US2877361A

US2877361A - Burglar alarm system

Info

Publication number: US2877361A
Application number: US652441A
Authority: US
Inventors: Leland H Chase
Original assignee: HOLMES ELECTRIC PROTECTIVE CO
Current assignee: HOLMES ELECTRIC PROTECTIVE CO
Priority date: 1957-04-12
Filing date: 1957-04-12
Publication date: 1959-03-10
Anticipated expiration: 1976-03-10

Description

March 10, 1959 L. H. CHASE 2,877,351

BURGLAR ALARM SYSTEM Filed April 12, 195i 2 SheetsSheet 1 52 POTENTIAL 5 42a 25 v 42 6a /4 1 /4a 56 /6 /o La 31 7 L24 (a 28 o 2/ POTENTIAL FIG. 2 Y 5 5/ I Z INVENTOR.

LELAND H. CHASE BY ATTORNEY I March 10, 1959 L. H. CHASE 2,877,361 BURGLAR ALARM SYSTEM Filed April 12, 1957 2 Shee-tsPShaei: 2.

OUTS I DE INVENTOR. LELAND H. CHASE ZQMM ATTORN EY Unitcd States Patent 2,877,361 BURGLAR ALARM SYSTEM Leland H. Chase, Port Washington, N. Y., assignor to Holmes Electric Protective Company, New York, N. Y., a corporation of New York Application April 12, 1957, Serial No. 652,441 7 Claims. (Cl. 307-112) tected area, the change given.

It is one object of this invention to provide a novel system of the kind described which is simple in construction, inexpensive to manufacture, easy to install and manipulate and durable in use.

One embodiment of the invention has a pair of switches whose contacts are movable between open and closed positions under the influence of a magnetic field and a biasing force. The switch contacts assume prein current therein so that the above changes in the switch positions due to movement of the door result in an alarm.

In an embodiment of the kind described the apparatus may be arranged so that a signal will be given in the event of an open contact in any part of the circuit or in the event of a failure of one of the switches to function properly.

In the drawings:

Fig. 1 is a partly schematic diagram of the burglar alarm system of this invention associated with a closure member which is shown in the closed position;

Fig. 2 is a view like Fig. 1 but howing the effect of opening of the closure member;

Fig. 3 is a perspective view from the inside of an area to be protected showing the location of the switches and magnets relative to a closed closure member; and

Fig. 4 is a partly schematic showing of another embodiment of the invention showing the closure member in closed position.

Referring now to Figs. 1 and 2 of the drawings, the numerals and 12 denote a pair of normally open switches each comprising

flexible reed contacts

14 and 16 and 18 and 20, respectively. The reed contacts in each switch are formed preferably of ferromagnetic material and, to combat environmental conditions such as dirt and moisture, are encased within a glass bulb which may be filled with an inert gas. These switches have the property that their normally open contacts close when they are in a magnetic field of a certain strength. A switch of this type is described in U. S. Patent 2,289,830 to Ellwood.

The switch elements 10 and 12 are mounted on a stationary part of a building structure adjacent a closure member for an opening in an enclosure, for example, on

the frame 21 of a door 22 to a room to be protected from unauthorized entry. A biasing magnet 23 is mounted above the upper switch 10 in fixed position relative to the switches. The magnetic field for the magnet 23 is represented by the lines of force emanating from and returning to magnet 23 as indicated by the lines L The numeral 24 denotes an actuator magnet which is of greater strength than magnet 23 and is mounted on the door which is attached to the frame by means of a hinge 28. The magnetic lines of force emanating from and re turning to magnet 24 are indicated by the lines L Terminal 16a of switch 10 is connected to terminal 20a of switch 12, and a resistance 30 is connected in series with

terminals

14a and 18a of switches 10 and 12, respectively. A line 32 having a source of potential 34 therein is connected to terminal 14a, while a line 36 is connected to the other terminal 16a.

Lines

32 and 36 lead into an alarm circuit denoted generally by the numeral 38. The alarm circuit 38 is sensitive to either a predetermined increase or a predetermined decrease in current through it to give an alarm. Any of a number of known alarm circuits 38 may be employed, one example being that shown in U. S. Patent 2,747,177 to Keller which exhibits the property that once there is a change in the steady state current through it, the alarm will continue regardless of any subsequent change.

