Patent Publication Number: US-11645897-B2

Title: Sensor assembly for use in a security alarm system and method of installing the same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sensor assembly and, in particular, to a magnetic field sensor with an indicator which indicates the presence or absence of a magnetic field or, to a security alarm sensor with an RFID reader which indicates when the sensor is within a predetermined distance of an RFID tag. 
     Description of the Related Art 
     It is known to provide a magnetic field sensor with a light-emitting diode indicator. U.S. Pat. No. 4,296,410, which issued on Oct. 20, 1981 to Higgs et al., discloses an integrated circuit including a Hall element and a threshold detector. The threshold detector is encased in a plastic housing with the plane of the Hall element parallel with a face of the housing to provide a two-state Hall element proximity sensor. A light-emitting diode is mounted in the housing and is connected to the output of the detector. This provides visual indication of the state of the sensor. A kit includes the sensor and a compatible magnet which may be used as a proximity sensor in a security alarm system, 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a magnetic field sensor having an indicator which indicates the presence or absence of a magnetic field, or a security alarm sensor with an RFID reader which indicates when the sensor in communication range with an RFID tag. 
     There is accordingly provided a magnetic field sensor comprising a microprocessor and an indicator which turns on when a magnetic field is sensed and turns off when a magnetic field is not sensed. The microprocessor renders the indicator inoperable a predetermined period of time after the magnetic field sensor is powered up. 
     There is also provided a magnetic proximity sensor including a magnet which generates a magnetic field and a magnetic field sensor. The magnetic field sensor comprises a microprocessor and an indicator which turns on when a magnetic field generated by the magnet is sensed and turns off when a magnetic field generated by the magnet is not sensed. The microprocessor renders the indicator inoperable a predetermined period of time after the magnetic field sensor is powered up. 
     There is further provided a magnetic field sensor comprising a housing having a lid. The magnetic field sensor includes a tamper switch which detects when the lid of the housing is opened. The magnetic field sensor includes a device which senses a presence or an absence of a magnetic field. The magnetic field sensor includes a power source disposed within the housing. The magnetic field sensor includes a pull strip positioned to inhibit the power source from providing power to the magnetic field sensor. The magnetic field sensor is powered up when the pull strip is removed from the power source. The magnetic field sensor includes a microprocessor. A signal is sent by the tamper switch to the microprocessor when the lid of the housing is open. A signal is sent by the device to the microprocessor when a magnetic field is sensed. The magnetic field sensor includes an indicator which indicates the presence or the absence of a magnetic field. The power source supplies current to the indicator, and the indicator turns on when a magnetic field is sensed and turns off when a magnetic field is not sensed. The indicator is initially operable following the pull strip being removed from the power source and the magnetic field sensor being powered up. The microprocessor renders the indicator inoperable a predetermined period of time after the pull strip is removed from the power source and the magnetic field sensor is powered up. The indicator remains inoperable until the lid of the housing is removed and the lid of the housing is closed at which point the indicator is operable until the microprocessor renders the indicator inoperable a predetermined period of time after the lid of the housing is closed. 
     There is yet also provided a proximity sensor including a magnet which generates a magnetic field and a magnetic field sensor. The magnetic field sensor includes a housing having a lid. The magnetic field sensor includes a tamper switch which detects when the lid of the housing is opened. The magnetic field sensor includes a device which senses a presence or an absence of a magnetic field. The magnetic field sensor includes a power source disposed within the housing. The magnetic field sensor includes a pull strip positioned to inhibit the power source from providing power to the magnetic field sensor. The magnetic field sensor is powered up when the pull strip is removed from the power source. The magnetic field sensor includes a microprocessor. A signal is sent by the tamper switch to the microprocessor when the lid of the housing is open. A signal is sent by the device to the microprocessor when a magnetic field is sensed. The magnetic field sensor includes an indicator which indicates the presence or the absence of a magnetic field. The power source supplies current to the indicator, and the indicator turns on when a magnetic field is sensed and turns off when a magnetic field is not sensed. The indicator is initially operable following the pull strip being removed from the power source and the magnetic field sensor being powered up. The microprocessor renders the indicator inoperable a predetermined period of time after the pull strip is removed from the power source and the magnetic field sensor is powered up. The indicator remains inoperable until the lid of the housing is removed and the lid of the housing is closed at which point the indicator is operable until the microprocessor renders the indicator inoperable a predetermined period of time after the lid of the housing is closed. 
