Patent Publication Number: US-9898906-B2

Title: RFID based event sensor

Description:
BACKGROUND 
     A sensor is an object whose purpose is to detect events or changes in its environment, and then provide a corresponding output. A sensor can be a type of a transducer and may provide various types of output, but typically uses electrical or optical signals. Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps which dim or brighten by touching the base, etc. Most presently used sensors need a power source, e.g., a battery or power supply from an electrical outlet, to perform their functions. 
     A radio-frequency identification (RFID) system can be used to track various types of events using one or more wireless means. RFID is the wireless use of electromagnetic fields to transfer data, for the purposes of automatically identifying and tracking tags attached to objects. The tags contain electronically stored information. Some tags are powered by electromagnetic induction from magnetic fields produced near an RFID reader. Some types of RFID systems have a local power, source such as a battery, and may operate at hundreds of meters from the reader. Other types of tags are passive, e.g., collect energy from the interrogating radio waves and use them to transmit signals. RFID tags are used in many industries. For example, an RFID tag attached to an automobile during production can be used to track its progress through the assembly line; RFID-tagged pharmaceuticals can be tracked through warehouses; implanting RFID microchips in livestock and pets allows positive identification of animals; implanting RFID tags in clothing or other products allow them to be tracked in shopping malls: etc. 
     Regardless of whether RFID tags are passive or active, the RFID tags described above are typically used for just reading data. They may not be used as sensors to detect events. The RFID tags cannot be turned off or on, e.g., based on the events happening in the environment they are used, to detect the events. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a sensor system for sensing events in an environment in which the sensory system is used. 
         FIG. 2  is a block diagram illustrating actuating a sensing switch of a sensor of  FIG. 1 , consistent with various embodiments. 
         FIG. 3A  is a block diagram illustrating an example of actuating the sensor of  FIG. 1  using a permanent magnet. 
         FIG. 3B  is a block diagram illustrating an example of actuating the sensor of  FIG. 1  using an electromagnetic field source. 
         FIG. 3C  is a block diagram illustrating an example of actuating the sensor of  FIG. 1  using an electromagnetic coil. 
         FIG. 3D  is a block diagram illustrating an example construction of the sensor of  FIG. 1  with the electromagnetic coil of  FIG. 3C . 
         FIG. 4A  is a block diagram of an example indicating a state of the sensor of  FIG. 1  that is employed in a security system to determine whether a window is open or closed, when the window is closed. 
         FIG. 4B  is a block diagram of an example indicating a state of the sensor of  FIG. 1 , when the window is opened. 
         FIG. 5A  is a block diagram of a power identification system in which the sensor of  FIG. 1  is employed to determine whether an electrical device is powered on or off, consistent with various embodiments. 
         FIG. 5B  is a block diagram of a cross section of a power cord wrapped with the power identification system of  FIG. 5A , consistent with various embodiments. 
         FIG. 6  is a flow diagram of a process for using a sensor of  FIG. 1  for tracking or monitoring events in various types of applications, consistent with various embodiments. 
         FIG. 7  is a flow diagram of a process for actuating a sensing switch of a sensor of  FIG. 1  using a magnetic field, consistent with various embodiments. 
         FIG. 8  is a block diagram of a processing system that can implement operations of the disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments are disclosed for a radio frequency (RF) based sensor (“sensor”) that generates an alert on an occurrence of an event. In some embodiments, the sensor tracks the occurrence of an event using a magnetic field. Such a sensor can be built using a radio frequency identification (RFID) module that generates a RF signal, a sensing switch, e.g., a reed switch, that is actuated by a magnetic field, and an antenna that can transmit and/or receive RF signals. In some embodiments, the sensor can be built using a standard RFID tag, e.g., by adding a reed switch to the standard RFID tag or its circuit. The sensing switch can change state when exposed to a magnetic field. In some embodiments, the sensing switch switches from a first state to a second state when exposed to the magnetic field, and switches back to the first state when the magnetic field ceases to exist in the proximity of the sensing switch. For example, the sensing switch switches from an open state to a closed state when exposed to the magnetic field, and switches back to the open state when the magnetic field ceases to exist in the proximity of the sensing switch. 
     When the sensing switch is in a closed state, a circuit of the sensor is complete, and an antenna in the sensor can transmit and/or receive, e.g., send a response to a request from a base station, e.g., an RFID reader. In some embodiments, the response from the sensor operates as a “heartbeat” signal that enables tracking of an event by the base station. In some embodiments, the sensor transmits the heartbeat signal as an RF signal. The base station can perform one or more functions, e.g., generate alerts indicating an occurrence of an event, based on a receipt or non-receipt of the heartbeat signal. 
