Abstract:
A door sensor system comprising door sensor circuit and a sensor support circuit. The door sensor circuit has the anode of a first LED and the anode of a second LED coupled to a supply voltage terminal. The door sensor circuit has one or more reed switches, each with a first terminal coupled to the first LED cathode and a second terminal coupled to a sensor output terminal. The door sensor circuit has a Zener diode with a cathode coupled to the second LED cathode and an anode coupled to the sensor output terminal. The sensor support circuit is configured to generate, based on the voltage at the senor output terminal, a first signal if the door is open, a second signal if the door is closed, and a third signal if tampering with the door sensor system is detected.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/250,376, filed 10 Apr. 2014, which claims the benefit of U.S. Provisional Application No. 61/810,577, “LED security sensor” filed 10 Apr. 2013, both incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to security systems. 
         [0004]    2. State of the Prior Art 
         [0005]    Security systems and/or security alarm systems often use magnetic switches attached to doors, windows, and other structures to detect the unauthorized opening and/or manipulation of the door, window, or other structures. However, many previous magnetic switch designs have been prone to tampering, have exhibited unacceptable reliability. 
         [0006]    Magnetic switches use the detection of a magnetic field and/or the absence of a magnetic field to indicate that a door, window, or other enclosure access has been opened. In its simplest form, a magnetic switch uses permanent magnet mounted to an enclosure access, e.g. a door to a room, and a magnetic sensor, such as a reed switch, to detect the presence or absence of the permanent magnet. If the magnet is detected, then the door is in the expected position. If no magnet is detected, then the door is not in the expected position. This go/no-go signal can be used as an input to an alarm system, automatic monitoring systems, and/or safety interlocks. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention will be described by way of exemplary embodiments, but not limitations, illustrated in the accompanying drawings in which like references denote similar elements, and in which: 
           [0008]    The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the detailed description, serve to explain the principles and implementations of the invention. 
           [0009]      FIG. 1  shows an exemplary embodiment of a door sensor system in accordance with the invention. 
           [0010]      FIG. 2  shows the door sensor circuit that is part of the door sensor system. 
           [0011]      FIG. 3  shows a comparator circuit that is part of the sensor support circuit. 
           [0012]      FIG. 4  shows an XOR Gate and an IC Switch, both part of the sensor support circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Before beginning a detailed description of the subject invention, mention of the following is in order. When appropriate, like reference materials and characters are used to designate identical, corresponding, or similar components in different figures. The figures associated with this disclosure typically are not drawn with dimensional accuracy to scale, i.e., such drawings have been drafted with a focus on clarity of viewing and understanding rather than dimensional accuracy. 
         [0014]    In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, such as compliance with application and business related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure. 
         [0015]    Use of directional terms such as “upper,” “lower,” “above,” “below”, “in front of,” “behind,” etc. are intended to describe the positions and/or orientations of various components of the invention relative to one another as shown in the various Figures and are not intended to impose limitations on any position and/or orientation of any embodiment of the invention relative to any reference point external to the reference. 
         [0016]      FIG. 1  shows an exemplary embodiment of a door sensor system  10  in accordance with the invention. The door sensor system  10  has a door sensor circuit  100  connected to a sensor support circuit  200  with a voltage supply wire  202  and a sensor output wire  204 . 
         [0017]      FIG. 2  shows the door sensor circuit  100 . The door sensor circuit  100  is typically enclosed in some type of housing, preferably with anti-tamper features. The housing with the enclosed door sensor circuit  100  is configured to be mounted to a door frame for the purpose of monitoring whether the door is open or shut. 
         [0018]    The door sensor  100  has a supply voltage terminal  114  and a sensor output terminal  116  configured to electrically connect with a sensor support circuit  200 , typically with a pair of wires. The supply voltage terminal  114  is configured to connect with a supply voltage V S  in the sensor support circuit  200 . In the exemplary embodiment, the supply voltage is +9 volts, but in other embodiments may be some other positive dc voltage. The sensor output terminal  116  is configured to carry a voltage signal back to the sensor support circuit  200  that indicates whether the door is open or closed. 
         [0019]    The door sensor circuit  100  has a LED module  102 , a Zener diode  104 , and one or more reed switches. The exemplary embodiment has two reed switches—a first reed switch  106  and a second reed switch  108 . The LED module  102  has at least two LEDs of different colors. In the exemplary embodiment, the LED module  102  has a first LED  110  configured to emitted light of a first color and a second LED  112  configured to emit light of a second color. In other embodiments the LED module  102  may have more than two LED, each of a different color. In the exemplary embodiment, the first color is blue and the second color is red, but in other embodiments, the LEDs may be of other colors. The anodes of the first LED  110  and second LED  112  are tied together and to connected the supply voltage terminal  114 . 