     There is yet further provided a method of installing a magnetic proximity sensor. The magnetic proximity sensor includes a magnet. The method includes providing the magnetic proximity sensor with an indicator that turns on for a predetermined period of time when a magnetic field generated by the magnet is sensed by the magnetic field sensor. The method includes positioning the magnetic field sensor in a desired location and positioning the magnet in a desired location relative to the magnetic field sensor. The indicator continues to be turned on during said predetermined period of time when the magnetic field generated by the magnet is sensed by the magnetic field sensor. The indicator is turned off during the predetermined period of time when the magnetic field generated by the magnet is not sensed by the magnetic field sensor. The indicator light thus assists in determining proper relative positioning of the magnet and the magnetic field sensor. If after the predetermined period of time more time is needed to install the magnetic proximity sensor, the method includes initiating another predetermined period of time by removing and replacing a lid of the magnetic proximity sensor. 
     In one example, the indicator may be a light-emitting diode. The power source may be a coin cell battery. The magnetic field sensor may include a supercapacitor. The magnetic field sensor may also include a tamper switch. 
     The sensors disclosed herein may be used together with a magnet as a proximity sensor, for example, as a door sensor or window sensor in a security alarm system. 
     There is yet additionally provided a security alarm sensor assembly comprising an RFID tag mountable on a first of a window or a door or framing thereof. The security alarm sensor assembly includes a sensor mountable on a second of the window or the door or said framing thereof. The sensor includes a housing having a lid. The sensor includes a tamper switch which detects when the lid of the housing is opened. The sensor includes a power source disposed within the housing. The sensor includes an RFID reader which emits an electromagnetic field. The sensor includes a microprocessor. A signal is sent by the tamper switch to the microprocessor when the lid of the housing is open. The microprocessor analyzes changes in signals from the RFID tag to determine when a distance between the RFID tag and the RFID reader is within a predetermined threshold. The sensor includes an indicator which indicates when the distance between the RFID tag and the RFID reader is within said predetermined threshold. The power source supplies current to the indicator. The indicator turns on when the distance between the RFID tag and the RFID reader is within said predetermined threshold. The indicator turns off when the distance between the RFID tag and the RFID reader is outside of said predetermined threshold. The indicator is initially operable upon the sensor being powered up. The microprocessor renders the indicator inoperable a predetermined period of time after the sensor being powered up. The indicator remains inoperable until the lid of the housing is removed and the lid of the housing is next closed at which point the indicator is operable until the microprocessor renders the indicator inoperable a predetermined period of time after the lid of the housing is closed. 