With the position of the door 22 and the polarity of the magnets as shown in Fig. 1, the reed contacts 14 and 16 of switch 10 remain in their normally open positions. It is believed that the flux lines from actuator magnet 24 (L24) oppose the flux lines from bias magnet 23 (L in the reed contacts 14 and 16 with the result that they cancel or annul one another. However, the magnet 24 is so close to the lower switch element 12 that its flux lines L acting on

reed contacts

18 and 20 are not annulled by flux lines from bias magnet 23 and they cause the contacts to close. Thus in the closed position of the door the opposed fields of

magnets

23 and 24 acting upon the switch 10 annul one another while the field of magnet 24 at switch 12 is strong enough to close this switch despite magnet 23.

The path taken by the current with the door in the closed position, as shown in Fig. 1, is as follows: source of potential 34, lead 32, resistance 30,

reed contacts

18 and 20, terminal 20a, terminal 16a, lead 36, alarm circuit 38 and back to the other side of the source of potential. In practice the alarm circuit 38 is adjusted so that in the position of the switches shown in Fig. 1 no alarm will. be given. For a given alarm circuit this adjustment will depend upon the value of the resistance 30, the resistances of the various leads and the value of the potential 34.

Fig. 2 of the drawings shows switches of movement of door 22 to the open position. Such movement carries the actuator magnet 24 away from the switches 10 and 12 and thereby weakens the influence of the field of this magnet on these switches. Accordingly, the

reed contacts

18 and 20 of the switch 12 assume their normally open position because magnet 23 is too far away from switch 12 to close these con tacts. However, the magnet 23 is close enough to switch 10 to close its contacts 14 and 16 when actuator magnet 24 is removed by opening the door. A new path now taken by the current is through lead 32, terminal 14a, reed contacts 14 and 16, terminal 16a and lead 36. The total current is greater than before because the resistance the effect upon the 30 has been lay-passed. This increased current causes the alarm circuit 38 to respond.

Because there is a critical balance between the fields of

magnets

23 and 24 when the door is closed, a relatively slight change in the magnetic forces acting upon theswitches will disturb the balance with a consequent alarm. As a result of the criticality, any successful attempt to prevent the normal operation of the switches and 12 by introducing an external magnetic field would be extremely difiicult. If such an external field were of one polarity and of too great a strength, it would aid the influence of bias magnet 23 on switch it) enough to result in its closure. This would cause the resistance 30 to be by-passed and increase the total current thereby giving an alarm. The introduction of such a magnet would also annul the infiuence of magnet 24 on switch 12, causing it to open.

If the external magnetic field so introduced were of the opposite polarity and also of too great a strength, it would aid the influence of magnet 24 on switch it again causing this switch to close and give an alarm. It the external magnet were too weak regardless of its polarity, its field would have no effect upon the switches it and 12. Thus an external magnet would have to be of both the proper polarity and strength in order to circumvent the initiation of an alarm.

In practice, the alarm circuit 38 as well as the source of potential 34 may be located at a point remote from the premises in which the protected area is located. For example this circuit and potential source may be at the central station" of a burglar protection company.

It is a feature of this invention that complete supervision of the system is realized. Thus, in the door closed position, if any of the leads which join the switches, resistor and alarm circuit become open, the current in the alarm circuit will fall to zero and an alarm will result.

If the door is opened and the switch 10 fails to close, an open circuit will again result because both switches will then be open. If the switch 12 should fail to open when the door is opened, an alarm condition will still result since the switch 10 will close and the current through alarm circuit 38 will increase even though switch 12 is also closed.

In Fig. 3 an actual installation of the invention is shown in perspective with all housing covers removed. The switches 10 and 12 and the biasing magnet 23 are mounted in a casing 50. This casing is in turn mounted on a plate 52, and the latter is secured above the inside of the frame 54 of a door 58 adapted to swing in both directions to open. The actuator magnet 24 is mounted on a bracket 56 which clips over the top of the door 53.

