Abstract:
A binary multi-code combination sensor employs a magnetically coded transmitter and corresponding receiver to add a level of security to conventional low-security alarm systems. The transmitter includes a pattern of magnetic and non-magnetic portions. The receiver includes a series arrangement of magnetically sensitive switches wherein normally open switches are positioned to be held closed by the magnetic portions of the transmitter and normally closed switches are positioned opposite the non-magnetic portions of the transmitter. Added security is provided by the normally closed switches which open in response to attempted tampering using an external magnetic field. Coded sensors increase the difficulty of successfully defeating the sensor using an external magnetic field.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to security systems used to secure the windows and doors of building structures against unauthorized entry. In particular, this invention relates generally to magnetically activated sensors used to detect unauthorized opening of windows or doors. 
     2. The Prior Art 
     It is known in the art of burglar alarm systems to use magnetically sensitive sensors to detect unauthorized entry. Standard magnetically sensitive sensors known in the art typically have two parts. The first part contains a magnet and is affixed to the moving portion of a window or door. The second part contains a magnetically sensitive switch and is affixed to the stationary part of the building, door jam or window sill. The stationary portion of the sensor is wired to an alarm system which is equipped to detect a change of state of the magnetically sensitive switch. Multiple windows or doors in a given room or area can be wired in a series loop where, when the area is secured, the loop is closed (conductive). If any of the sensors in a secured area are disturbed, the loop becomes open, activating the alarm. Typically, this type of system is used in a low risk level security system such as a typical home burglar alarm system. 
     Single switch magnetic sensors have the significant disadvantage of being easily defeated by an external magnetic field. A powerful magnetic field placed outside the window or door adjacent to the position of the sensor will hold the magnetically sensitive switch in a closed position even if the window or door is opened, thus defeating the ability of the sensor to detect movement of the window or door. 
     The sensors previously described are known as a Class  1 , or low security level sensor. More sophisticated sensors are used in higher security systems. For example, a Class  2  sensor consists of a reed switch and bias magnet positioned in a receiver and an actuator magnet located in the transmitter. The reed switch is manufactured with the common leg touching the normally closed leg. Installation requires biasing the sensor by carefully positioning the bias magnet to move the common leg of the sensor out of contact with the normally closed wire and into contact with the normally open wire. The actuator magnet is capable of moving the switch back to the normally closed position. This switch is balanced and is held in the operational state only as long as the actuator magnet remains in a specific zone near the switch. If the actuator magnet is moved closer to the switch or further away, the switch becomes unbalanced, causing a change of state. Approaching a Class  2  switch with a magnet of the wrong polarity or of improper strength will cause the switch to change state and trigger the alarm. Class  2  switches offer a medium level of security and are used in commercial, industrial and prison security applications. 
     Switches of Classes  3 - 6  are known in the art, each providing a progressively higher level of security, complexity and expense. Class  2  switches, for example, are significantly more expensive than Class  1  switches and require extensive time and sophistication for proper installation and operation, making them impractical for inexpensive low security installations. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the invention in a preferred form comprises a multi-combination magnetism transmitter and a multi-code combination sensor. The multi-code combination magnetism transmitter (hereinafter “transmitter”) comprises a unit containing magnetic portions and non-magnetic portions arranged in a pattern. The multi-code combination sensor (hereinafter “receiver”) comprises multiple magnetically sensitive switches positioned to respond to magnetic fields in the transmitter. In practice, a magnetically sensitive normally open switch is positioned opposite the magnetic field of a magnetic portion in the transmitter. When correctly positioned, the magnetic field will close the magnetically sensitive, normally open switch. A magnetically sensitive normally closed switch is positioned opposite a non-magnetic portion of the transmitter. This normally closed switch will remain closed when it is positioned adjacent to the non-magnetic portion of the transmitter. 
     A pattern of magnetic and non-magnetic portions of the transmitter correspond to magnetically sensitive normally open and normally closed switches on the receiver, respectively. A matched transmitter and receiver form a sensor which can replace the single switch sensor and magnet of a standard Class I sensor while adding an additional level of security. Added security is provided by the magnetically sensitive normally closed switches. These normally closed switches will respond to an externally applied magnetic field used in an attempt to defeat the sensor. 
     The pattern of magnetic and non-magnetic portions in the sensor can be arranged in linear fashion where it will create a ‘binary’ pattern of magnetic and non-magnetic portions. The magnetic, non-magnetic pattern may alternatively be more geometric, forming a grid of magnetic and non-magnetic portions. Whatever the form, the cost and installation complexity of the binary multi-code combination sensor is comparable to a standard Class  1  switch. The binary multi-code combination sensor is a direct replacement for Class  1  switches in existing low security alarm systems. 