    
    
     
       BRIEF DESCRIPTIONS OF DRAWINGS 
       The invention will be more readily understood from the following description of the embodiments thereof given, by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1    is a perspective view of a sensor assembly according to a first aspect, the sensor assembly comprising a magnet and a first magnetic field sensor; 
         FIG.  2    is another perspective view of the magnet and the magnetic field sensor of  FIG.  1   ; 
         FIG.  3    is a further perspective view of the magnet and the magnetic field sensor of  FIG.  1   ; 
         FIG.  4    is an exploded view of the magnet and the magnetic field sensor of  FIG.  1   ; 
         FIG.  5    is a schematic diagram of the magnetic field sensor of  FIG.  1   ; 
         FIG.  6    is a bottom plan view of the magnetic field sensor of  FIG.  1   ; 
         FIGS.  7 A to  7 E  are circuit diagrams of the magnetic field sensor of  FIG.  1   ; 
         FIG.  8    is a perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window closed; 
         FIG.  9    is another perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window closed; 
         FIG.  10    is a further perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window closed; 
         FIG.  11    is a perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window opened a distance D1; 
         FIG.  12    is another perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window opened a distance D1; 
         FIG.  13    is a further perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window opened a distance D1; 
         FIG.  14    is a perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window opened a distance D2; 
         FIG.  15    is another perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window opened a distance D2; 
         FIG.  16    is a further perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window opened a distance D2; 
         FIG.  17    is a perspective view showing the magnet and the magnetic field sensor of  FIG.  1    being used as a window sensor in a security alarm system with the window opened a distance D3; 
         FIG.  18    is a flow chart showing the logic of installing the magnet and the magnetic field sensor of  FIG.  1    relative to each other; 
         FIG.  19    is a flow chart showing the logic of a software algorithm which drives the magnetic field sensor of  FIG.  1   ; 
         FIG.  20    is a perspective view of a sensor assembly according to a second aspect, the sensor assembly comprising a magnet and a magnetic field sensor; 
         FIG.  21    is another perspective view of the magnet and the magnetic field sensor of  FIG.  20   ; 
         FIG.  22    is a perspective view of a sensor assembly according to a third aspect, the sensor assembly comprising a magnet and a magnetic field sensor; 
         FIG.  23    is a perspective view of a sensor assembly according to a fourth aspect, the sensor assembly comprising a magnet and a magnetic field sensor; 
         FIG.  24    is a perspective view of a sensor assembly according to a fifth aspect, the sensor assembly comprising a magnet and a magnetic field sensor; 
         FIG.  25    is a front, right side exploded view of a sensor assembly according to a sixth aspect, the sensor assembly comprising a magnet and a magnetic field sensor, with a door frame and door to which the magnet field sensor and magnet are to be connected also being shown, the door frame and door being shown in fragment; 
         FIG.  26    is a rear, left side perspective view of the magnetic field sensor of  FIG.  25   , with a pull strip thereof in the process of being removed to enable a power source thereof to power the magnetic field sensor; 
         FIG.  27    is a flow chart showing the logic of installing the magnet and the magnetic field sensor of  FIG.  25    relative to each other; 
         FIG.  28    is a flow chart showing the logic of a software algorithm which drives the magnetic field sensor of  FIG.  1   ; 
         FIG.  29    is a perspective view of a sensor assembly according to a seventh aspect, the sensor assembly comprising an RFID tag and a sensor with an RFID reader, the sensor being shown in a perspective, exploded view; 
         FIG.  30    is a perspective view showing the RFID tag and the sensor of  FIG.  29    being used as a window sensor in a security alarm system with the window closed; 
         FIG.  31    is a perspective view showing the RFID tag, the sensor and the window of  FIG.  30    with the window being open a distance of D1.6; 
         FIG.  32    is a perspective view showing the RFID tag, the sensor and the window of  FIG.  30   , the window being open a distance of D2.6; and 
         FIG.  33    is a rear, left side perspective view of a sensor assembly according to an eighth aspect, the sensor assembly being similar to that shown in  FIG.  29    with the exception that the sensor assembly includes a pull strip with the removal thereof enabling a power source thereof to power the sensor thereof. 
     
    
    
     DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings and first to  FIGS.  1  to  3   , there is a provided a sensor assembly  9 . The sensor assembly includes a first subassembly, in this embodiment in the form of a magnet  10 . The sensor assembly  9  includes a second subassembly, in this embodiment a sensor, in this example a magnetic field sensor  14 . A field, in this example a magnetic field  12 , is generated by the magnet  10  and said magnetic field is sensed by the magnetic field sensor. 
     The magnetic field sensor  14  includes an indicator which, in this example, is a visual indicator in the form of an indicator light  16  that turns on when the magnetic field sensor  14  is within the magnetic field  12  as shown in  FIGS.  1  and  2   . The indicator light  16  turns off when the magnetic field sensor  14  is outside the magnetic field  12  as shown in  FIG.  3   . The indicator light  16  may accordingly provide a visual indication as to the presence or absence of a magnetic field. Together with magnet  10  and magnetic field sensor  14  may be used as a magnetic proximity sensor. 