The possibility of defeat of the system by the introduction of a third magnetic source is extremely small as previously mentioned, but it is theoretically possible if such a third magnetic source of proper strength is oriented with respect to the switches so that the polarity of its field corresponds to the polarity of the actuator magnet on the door. In practice such a manipulation would be very nearly impossible, but nevertheless Fig. 4 of the drawings discloses an embodiment which makes even such an unlikely defeat of the system virtually impossible. Fig. 4 shows schematically a second embodiment of the invention which utilizes a third magneticallyresponsive switching element. The circuit of this embodiment is the same as previously described except that a third switch element 11, identical with switching elements 10 and 12, is connected directly across leads 32 and 36 (see Figs. 1 and 2). As shown in Fig. 4, switch 11 is located nearer the door frame 54 than switches 10 and 12. .In the closed position of the door the switch 10 is open and the switch 12 is closed (as in the embodiment of Figs. 1 and 2) under the influence of

magnets

23 and 24. The third switch 11 is also open since, due to its location relative to

magnets

23 and 24, neither magnetic field is strong enough to cause its reed contacts to close.

The operation of this embodiment is the same as that of the first embodiment and possesses all of the latters advantages. It additionally provides even greater pro- 4 tection'against defeat of the system by the attempted introduction of a third magnetic field. If such a field were introduced from the outside of the door the reed contacts of switch 11 would close, the current in alarm circuit 38 would again be increased, and an alarm would be given.

I claim:

1. In a burglar alarm system: first and second switches each having an arm which is movable between open and closed positions, which is normally in open position and which is responsive to the force of a magnetic field to move in one direction to closed position; means normally exerting a biasing force on the arm of one switch to move said arm in the opposite direction to closed position; a source of magnetic field located adjacent the switches, the force from said source balancing the effect of the biasing force on the said one switch arm to allow said arm to move to its normally open position, and said source acting upon said normally open second switch arm to move it to closed position.

2. In a burglar alarm system: first and second switches each having at least one arm movable between open and closed positions under the influence of a magnetic field; a first source of magnetic field adjacent the first switch and normally holding the arm of the first switch in closed position, said first switch arm moving to open position when the efiect of the magnetic field from said first source is removed, said first source being sufficiently remote from said second switch so that the arm of said second switch is unafiected by the field of said first source; a second source of a magnetic field adjacent the switches and normally holding the arm of the second switch in closed position, said second source being oriented so that its polarity is opposite to the polarity of said first source and being suificiently close to said first switch so that the efiects of said two magnetic fields on said first switch arm substantially annul each other and said first switch remains open.

3. The structure of claim 2 wherein a resistance is in series with the second switch and the first switch when closed bypasses said resistance and second switch.

4. The structure of claim 3 wherein an alarm circuit responsive to a change of current through it is in series with both the first and second switches.

5. The structure of claim 4 including a third normally open switch having at least one arm movable between open and closed positions under the influence of a magnetic field, said third switch being connected across said first switch and being adjacent the said first and second switches.

6. In a burglar alarm system: a normally open switch having contacts of magnetic material at least one of which is movable between open and closed positions under the influence of a magnetic field; a first magnetic source adjacent the switch, magnetic lines of force from said source passing between the contacts in one direction, the number of said lines being sufiicient to normally move said movable contact to closed position, a second magnetic source also adjacent the switch, magnetic lines of force from said second source passing between said contacts in a direction opposite to said one direction and being sufficient in number to annul the effect on the movable switch contact of the lines of force from the first magnetic source.

7. The structure of claim 6 including a second switch having at least one contact movable between open and closed positions under the influence of a magnetic field, said second magnetic source being adapted to be mounted on a movable closure member movable from a first closed position in which the said second magnetic source is adjacent the switches to a second open position in which the second source is remote from the switches, the second switch having its movable contact moved into closed position by the magnetic lines of force from the second magnetic source when said second magnetic source is adjacent said switches and having its movable contact moved into open posit-Eon when said second source is remote from said switches, whereby both switch contacts change position upon movement of the closure member from closed position to open position.

References Cited in the file of this patent UNITED STATES PATENTS