         [0020]    The cathode of the first LED  110  is connected to one terminal of the first reed switch  106  and one terminal of the second reed switch  108 . If the door sensor circuit  100  has more than one reed switch, then the reed switches  106 ,  108  are connected in parallel. The other terminals of the reed switches  106 ,  108  are connected to the sensor output terminal  116 . The cathode of the second LED  112  is connected to the cathode of the Zener diode  104 . 
         [0021]      FIG. 3  shows a comparator circuit  230  that is part of the sensor support circuit  200 . The comparator circuit  230  has a comparator input  232  that is configured to connect with the sensor output terminal  116  via the sensor output wire  204 . A first resistor  234  is connected between the comparator input  232  and ground  236 . The first resistor  234  limits current drawn from the sensor output terminal  116 . The comparator circuit  230  has a first op amp  238  and a second op amp  240 . The comparator input  232  is connected to the positive inputs of the first op amp  238  and second op amp  240 . A third resistor  246 , a fourth resistor  248  and a fifth resistor  250  provide appropriate biasing for the op amps so they trigger at the desired input voltages. A sixth resistor  252  and seventh resistor  254  provide current limiting for a first op amp output  256  and second op amp output  258 , respectively. 
       Operation 
       [0022]    The first LED  110  has a first LED on-voltage V O1  and the second LED  112  has a second LED on-voltage V O2 . When the voltage across an LED is at or above its on-voltage, the LED illuminates, but below its on-voltage, the LED does not illuminate. In addition, the Zener diode  104  has a breakdown voltage V B  above which the Zener diode  104  will pass current, but below which, it will block current. The values of the first LED on-voltage V O1 , second LED on-voltage V O2 , the supply voltage V S , and the Zener diode breakdown voltage V B  are selected such that the combination of the breakdown voltage V B  with the second LED on-voltage V O2  is less than the supply voltage V S , but greater than the first LED on-voltage V O1 . Thus when either first reed switch  106  or second reed switch  108  is closed, the first LED  110  illuminates but the second LED  112  is unilluminated. When both the first reed switch  106  and second reed switch  108  are open, the second LED  112  illuminates but the first LED  110  is unilluminated. The switch open condition will produce a greater voltage drop from the supply voltage terminal  114  to the sensor output terminal  116  than if one of the switches  106 ,  108  is closed. Thus the voltage at the sensor output terminal  116  and comparator input  232  is lower for the open switch condition than it is for the closed switch position. 
         [0023]    If the voltage supply wire  202  and sensor output wire  204  connecting the door sensor circuit  100  to the sensor support circuit  200  are shorted together in an attempt to tamper and bypass the sensor, neither the first LED  110  nor second LED  112  will illuminate. Also, the voltage at the comparator input  232  will be the undiminished supply voltage V S , which will allow the sensor support circuit  200  to distinguish between the switch open/switch closed conditions and a tempering condition. 
         [0024]    For a “door open” event both the first op amp  238  comparator and second op amp  240  comparator give a “low” signal value. For a “door closed” event one op amp comparator gives a “low” signal value while the other gives a “high” signal value. If the wires  202 ,  204  between the door sensor circuit  100  and sensor support circuit  200  are tampered and shorted together both comparators return a “high” signal value. 
         [0025]      FIG. 4  shows an XOR Gate  260  and an IC Switch  262 , both part of the sensor support circuit  200 . The first op amp output  256  is connected to a first XOR Gate input  264  and the second op amp output  258  is connected to a second XOR Gate input  266 . An XOR gate output  268  will only output a high signal value for a closed door event. The XOR Gate output  268  is connected to an IC Switch input  270 . The IC Switch input  270  has an IC Switch first output  272  and an IC Switch second output  274 . The IC Switch first output  272  and IC Switch second output  274  close when there is a door closed event and open during an open door or tampered event. These two outputs are wired out to the security controller zone inputs to accurately communicate to the controller the status of the door sensor circuit. 
         [0026]    Those skilled in the art will recognize that numerous modifications and changes may be made to the preferred embodiment without departing from the scope of the claimed invention. It will, of course, be understood that modifications of the invention, in its various aspects, will be apparent to those skilled in the art, some being apparent only after study, others being matters of routine mechanical, chemical and electronic design. No single feature, function or property of the preferred embodiment is essential. Other embodiments are possible, their specific designs depending upon the particular application. As such, the scope of the invention should not be limited by the particular embodiments herein described but should be defined only by the appended claims and equivalents thereof.