     The magnet  10  is shown in greater detail in  FIG.  4    and, in this example, is a bar magnet  11  which is disposed in a housing  13  provided with a cover  15 . The magnetic field sensor  14  is also shown in greater detail in  FIG.  4    and, in this example, is a substantially rectangular parallelepiped with rounded corners but may be other shapes. The magnetic field sensor  14  includes a housing  18  and a circuit board  20  disposed within the housing. The housing  18  is provided with a lid  19  that has a window  21  to facilitate viewing of the indicator light  16  which is mounted on the circuit board  20 . The window  21  may be an aperture in the lid  19  or a translucent portion of the lid  19 . A microprocessor  22 , a power source which is in the form of a coin cell battery  24 , and a device which senses a magnetic field which is in the form of a reed switch  26  are also mounted on the circuit board  20 . It will however be understood by a person skilled in the art that any AC or DC power source may be used. Likewise any device which senses a magnetic field, such as a magnetoresistive sensor or Hall Effect sensor or MAGNASPHERE™ may be used in place of the reed switch. 
     In this example, the indicator light  16  is a light-emitting diode package and includes a blue light-emitting diode, a green light-emitting diode, and a red light-emitting diode. The reed switch  26  is actuated by a magnetic field and the microprocessor  22  monitors the change of state of the reed switch  26  by periodically sampling the reed switch  26  to sense a magnetic field. If a magnetic field is sensed then the microprocessor  22  turns on the indicator light  16 . In the absence of a magnetic field, the microprocessor  22  turns the indicator light  16  off. The sampling of the reed switch  26  by the microprocessor  22  may be done, for example, four times per second or as many times per second as required. The sensitivity of the magnetic field sensor  14  may be adjusted by selecting different types of reed switches. This may be used to set a maximum or minimum distance at which the magnetic field sensor  14  is able to sense a magnet. If another device which senses a magnetic field is used in the magnetic sensor, such as a magnetoresistive sensor or Hall Effect sensor or MAGNASPHERE™, then the sensitivity of the magnetic field sensor may be adjusted based on measured analog and/or digital output. 
     Referring now to  FIG.  5   , in this example, the magnetic field sensor  14  includes a radio, which is in the form of a radio chip  28  in this example, and an antenna  30  that allows the magnetic field sensor  14  to transmit and receive radio signals. The antenna  30  may communicate with a control panel  40  as part of a security alarm system  31 . There is also a battery detection circuit  32 , a tamper switch  34 , and a supercapacitor  36 . The battery detection circuit  32  and tamper switch  34  are both conventional and in communication with the microprocessor  22 . The supercapacitor  36  may be used to assist the coin cell battery  24  as the power source. Without the supercapacitor  36  the coin cell battery  24  may not be able to provide the surge current required when the radio chip  28  and the antenna  30  transmit and receive radio signals. This is due to the internal resistance of a coin cell battery. A coin cell battery generally cannot be used in applications requiring current in excess of about 20 to 30 milliamperes. The internal resistance of the coin cell battery causes a voltage drop when larger currents are required. This may cause the terminal voltage to drop below a minimum acceptable level of, for example, 2.2 volts. 