     The sensor can be used with the base station as a sensor system for tracking events in a variety of applications, e.g., Internet of Things (IoT) applications, security applications, power identification systems. For example, the sensor system can be used to determine whether a window of a building is open and generate an alert if the window is open. In some embodiments, a magnetic source, e.g., a permanent magnet, is installed on a window and the sensor is installed on a window sill. A RFID reader that polls the sensor for the heartbeat signal can be installed in any suitable location, e.g., a location at which the RFID reader can receive the heartbeat signal from the sensor via radio transmissions. When the window is closed, the window is in the proximity of the window sill and a magnetic field generated by the magnetic source in the window causes the sensing switch to be in closed state, thereby enabling the sensor to respond to the polling request from the RFID reader by sending the heartbeat signal. When the window is opened, the magnetic source installed on the window moves out of the proximity of the sensor installed on the window sill causing the sensing switch to switch to the open state and therefore, disabling the sensor from sending the heartbeat signal to the RFID reader. On not receiving the heartbeat signal from the sensor, e.g., for a specified duration, the RFID reader can then perform one or more functions such as generate an alert, e.g., send a notification to a recipient via email or a text message, generate a message in an application, trigger an audio alarm, place a telephone call to one or more recipients. Note that the above example is described using a normally open switch. However, the same can be implemented using a normally closed switch with appropriate modifications. 
     The sensing switch can be actuated in a number of ways. In some embodiments, the sensing switch is actuated using an external magnetic field from an electromagnet and/or a permanent magnet. In some embodiments, the sensing switch is actuated using an electromagnetic coil wound around the sensing switch. For example, a set of inductors connected in series and to a sensor can be printed onto a sticker and wrapped around a power chord of an electrical device. When the sticker is wrapped around the power cord, the set of inductors form a coil around the sensing switch of the sensor. When electrical power, e.g., an alternating current (AC), flows through the power cord, the electrical power generates a time varying magnetic field around the power cord, which generates a voltage in the set of inductors, which in turn can generate a magnetic field around the sensing switch that is strong enough to actuate the sensing switch. Such a sensor can be used in a power identification system to determine whether a particular electrical device is powered on. 
     The sensor can be built in various form factors. In one example, the form factor of the sensor (e.g., the RFID tag, sensing switch, and the antenna) is a sticker that can be affixed to a variety of surfaces, equipment, etc. In another example, the sensor can be installed in a housing, which can then be installed on various surfaces with appropriate mounting hardware. The sensing switch can be of an open type (i.e., normally open switch) or a close type (i.e., normally closed switch). 
     In some embodiments, the sensor does not have its own power source, e.g., required to generate, transmit and/or receive, radio signals. The sensor derives the necessary power from the request signal received as a RF signal from the base station. 
     Turning now to the figures,  FIG. 1  depicts a block diagram illustrating a sensor system  100  for sensing events in an environment in which the sensory system is used. As described above, the sensor system  100  can be used to track events in various applications, e.g., IoT applications, security systems, power identification systems, robotics, toys and games. The sensor system  100  includes a sensor  150  and a base station  120 , e.g., an RFID reader. The sensor  150  and the base station  120  communicate via RF signals. The sensor  150  can transmit an RF signal to the base station  120 , which operates as a “heartbeat” to be used by the base station  120  to track an occurrence of an event in an environment in which the sensor system  100  is used. The base station  120  can perform one or more functions, e.g., generate an alert, based on the receipt or non-receipt of the heartbeat signal. 
     The sensor  150  can include an RFID module  105 , a sensing switch  110  and an antenna  115 . The RFID module  105  can store and process sensor data. For example, the RFID module  105  can store information associated with an environment, e.g., an object, with which the sensor  150  is used and/or information necessary to generate the heartbeat signal. The sensor data may include a unique serial number of the sensor  150 , a product-related information such as a stock number, a lot or batch number, a production date associated with the object. Using the serial numbers, the base station  120  can discriminate among several sensors  150  that might be within the range of the base station  120  and read them sequentially or simultaneously. The base station  120  can transmit an encoded radio signal (“probe signal”) to probe the sensor  150 . The sensor  150  receives the probe signal and then responds with its identification and other information, e.g., as a heartbeat signal, to the base station  120 . The sensor data can be stored in a non-volatile memory. The RFID module  105  can include either a fixed or a programmable logic for processing the transmission data and sensor data, respectively. The RFID module  105  can include a memory, e.g., erasable programmable read-only memory (EPROM), to store the sensor data. 