     An object of the present invention is to provide a new and improved binary multi-code combination sensor which cost effectively provides an added level of security to standard burglar alarm systems. 
     Another object of the present invention is to provide a new and improved binary multi-code combination sensor equipped with multiple tamper detection switches. 
     These and other objects, features, and advantages of the invention will become readily apparent to those skilled in the art upon reading the description of the preferred embodiments, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an alarm system incorporating a binary multi-code combination sensor in accordance with the present invention; 
     FIG. 2 is a block diagram of the alarm system of FIG. 1, illustrating the binary multi-code combination sensor in an installed configuration; 
     FIG. 3 is a block diagram of the alarm system of FIG. 1, schematically illustrating the binary multi-code combination sensor configuration in response to an exterior magnetic field; 
     FIG. 4 is a schematic representation of a two digit binary multi-code combination sensor in accordance with the present invention; 
     FIG. 5 is a schematic representation of a three digit binary multi-code combination sensor in accordance with the present invention; 
     FIG. 6 is a schematic representation of a grid pattern four digit binary multi-code combination sensor in accordance with the present invention; 
     FIG. 7 is a schematic representation of an in line four digit binary multi-code combination sensor in accordance with the present invention; 
     FIG. 8 is a schematic representation of an eight digit binary multi-code combination sensor in accordance with the present invention; and 
     FIGS. 9-11 are line drawings of alternative configurations of an eight digit binary multi-code combination sensor in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-3 are simplified illustrations of an alarm system incorporating a six digit binary multi-code combination sensor in accordance with the present invention. In a preferred form, the binary multi-code combination sensor comprises a multi-code magnetism transmitter  12  (hereinafter “transmitter”) and a multi-code combination sensor  14  (hereinafter “receiver”). The transmitter  12  comprises a unit containing magnetic portions  16  and non-magnetic portions  18  arranged in a pattern. The sensor  14  comprises a pattern of two types of magnetically sensitive switch. The magnetically sensitive switches are typically reed-type switches, but any switch which reliably reacts when exposed to a magnetic field is appropriate. 
     A magnetically sensitive normally open switch  26  is constructed to be normally open but may be held in a closed position by the presence of a magnetic field. The normally closed switches  28  are also magnetically sensitive but are constructed to remain closed in the absence of a magnetic field. In accordance with one aspect of the invention, the normally closed switches  28  may have a greater sensitivity to magnetic fields than the normally open switches  26 . The two types of switches  26 ,  28  are arranged in a pattern corresponding to the pattern of magnetic portions  16  and non-magnetic portions  18  in the transmitter  12 . As can be seen from FIG. 1, the normally open switch  26  is positioned opposite a magnetic portion  16 , whereas normally closed switch  28  is positioned opposite a non-magnetic portion  18 . 
     As is typical with such installations, the transmitter  12  is affixed to the moving part of a window or door, in this case the sash  20  of a window. The receiver  14  is affixed to the stationary part of a window or door, in this case the sill  22  of a window. FIG. 1 illustrates the positions of the switches in the receiver  14  with the window sash  20  in a raised or open position. When the window is open, the transmitter  12  is too far away to influence the positions of the magnetically sensitive switches in the receiver  14 . Therefore, the normally open switches  26  are in their normally open position and the normally closed switches  28  are in their closed position. In this condition, there is no continuity through the receiver  14  due to the open configuration of the normally opened switches in the first, third and fifth portions. 
     Of course it is possible and within the skill in the art to reverse the position of the transmitter  12  and receiver  14  so that the transmitter is fixed to the moving part and the receiver is fixed to the stationary part of the window or door. 
     FIG. 1 illustrates the other components of a simple alarm system in block form. An alarm module  40  is equipped to monitor a sensor loop including binary multi-code combination sensors  10   a,    10   b  and  10   c.  The alarm module  40  is equipped to detect a change of state in any of the switch portions  26 ,  28  of the receivers  14 . 
     FIG. 2 illustrates the switch configurations in the receiver  14  when the sash has been lowered to bring the transmitter  12  in close proximity to the receiver  14 . When the sash  20  is lowered, the magnetic portions  16  of the transmitter  12  are brought close enough to the normally open portions  26  of the receiver  14  to hold the normally open switches  26  in a closed and conductive position. Assuming sensors  10   b  and  10   c  are also closed, the loop including sensors  10   a,    10   b  and  10   c  will present a continuous (conductive) loop to the alarm module  40 . In the configuration illustrated in FIG. 2, the alarm could be armed. Thereafter, if any of the three sensors  10   a,    10   b,    10   c  are disturbed to the extent that their receiver switches change state, the loop will no longer be continuous and the alarm will be triggered. A typical alarm system will have multiple such loops, each loop protecting rooms or floors, or portions of a building from unauthorized entry. 