     The supercapacitor  36  may have a low profile which, in combination with use of the coin cell battery  24 , allows the magnetic field sensor  14  to be relatively small. The supercapacitor  36  allows for high short term current draws while still providing a terminal voltage of, for example, 3.0 volts. Without the supercapacitor  36  a larger battery may have to be used as a power source. The supercapacitor  36  may have a sufficient residual charge to prevent the microprocessor  22  from properly detecting the removal of the coin cell battery  24  during battery replacement. However, the battery detection circuit  32  allows the microprocessor  22  to shut down properly when the coin cell battery  24  is removed. There may also be a reverse voltage protection circuit  38 , which may be a diode or P-channel mosfet, connected in series between the coin cell battery  24  and the supercapacitor  36  to ensure that the coin cell battery  24  is not reverse charged if the supercapacitor  36  has a higher voltage. The tamper switch  34  may be internal or external of the housing  18  and detects when the lid  19  of the housing  18  is removed and sends a signal to the microprocessor that the lid  19  of the housing  18  has been removed and someone is tampering with the magnetic field sensor  14 . The tamper switch  34  also sends a signal to the microprocessor  22  to restart an algorithm related to the sensing of a magnetic field when the tamper switch  34  detects that the lid  19  of the housing  18  has been removed. In other examples, the magnetic field sensor may not have a tamper switch and the microprocessor may be signalled to restart the algorithm related to the sensing of a magnetic field when the coin cell battery is inserted. The microprocessor may alternatively be signalled to restart the algorithm related to the sensing of a magnetic field when an ON/OFF switch is actuated. Such an ON/OFF switch may be used turn the indicator on and off. 
     The magnetic field sensor  14  is further provided in this example with a MEMS oscillator  42  which may be programmed to a plurality of discrete frequencies and, in this case, to at least four discrete frequencies which are feed to the radio chip  28  to generate an output frequency ranging between 250 MHz and 1 GHz. The MEMS oscillator  42  is able to provide the at least four discrete frequencies to the radio chip  28  without an additional phase locked loop being required to generate the output frequency, or output signal  41 , because the radio chip  28  is provided with a single phase-locked loop  29 , for example, a x32 multiplier to generate the output frequency. There is also a dip switch  44  which, in this example, is a four position dip switch. Referring now to  FIG.  6   , the dip switch  44  is mounted on a side of the circuit board  20  opposite of the indicator light  16 . This allows the dip switch  44  to be accessed through an aperture  46  in the housing  18  as shown in  FIG.  6   . The M EMS oscillator and dip switch are not strictly required and the magnetic field sensor may not have such components in other embodiments. 
     The microprocessor  22  is programmed with a plurality of data protocols and each output frequency may operate on at least one of the data protocols. The dip switch  44  is actuated to provide a code to the microprocessor  22  and a data protocol is implemented by the microprocessor  22  based on the code. The MEMS oscillator  42  is programmed to a discrete frequency based on the data protocol implemented by the microprocessor  22 . The MEMS oscillator  42  then provides the discrete frequency to the radio chip  28  which an output signal  41  based on the discrete frequency. This allows an installer to select a discrete frequency to match the protocol of a given alarm system. Respective ones of digitally tuned capacitor chips  48   a  and  48   b  are disposed at each terminal of the antenna  30 . The capacitor chips  48   a  and  48   b  are used in a shunt mode rather than a series mode to prevent a degradation of antenna performance resulting due to stray capacitance issues when the capacitor chips  48   a  and  48   b  are used in series. Using the capacitor chips  48   a  and  48   b  in a shunt configuration may allow the antenna  30  to be tuned. The supercapacitor  36  may maintain a maximum output signal by maintaining the voltage at its maximum value during transmission of the output signal. A circuit diagram of the magnetic field sensor is shown in  FIGS.  7 A to  7 E . 