     The RFID module  105  can modulate and/or demodulate the RF signal received from and/or transmitted to the base station  120 . In some embodiments, the sensor  150  is a passive RFID sensor, e.g., does not have its own power source. The RFID module  105  can create power, e.g., direct current (DC) power, from the probe signal it received from the base station  120  and use the power for performing the necessary functions, e.g., generating the heartbeat signal and powering the antenna  115  to transmit radio signals. 
     The antenna  115  can receive and/or transmit radio signals from and/or to the base station  120 . In some embodiments, the antenna  115  can be configured as a loop. The design of the antenna  115 , e.g., its length, shape, sensitivity, can be determined as a function of one or more of frequency of the RF signals with which the base station  120  and the sensor  150  communicate, and/or an expected communication range between the base station  120  and the sensor  150 . 
     The sensing switch  110  facilitates enabling or disabling of transmission of the heartbeat signal to the base station  120 . In some embodiments, the sensing switch  110  is a reed switch. The sensing switch  110  can be an electrical switch operated by a magnetic field. The contacts of the sensing switch  110  may be normally open (e.g., open state), closing when a magnetic field is applied (e.g., closed state), or normally closed and opening when a magnetic field is applied. In the embodiment of  FIG. 1 , the sensing switch  110  is a normally open switch. The sensing switch  110  may be actuated by a magnetic field in the proximity of the sensing switch  110 . Once the magnet field ceases to exist in the proximity of the sensing switch  110  with enough force to open or close the sensing switch  110 , the sensing switch  110  will return to its original state. 
     In some embodiments, the sensor  150  can be built using a standard RFID tag, e.g., by adding a reed switch to the standard RFID tag. The sensor  150  can be built in various form factors, e.g., depending on its intended application. For example, the sensor  150  can built as a sticker having the necessary circuitry for the sensor  150  to be placed onto an object that is being tracked. In another example, the sensor  150  can be installed in a housing that can be placed, fixed, and/or installed in association with an object that is being monitored. 
     The specification, e.g., mechanical and/or electrical specification, of the sensing switch  110  to be used in the sensor  150  depends on various factors, e.g., the application in which the sensor  150  is used, a strength of the magnetic field to be applied to actuate the sensing switch  110 , a sensing distance, which is the maximum distance between the magnet and the sensing switch  110  at which the sensing switch  110  functions satisfactorily. The sensing switch  110  can be of a compact size, low weight, have a quick response time, a long life and of low cost. 
       FIG. 2  is a block diagram illustrating actuating of the sensing switch  110  of the sensor  150  of  FIG. 1 , consistent with various embodiments. When a magnetic field  205  is applied to the sensing switch  110  the sensing switch  110  changes its state. As described above, in the embodiment of  FIG. 1 , the sensing switch  110  is a normally open switch, and therefore, when the magnetic field  205  is applied to the sensing switch  110 , the sensing switch  110  switches to a closed state, as illustrated in  FIG. 2 . When the sensing switch  110  switches to the closed state, the circuit of the sensor  150  is completed and the sensor  150  can now transmit a heartbeat signal  210  to the base station  120 , e.g., when the base station  120  sends a probe signal to the sensor  150 . 
     When the magnetic field  205  ceases to exist or decreases in strength, the sensing switch  110  returns to an open state, e.g., as illustrated in  FIG. 1 , thereby causing the circuit of the sensor  150  to be incomplete and ceasing to transmit the heartbeat signal  210  to the base station  120 . 
     In some embodiments, the sensor  150  sends the heartbeat signal  210  in response to a probing request from the base station  120  for the heartbeat signal  210 . The sensor  150  may not transmit the heartbeat signal  210  unless the base station  120  requests the sensor  150  for the heartbeat signal  210 . The sensor  150  can receive the probing request and/or transmit the heartbeat signal  210 , e.g., in response to the probing request, only if the circuit of the sensor  150  is complete, e.g., the sensing switch  110  is closed. 
     The base station  120  can be configured to perform one or more functions based on the receipt or non-receipt of the heartbeat signal  210 . For example, if the sensor  150  is employed in a security system as a proximity switch to indicate whether a door is open or shut, the base station  120  can indicate that the door is shut if the base station  120  receives the heartbeat signal  210 , and can indicate, e.g., generate an alert, that the door is open if the base station  120  does not receive the heartbeat signal  210 , e.g., for a specified period. The base station  120  and/or a computer device (not illustrated) connected to the base station  120  can have the necessary logic, components, circuitry and/or modules to interpret the meaning of a receipt or non-receipt of the heartbeat signal  210  and to perform the necessary functions based on the interpretation. In some embodiments, the interpretation and the performing of the necessary functions based on the interpretation may be distributed between the base station  120  and the computer device. 
