduce the overall amount of
material between magnet 18 and alarm switch 16 (as compared to a slot with
straight
parallel surfaces). Sloped surfaces 26c promote smooth latch 10 operation as
latch
10 passes through housing 14 when being closed.
In an alternative embodiment, slot 25 is defined only part of the way through
housing 14. In other words, slot 25 is closed such that it does not extend
through
the front surface 14a of housing 14. This completely prevents a "dummy" latch
from
being inserted into opening 25.
Switch assembly 12 can be sized to fit any size door. latch. It will be
appreciated by those skilled in the art that magnet 18 and alarm switch 16 can
be
configured for maximum sensitivity as desired.
It will be understood that the present invention is more difficult to defeat
than
the prior art.- The increased sensitivity and the resistance to insertion of a
"dummy"
latch are advantages over the prior art. It is easy to fit a readily available
size
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"dummy" latch into the space between the housing and latch in prior art alarm
systems (see Fig.2). Only a very thin "dummy" latch can be used to attempt to
defeat the present invention. The increased sensitivity causes the alarm to be
activated as the top of latch 10 is withdrawn from between surfaces 26b.
Referring to FIG. 10, in an alternative embodiment, housing 114 includes first
116, second 118 and third 120 portions. At least one blocking portion 122
extends
inwardly from one or both of the first 116 and third 120 portions. Blocking
portion
122 preferably extends inwardly across at least 25% of the width of slot 25,
more
preferably blocking portion 122 extends inwardly across at least 75% of the
width
of slot 25, and most preferably blocking portion 122 extends inwardly across
100%
of the width of slot 25. The blocking portion 122 substantially prevents a
"dummy"
latch from being inserted from the top of housing 114, as is possible in the
prior art
(see FIG. 2). In order to insert a "dummy" latch from the top, the "dummy"
latch
would have to be flexible. The added time it would take a thief to bend the
"dummy"
latch to get it past the blocking portion 122 may prevent the crime from
occurring.
It will be understood that a blocking portion can be included on the bottom or
sides
also.
The embodiments of the present invention recited herein are intended to be
merely exemplary and those skilled in the art will be able to make numerous
modifications to them without departing from the spirit of the present
invention. For
example, the sloping surfaces 26a and 26c can be omitted and the magnet and
switch chambers 28 and 29 can be completely enclosed in said housing (i.e.,
the
switch and magnet can be molded in the housing). The slot does not have to be
enclosed on all sides. All such modifications are intended to be within the
scope of
the present invention as defined by the claims appended hereto.