     Referring now to  FIGS.  8  to  17   , the magnet  10  and magnetic field sensor  14  are shown in use as a proximity sensor in the form of a window sensor of a security alarm system. It will however be understood by a person skilled in the art that the magnet  10  and magnetic field sensor  14  may also be used as a door sensor or in any other proximity sensor application. The magnet  10  and magnetic field sensor  14  are each mounted on a window  50  with the magnetic field sensor  14  generally being mounted first, although this is not strictly required. The magnet  10  is mounted on a stile  52  of the window  50  while the magnetic field sensor  14  is mounted on a side jamb  54  of the window near a sill  56  thereof. The window  50  is fully closed in  FIGS.  8  to  10    with a bottom rail  58  of the window abutting the sill  56  thereof. When the window  50  is fully closed, the magnetic field sensor  14  is able to sense a magnetic field generated by the magnet  10  when the magnet is mounted along the stile  52  as indicated by the indicator light  16  which is turned on in  FIGS.  8  to  10   . The indicator light  16  of the magnetic field sensor  14  is turned on when the magnet  10  is mounted on the stile  52  at a first position adjacent to the bottom rail  58  of the window  50  as shown in  FIG.  8   , at a second position away from the bottom rail  58  of the window  50  as shown in  FIG.  9   , and at a third position further away from the bottom rail  58  of the window  50  as shown in  FIG.  10   . This provides a visible confirmation to an installer that, when the magnetic field sensor  14  is mounted on the side jamb  54  of the window  50  near the sill  56  thereof, the magnet  10  may be mounted anywhere on the stile  52  between the first position and the third position for the magnetic field sensor to still be able to sense a magnetic field generated by the magnet. An alarm will accordingly not be triggered when the window  50  is fully closed and the magnet  10  and magnetic field sensor  14  are positioned relative to one another as shown in  FIGS.  8  to  10   . 
     However, it may be desirable for an alarm to not be triggered when the window  50  is not fully closed. This would allow the window  50  to be partially opened for ventilation but not enough to allow an intruder to enter through the window. For example, as shown in  FIGS.  11  to  13   , it may be desired to allow the window  50  to be opened a distance of D1 without triggering an alarm.  FIG.  11    shows that the magnetic field sensor  14  is able to sense the magnetic field generated by the magnet  10 , as visually indicated by the indicator light  16  which is turned on, when the magnet  10  is mounted on the stile  52  adjacent to the bottom rail  58 .  FIG.  12    shows that the magnetic field sensor  14  is also able to sense the magnetic field generated by the magnet  10 , as visually indicated by the indicator light  16  which is turned on, when the magnet  10  is mounted on the stile  52  away from the bottom rail  58 .  FIG.  13    shows that the magnetic field sensor  14  is unable to sense the magnetic field generated by the magnet  10 , as visually indicated by the indicator light  16  which is turned off, when the magnet  10  is mounted on the stile  52  further away from the bottom rail  58 . The indicator light  16  accordingly provides a visual indication to an installer as to where on the stile  52  the magnet  10  may be mounted to allow the window  50  to be opened a distance of D1 without triggering an alarm. 
       FIGS.  14  to  16    show where the magnet  10  may be positioned to avoid triggering an alarm when the window is opened a distance of D2 which is greater than D1.  FIG.  14    shows that the magnetic field sensor  14  is able to sense a magnetic field generated by the magnet  10 , as visually indicated by the indicator light  16  which is turned on, when the magnet  10  is mounted on the stile  52  adjacent to the bottom rail  58 .  FIG.  15    shows that the magnetic field sensor  14  is unable to sense the magnetic field generated by the magnet  10 , as visually indicated by the indicator light  16  which is turned off, when the magnet  10  is mounted on the stile  52  away from the bottom rail  58 .  FIG.  16    shows that the magnetic field sensor  14  is also unable to sense the magnetic field generated by the magnet  10 , as visually indicated by the indicator light  16  which is turned off, when the magnet  10  is mounted on the stile  52  further away from the bottom rail  58 . The indicator light  16  accordingly provides a visual indication to an installer that the magnet  10  should be mounted on the stile  52  adjacent to the bottom rail  58 , allowing the window  50  to be opened a distance of D2 without triggering an alarm. However, as shown in  FIG.  17   , opening the window a distance of D3 which is greater than D2 will trigger an alarm even if the magnet  10  is mounted on the stile  52  adjacent to the bottom rail  58 . The indicator light  16  accordingly provides an installer with a visual indication to verify correct placement of the magnet  10  to allow a maximum threshold opening of the window  50 . It will be understood by a person skilled in the art that mounting the magnet  10  on the stile  52  of the window  50  and mounting the magnetic field sensor  14  on the side jamb  54  of the window is only an example. The magnet  10  and the magnetic field sensor  14  may be mounted anywhere provided there is relative movement of the magnet  10  and the magnetic field sensor  14  when the window  50  is opened. 