     The sensing switch  110  described in  FIGS. 1 and 2  is a normally open switch. However, in some embodiments, the sensing switch  110  is a normally closed switch, which opens when the magnetic field  205  is applied. In the normally closed sensing switch  110 , the sensor  150  transmits the heartbeat signal  220  in the absence of the magnetic field  205  and ceases to transmit the heartbeat signal  210  in the presence of the magnetic field  205 . That is, the behavior of the normally closed sensing switch  110  is opposite to that of the normally open sensing switch  110 . The base station  120  may have to be configured accordingly to track the events appropriately. Continuing with the above example of the door, in the case of the normally closed sensing switch  110 , the base station  120  may be configured to indicate that the door is shut if the base station  120  does not receive the heartbeat signal  210 , e.g., for a specified period, and indicate that the door is open if the base station  120  receives the heartbeat signal  210 . 
     In some embodiments, actuating the sensing switch  110  using the magnetic field  205  depends on various factors, e.g., strength of the magnetic field  205  and a sensing distance of the sensing switch  110 . 
       FIGS. 3A-3C  are block diagrams illustrating various ways of actuating the sensing switch of the sensor of  FIG. 2 , consistent with various embodiments. The sensing switch  110  can be actuated in a number of ways.  FIG. 3A  is a block diagram illustrating an example  300  of actuating the sensor of  FIG. 1  using a permanent magnet  305 . The sensing switch  110  can be actuated using a permanent magnet  305 . The permanent magnet  305  can generate a magnetic field, e.g., magnetic field  205 , that can be used to actuate the sensing switch  110 . The permanent magnet  305  can be of any size and shape, which can be determined based on the intended application of the sensor  150 . The permanent magnet  305  to be used, e.g., size, shape, strength, can also be determined based on factors including a strength of the magnetic field  205  required to actuate the sensing switch  110 , the sensing distance of the sensing switch  110 , a specification of the sensing switch  110 , etc. 
       FIG. 3B  is a block diagram illustrating an example  325  of actuating the sensor of  FIG. 1  using an electromagnetic field source. The sensing switch  110  can be actuated using an electromagnet  310 . In some embodiments, an electromagnet is a type of magnet in which a magnetic field, e.g., the magnetic field  205 , is produced by an electric current. The magnetic field  205  disappears when the current is turned off. Electromagnets usually consist of a large number of closely spaced turns of wire that create the magnetic field in response to an application of current. One of the advantages of an electromagnet over a permanent magnet is that the magnetic field  205  can be quickly changed by controlling the amount of electric current flowing in the coil. Electromagnets are used as components of various electrical devices, e.g., motors, generators, relays, loudspeakers, hard disks, scientific instruments, and magnetic separation equipment. In some embodiments, the sensors  150  are used to detect presence or absence of such electromagnetic fields. 
       FIG. 3C  is a block diagram illustrating an example of actuating the sensor of  FIG. 1  using an electromagnetic coil  350 . The electromagnetic coil  350  can produce an electromagnetic field, e.g., magnetic field  205 , which can be used to actuate the sensing switch  110 . In some embodiments, the electromagnetic coil  350  can be formed using a magnetic core  365 , e.g., a ferromagnetic core, and a set of inductors  370  connected in series. When an electrical current flows through the set of the inductors  370 , a magnetic field is generated in response to the current flow. This magnetic field can be used to actuate the sensing switch  110 . In some embodiments, the magnetic field is generated in a direction perpendicular to the direction of flow of current in the set of inductors  370 . 
     In some embodiments, the intensity of the magnetic field generated by the electromagnetic coil  350  can be controlled by the amount of electric current flowing through the electromagnetic coil  350 . 
       FIG. 3D  is a block diagram illustrating an example construction of the sensor of  FIG. 1  with the electromagnetic coil of  FIG. 3C . The electromagnetic coil  350  and the sensor  150  can be built in various form factors. In some embodiments, the electromagnetic coil  350  and the sensor  150  are built into a sticker  375  (first sticker portion  375   a  and a second sticker portion  375   b  are collectively referred to as sticker  375 ). The set of inductors  370  are printed as wire traces in the sticker  375 , and a metal plate or a core like material that acts as the magnetic core  365  can be affixed to one of the sticker portions. The wire traces form a coil when the first sticker portion  375   a  and the second sticker portion  375   b  are affixed to one another. The sticker  375  can be affixed to various surfaces, and can be used for a variety of applications. The sensor  150  can also printed onto one of the portions of the sticker  375 . 