     FIG. 3 illustrates the alarm system of FIGS. 1 and 2 with the sash lowered as illustrated in FIG. 2 with the added element of an attempt to defeat sensor  10   a  by use of an externally applied magnetic field  52  from horseshoe magnet  50 . An externally applied magnetic field is the most common method of defeating a low security or Class  1  type of sensor. The normally open switches  26  are still held in their closed position by the magnetic portions of the transmitter  12  affixed to the closed window sash  20 . If only one magnetically sensitive normally open switch  26  were used in the sensor, as is typical, the externally applied magnetic field  52  would hold the normally open switch  26  in a closed position even after the sash  20  had been raised. 
     A binary multi-code combination sensor  10   a  in accordance with the present invention avoids defeat by incorporating magnetically sensitive normally closed switch portions  28 . An externally applied magnetic field  52  will cause the magnetically sensitive normally closed switch portions  28  to open, changing the state of the receiver  14  from conductive to non-conductive and triggering the alarm module  40 . An externally applied magnetic field  52  may not open all the normally closed portions  28  of a receiver  14  as illustrated in FIG.  3 . However, only one portion  28  needs to open to change the state of the receiver and trigger the alarm  40 . 
     FIG. 4 illustrates a two digit binary multi-code combination sensor. Sensor  10   d  comprises a transmitter having one magnetic portion  16  and one non-magnetic portion  18 . Switch portions  26  and  28  are positioned to respond to the arrangement of transmitter portions  16  and  18 . FIG. 5 illustrates a three digit binary multi-code combination sensor  10   e.  Increasing the number of digits in the binary multi-code combination sensor allows the pattern of magnetic and nonmagnetic portions to be varied, creating a multitude of combinations which can be used to match transmitters  12  with receivers  14 . 
     It should be noted that, no matter how many portions are used in the binary multi-code combination sensor, the pattern of all normally open or all normally closed switches must be eliminated for the practical reasons that the former does not improve security over the prior art, while the latter would only detect an externally applied magnetic field and would have no way of detecting the condition of the door or window to which it is applied. For example, in the case of the two digit binary multi-code combination sensor  10   d,  there are two possible patterns. The first pattern is illustrated in FIG. 4 wherein the left hand switch portion is magnetically sensitive and the left hand transmitter portion is magnetic. The alternative pattern is a mirror image of the configuration illustrated in FIG.  4 . 
     An increase from tow switch portions and two transmitter portions to three switch portions and three transmitter portions results in six possible combinations, an increase to four switch portions and four transmitter portions result in fourteen possible combinations and so on. Matched transmitters and receivers having three or more switch portions resist tampering by providing multiple patterns of switch and magnetic portions. Defeat of such a sensor may only be accomplished using a device which will accurately reproduce both the pattern and the relative intensity of the magnetic portions of the transmitter  12 . This is extremely difficult because magnetic fields fade quickly as the distance between the magnet and the object to be influenced increases. As a result, a magnet powerful enough to hold the normally open switch portions  26  in a closed position is very likely to open the normally closed switch portions  28 . 
     FIGS. 6 and 7 illustrate alternative configurations of a four digit binary multi-code combination sensor  10   f.  FIG. 6 illustrates a grid pattern in which receiver switch portions  26  and  28  are arranged in two rows of two, one over the other. The magnetic portions of the transmitter  12  are arranged in a corresponding pattern. The switch portions  26 ,  28  are still arranged in series within the receiver  14 . 
     FIG. 7 illustrates a simple linear pattern for a four digit binary multi-code combination sensor  10   g.  FIG. 8 illustrates an eight digit binary multi-code combination sensor. As the number of digits in a binary multi-code combination sensor increases, the corresponding length of a linear arrangement of portions may increase beyond what is practical for a particular application. Alternative configurations of the eight switch and magnetic portions of an eight digit binary multi-code combination sensor are illustrated in FIGS. 9-11. Any of these alternative configurations may be used to alleviate space constraints and/or to suit a particular application. 
     A binary multi-code combination sensor in accordance with the present invention resists tampering in two ways. First, the magnetically sensitive normally closed switch portions will open in response to the externally applied magnetic fields typically used to defeat such sensors. Second, the pattern of normally open and normally closed switches in the receiver can vary in accordance with the number of digits. Even a sophisticated burglar can never be sure what the pattern of switches will be, making defeat of a multi-portion binary multi-code combination sensor unlikely. 
     While preferred embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing descriptions should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.