     In some embodiments, an actuation method to be used can be determined based on an application for which the sensor  150  is used.  FIGS. 4A, 4B and 5  illustrate examples of usage of the sensor of  FIG. 1  in various applications. 
       FIGS. 4A and 4B  are block diagrams illustrating the sensor of  FIG. 1  employed in a security system as a proximity switch to indicate whether a window is open or closed.  FIG. 4A  is a block diagram of an example  400  indicating a state of the sensor, when the window is closed. In some embodiments, the sensor  150  is installed on a window sill or a frame  405 . The sensor  150  can be built as a sticker, and affixed to the window frame  405 . A magnetic source, e.g., permanent magnet  305 , can be installed on a portion of the window, e.g., the door  410  of the window, that moves with respect to the window frame  405  when the window opens. The door  410  can be opened in any of various ways, e.g., the door  410  can be a vertically sliding door, a horizontally sliding door, rotates around a hinge, etc. The sensor  150  and the permanent magnet are installed accordingly. In some embodiments, the sensor  150  and the permanent magnet  305  are installed at appropriate locations of the window such that when the door  410  is shut, the magnetic field  205  generated by the permanent magnet  305  is in the proximity of the sensor  150 , that is, the permanent magnet  305  is within the sensing distance of the sensor  150  (e.g., sensing switch  110  of the sensor  150 ), and when the door is opened, e.g., by a specified amount, the magnetic field  205  is not in the proximity of the sensor  150 , that is, the permanent magnet  305  is beyond the sensing distance. 
     In some embodiments, the sensor  150  has a normally open sensing switch  110 . When the door  410  is closed, the door  410  is near the window sill  405  such that the magnetic field  205  generated by the permanent magnet  305  is within the proximity of the sensor  150 , causing the sensing switch  110  of the sensor  150  to be in the closed state. When the sensing switch  110  is in the closed state, the circuit of the sensor  150  is complete, thereby enabling the sensor  150  to respond to a probing request it receives from an RFID reader  420  by sending a heartbeat signal  425 . In some embodiments, the RFID reader  420  is similar to the base station  120  of  FIG. 1 . Upon receiving the heartbeat signal  425 , the RFID reader  420  can indicate that the window is closed. The indication can be provided in various ways. For example, the RFID reader  420  can be configured to provide an indication to an application (“app”) installed on a computing device of a user that the window is closed. In another example, the RFID reader  420  can be configured to update the status of the window in a website to which the user may login to view the indication. In yet another example, the RFID reader  420  can be configured to provide the indication to a security agency, e.g., to a server computing device at the security agency via a computer network. 
       FIG. 4B  is a block diagram of an example  450  indicating a state of the sensor of  FIG. 1 , when the window is opened. When the door  410  is opened, e.g., by a specified amount, such that the magnetic field  205  generated by the permanent magnet  305  is not in the proximity of the sensor  150  or does not have the strength required to close the sensing switch  110 , the sensing switch  110  switches to an open state causing the circuit of the sensor  150  to be incomplete. Therefore, the sensor  150  is prevented from responding to the probe request of the RFID reader  420 . When the RFID reader  420  does not receive the heartbeat signal  425  from the sensor  150 , e.g., for a predefined period, the RFID reader  420  infers that the window is open and can indicate so. In some embodiments, the RFID reader  420  is configured to generate an alert  430 , e.g., trigger an audio alarm at the premises where the window is installed, trigger a silent alarm at a security agency or a law enforcement agency, send a notification to a user via email or text message, and/or proved an indication to the app and/or the website. 
     In some embodiments, the sensor  150  may be installed on multiple windows, and the RFID reader can distinguish between the windows using the sensor data sent via the heartbeat signal  425 . For example, the sensor data can include a unique identification number that identifies a specified sensor uniquely, and the RFID reader  420  can distinguish between different windows using the unique identification number of the sensors installed at the corresponding windows. 
     The sensor  150  can be used in various such security systems. In some embodiments, the sensor  150  does not have its own power source. The sensor  150  draws the power, e.g., required to generate and transmit the heartbeat signal  425 , from the probing request received from the RFID reader  420 . The RFID reader  420  and the sensor can communicate using various RF ranges, e.g., a low frequency range of 120-150 kHz, a high-frequency range of 13.56 MHz, an ultra-high frequency range of 433 MHz and 902-928 MHz. Different frequency ranges have different communication ranges, data speeds, and different costs. A particular frequency range can be chosen based on the application for which the sensor is used. 
       FIG. 5A  is a block diagram of a power identification system  500  in which the sensor of  FIG. 1  is employed, consistent with various embodiments. The power identification system  500  can employ the sensor  150  to determine whether an electrical device  505  is powered on or off. In some embodiments, the power identification system  500  employs the sensor  150  in which the sensing switch  110  is normally open. 
     In some embodiments, the power identification system  500  is built as a sticker. The power identification system  500  includes a first sticker portion  520  and a second sticker portion  550 . The power identification system  500  can be wrapped around a power cord  515  of the electrical device  505  that is being monitored. The first sticker portion  520  includes a wire trace that forms a coil  510  when wrapped around the power cord  515 . The second sticker portion  550  can be the sensor sticker  375 , e.g., as illustrated in  FIG. 3D . The electromagnetic coil  510  is connected to the electromagnetic coil  370  of the sensor sticker  375 . The electromagnetic coils  510  and  370  can produce an electromagnetic field, e.g., magnetic field  205 , which can be used to actuate the sensing switch  110 . 
     The electrical device  505  can be powered using the power supply  560 , which can supply either alternating current (AC) or direct current (DC). When the power supply  560  is switched on, current flows through the power cord  515  creating a magnetic field around the power cord  515 , e.g., time-varying magnetic field in the interior of the coil  510 , which generates a current in the coil  510 . When the current flows from the coil  510  to coil  370 , the current generates a magnetic field in the proximity of the coil  370 . The magnetic field can be generated in a direction perpendicular to the direction of flow of current in the coil  370 . In some embodiments, the magnetic field is amplified by the core  365  and is concentrated around the core  365 . The magnetic field thus created causes the sensing switch  110  of the sensor  150  to switch to a closed state. Closing of the sensing switch  110  causes the circuit of the sensor  150  to be complete, thereby enabling the sensor  150  to respond to a probing request from the RFID reader  420  by sending a heartbeat signal, e.g., heartbeat signal  425 . 
     Upon receiving the heartbeat signal  425 , the RFID reader  420  can indicate that the electrical device is powered on. The indication can be provided in various ways. For example, the RFID reader  420  can be configured to provide an indication to the app installed on a computing device of a user that the electrical device  505  is powered on. In another example, the RFID reader  420  can be configured to update the status of the electrical device  505  in a website to which the user may login to view the indication. In yet another example, the RFID reader  420  is configured to generate an alert, e.g., send a notification to a user via email, text message or an app. 
     When the electrical device is powered off, the power stops flowing through the power cord  515  causing the magnetic field generated by the power cord  515  to cease, which stops the current flow in the coils  510  and  370  causing the magnetic field to cease, thereby causing the sensing switch  110  to switch to an open state. Opening of the sensing switch  110  causes the circuit of the sensor  150  to be incomplete, thereby preventing the sensor  150  from responding to a probing request from the RFID reader  420 . When the RFID reader  420  does not receive the heartbeat signal  425  for a specified duration, it infers that the electrical device  505  is powered off and generates an indication indicating so. For example, the RFID reader  420  is configured to generate an alert, e.g., send a notification to a user via email, text message or an app. 
     In an event the power supplied from the power supply  560  is AC, the current generated in the coil  510 , and therefore in the coil  370 , can also be AC, which causes the magnetic field thus generated to vary repeatedly causing the sensing switch  110  to open and close repeatedly, e.g., at a frequency based on the frequency of the AC. The power identification system  500  can solve the problem caused due to such a situation using various methods, and can continue to provide accurate notifications. For example, the power identification system  500  can employ a diode that filters AC and outputs DC to the coil  370 , thereby causing the magnetic field thus generated to be constant. In another example, the timeout period for receiving a heartbeat signal can be set such that that the RFID reader  420  infers that the electrical device  505  is powered on if the RFID reader  420  has received a predefined number of heartbeat signals within a specified period. 
     In the embodiment shown in  FIG. 5A , the sensor  150  does not have its own power source. The sensor  150  (e.g., the RFID module  105 ) draws the power, e.g., required to generate and transmit the heartbeat signal  425 , from the probing request received from the RFID reader  420 . 
     The first sticker portion  520  can built as a multilayered sticker.  FIG. 5B  is a block diagram of a cross section  575  of the power cord  515  wrapped with the power identification system  500 , consistent with various embodiments. As illustrated in the cross section  575 , the first layer  535 , which includes the wire trace that forms the coil  510 , is the most proximate layer to the power cord  515 , when the power identification system  500  is wrapped around the power cord  515 . The second layer  530 , which is above the first layer  535  and farther from the power cord  515 , includes a flexible metal that acts like a magnetic core to strengthen the magnetic field generated in response to a current flowing through the coil  510 . In some embodiments, the flexible metal layer is used to supplement the strength of the magnetic field generated by the coil  370  of the second sticker portion  550 . The third layer  525 , which is the farthest layer from the power cord  515 , can be any material, e.g., paper, that can be used to conceal the flexible metal. The “H”, “N” and “G” in the power cord  515  indicate the hot, neutral and ground wires of the power cord  515 . 
       FIG. 6  is a flow diagram of a process  600  for using a sensor of  FIG. 1  for tracking or monitoring events in various types of applications, consistent with various embodiments. In some embodiments, the process  600  can be implemented using the sensor system  100  of  FIG. 1 . The process  600  begins at block  605 , and at block  610 , the base station  120  transmits a probe request signal to the sensor  150 . In some embodiments, the probe request signal is an encoded RF signal that requests a sensor to respond by sending a response, e.g., heartbeat signal. The heartbeat signal typically serves as a signal to indicate that a sensor is active and/or online. The heartbeat signal can also include sensor data, e.g., information regarding an object with which the sensor  150  is used, a unique serial number of the sensor  150  and/or the object, a product-related information such as a stock number, a lot or batch number, a production date associated with the object. 
     At determination block  615 , it is determined if the sensing switch  110  of the sensor  150  is closed. If the sensing switch  110  is closed, at block  620 , the sensor  150  transmits a heartbeat signal, e.g., heartbeat signal  210  of  FIG. 2 , to the base station  120 . In some embodiments, the sensor  150  is a passive sensor, that is, the sensor  150  may not have its own power source and transmits the heartbeat signal only in response to a probe request from the base station  120 . 
     Upon receiving the heartbeat signal, at block  625 , the base station  120  performs a first function, e.g., indicate the occurrence of a first event indicative of the sensing switch being closed. 
     Referring back to the determination block  615 , if the sensing switch  110  is not closed, the sensor  150  is not active, e.g., the sensor  150  cannot receive the probe request from the base station  120 , and therefore, does not transmit the heartbeat signal to the base station  120 . Upon failure to receive the heartbeat signal for a predefined period, at block  630 , the base station  120  performs a second function, e.g., indicate the occurrence of a second event associated with the non-receipt of the heartbeat signal. 
     In some embodiments, the first and second events and the first and second functions performed by the base station depends on the application in which the sensor  150  is employed. For example, if the sensor  150  is employed as a proximity switch in a security system to indicate whether a door is open or shut, and the sensor includes a sensing switch that is normally open, the first event could be an event indicating the door is shut and the second event could be an event indicating the door is open. The first function can be a function for indicating that the door is shut, in an app installed on a computing device of a user. The second function can be a function for generating an alert, e.g., sending an email or a text message to the user or triggering a silent alarm at a law enforcement agency, to indicate that the door is open. 
     Further, if the sensing switch  110  is a normally closed type, the first function and the second function and the first and second events could be a reverse of what is described above. 
     The base station  120  and/or a computer device (not illustrated) connected to the base station  120  can have the necessary logic, components, circuitry and/or modules to interpret the meaning of a receipt or non-receipt of the heartbeat signal  210 , and to perform the necessary functions based on the interpretation. 
       FIG. 7  is a flow diagram of a process  700  for actuating a sensing switch of a sensor of  FIG. 1  using a magnetic field, consistent with various embodiments. In some embodiments, the process  700  can be implemented using the sensor system  100  of  FIG. 1 . The process  700  is described with reference to a sensor having a sensing switch that is normally open. However, the process  700  is not restricted to a sensor having a sensing switch that is normally open and can be implemented for a sensor having a sensing switch that is normally closed. The process  700  begins at block  705 , and at block  710 , it is determined if a magnetic field, e.g., magnetic field  205 , exists in the proximity of the sensing switch  110 . If the magnetic field exists, at block  715 , the sensing switch  110  switches to a closed state, which completes the circuit of the sensor  150 , thereby enabling the sensor  150  to transmit the heartbeat signal  210  to the base station  120  in response to a probe request from the base station  120 , e.g., as described at least with reference to  FIG. 6 . 
     If the magnetic field does not exist or is not strong enough to close the switch  110  in the proximity of the sensor  150 , at block  720 , the sensing switch  110  switches remains in an open state, which causes the circuit of the sensor  150  to be incomplete, thereby preventing the sensor  150  from transmitting the heartbeat signal  210  and receiving the probe request from the base station  120 , e.g., as described at least with reference to  FIG. 6 . 
     In some embodiments, the magnetic field can be generated using a permanent magnet and/or an electromagnet. In some embodiments, the magnetic field is determined to be in the proximity of the sensor if the strength of the magnetic field near the sensing switch  110  is strong enough to cause the sensing switch to change state. In some embodiments, the magnetic field is determined to be in the proximity of the sensor if the magnet that generates the magnetic field is within the sensing distance of the sensor (e.g., sensing switch  110  of the sensor  150 ). 
       FIG. 8  is a block diagram of a computer system as may be used to implement features of the disclosed embodiments. The computing system  800  may be used to implement any of the entities, components or services depicted in the examples of the foregoing figures (and any other components and/or modules described in this specification). The computing system  800  may include one or more central processing units (“processors”)  805 , memory  810 , input/output devices  825  (e.g., keyboard and pointing devices, display devices), storage devices  820  (e.g., disk drives), and network adapters  830  (e.g., network interfaces) that are connected to an interconnect  815 . The interconnect  815  is illustrated as an abstraction that represents any one or more separate physical buses, point to point connections, or both connected by appropriate bridges, adapters, or controllers. The interconnect  815 , therefore, may include, for example, a system bus, a Peripheral Component Interconnect (PCI) bus or PCI-Express bus, a HyperTransport or industry standard architecture (ISA) bus, a small computer system interface (SCSI) bus, a universal serial bus (USB), IIC (I2C) bus, or an Institute of Electrical and Electronics Engineers (IEEE) standard 1394 bus, also called “Firewire”. 
     The memory  810  and storage devices  820  are computer-readable storage media that may store instructions that implement at least portions of the described embodiments. In addition, the data structures and message structures may be stored or transmitted via a data transmission medium, such as a signal on a communications link. Various communications links may be used, such as the Internet, a local area network, a wide area network, or a point-to-point dial-up connection. Thus, computer readable media can include computer-readable storage media (e.g., “non-transitory” media) and computer-readable transmission media. 
     The instructions stored in memory  810  can be implemented as software and/or firmware to program the processor(s)  805  to carry out actions described above. In some embodiments, such software or firmware may be initially provided to the processing system  800  by downloading it from a remote system through the computing system  800  (e.g., via network adapter  830 ). 
     The embodiments introduced herein can be implemented by, for example, programmable circuitry (e.g., one or more microprocessors) programmed with software and/or firmware, or entirely in special-purpose hardwired (non-programmable) circuitry, or in a combination of such forms. Special-purpose hardwired circuitry may be in the form of, for example, one or more ASICs, PLDs, FPGAs, etc. 
     An RFID programmer can program the above described sensor  150 , e.g., RFID module  105  in the sensor, using the I/O device  825 . For example, the programmer can use the I/O device  825  to store the sensor data in the RFID module  105 . 
     Remarks 
     The above description and drawings are illustrative and are not to be construed as limiting. Numerous specific details are described to provide a thorough understanding of the disclosure. However, in some instances, well-known details are not described in order to avoid obscuring the description. Further, various modifications may be made without deviating from the scope of the embodiments. Accordingly, the embodiments are not limited except as by the appended claims. 
     Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not for other embodiments. 
     The terms used in this specification generally have their ordinary meanings in the art, within the context of the disclosure, and in the specific context where each term is used. Terms that are used to describe the disclosure are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the disclosure. For convenience, some terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term is the same, in the same context, whether or not it is highlighted. It will be appreciated that the same thing can be said in more than one way. One will recognize that “memory” is one form of a “storage” and that the terms may on occasion be used interchangeably. 
     Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for some terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any term discussed herein is illustrative only, and is not intended to further limit the scope and meaning of the disclosure or of any exemplified term. Likewise, the disclosure is not limited to various embodiments given in this specification. 
     Those skilled in the art will appreciate that the logic illustrated in each of the flow diagrams discussed above, may be altered in various ways. For example, the order of the logic may be rearranged, substeps may be performed in parallel, illustrated logic may be omitted; other logic may be included, etc. 
     Without intent to further limit the scope of the disclosure, examples of instruments, apparatus, methods and their related results according to the embodiments of the present disclosure are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the disclosure. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. In the case of conflict, the present document, including definitions will control.