       FIG.  18    is a flow chart showing the logic of installing the magnet  10  and magnetic field sensor  14  relative to each other. In this embodiment the coin cell battery  24  is first inserted into the magnetic field sensor  14 , as shown by box  53 . It is next determined if the indicator light flashes, as shown by box  55 , to indicate that the magnetic field sensor  14  seen in  FIG.  17    has powered up and is functioning properly. If the indicator light  16  does not flash, then the magnetic field sensor  14  is not functioning properly and the coin cell battery  24  is removed and replaced, as shown by box  57  in  FIG.  18   . 
     Referring to  FIGS.  8  and  18   , the magnetic field sensor  14  is next positioned in a desired location, as shown by box  59 , and the magnet  10  positioned in a desired location relative to the magnetic field sensor  14 , as shown by box  61 . It is next determined if the indicator light turns on based on whether the magnet is within range, as shown by box  63 . The indicator light  16  will turn on when a magnetic field generated by the magnet  10  is sensed by the magnetic field sensor  14 , as seen in  FIGS.  8  to  12  and  14    and the indicator light  16  will turn off when a magnetic field generated by the magnet  10  is not sensed by the magnetic field sensor  14 , as seen in  FIGS.  13  and  15  to  17   . The indicator light  16  accordingly assists an installer in determining proper relative positioning of the magnet  10  and magnetic field sensor  14 . This allows the window  50  to be opened a certain distance without triggering the alarm. Once the installer is satisfied with the relative positioning of the magnet  10  and magnetic field sensor  14  as shown by box  65  in  FIG.  18   , the lid  19  of the magnetic field sensor  14  is closed, as shown by box  67 . The indicator light  16  will then remain on for a predetermined period of time when a magnetic field generated by the magnet  10  is sensed by the magnetic field sensor  14 , as shown by box  69 . 
     Referring to  FIG.  4   , the magnetic field sensor  14  is further provided with software having an algorithm which turns off the indicator light  16  after a predetermined period of time even if the magnetic field sensor  14  senses a magnetic field. This conserves the coin cell battery  24  of the magnetic field sensor  14  and does away with any visual annoyance resulting from the indicator light  16  being turned on when the window  50  is closed or opened less than a maximum threshold opening required to trigger an alarm. Referring to  FIGS.  8  and  13   , the magnetic field sensor  14  will however continue to otherwise operate normally and transmit a signal to trigger an alarm when the window  50  is opened and the magnetic field sensor  14  no longer senses a magnetic field generated by the magnet  10 . The indicator light  16  is accordingly operable during installation of the window sensor and assists an installer in determining the relative positioning of the magnet  10  and magnetic field sensor  14  so as to allow the window  50  to be opened a certain distance without triggering the alarm. If an installer is unable to position the magnet  10  and magnetic field sensor  14  within the predetermined period of time before the software turns off the indicator light  16  then the lid  19  of the magnetic field sensor  14  can be removed to restart the algorithm. Replacing of the lid  19  will result in another predetermined period of time in which the indicator light  16  is operable to assist an installer in determining the relative positioning of the magnet  10  and magnetic field sensor  14  so as to allow the window  50  to be opened a certain distance without triggering the alarm. 
       FIG.  19    is a flow chart showing the logic of the algorithm. In this embodiment the coin cell battery  24  seen in  FIG.  4    is first inserted into the magnetic field sensor  14 , as shown by box  71  in  FIG.  19   . Referring to  FIGS.  8  and  19    and as shown by box  73  the green light-emitting diode of the indicator light  16  thereafter turns on for a period of time, for example, five seconds. This indicates that the magnetic field sensor  14  has powered up and is functioning properly. The blue light-emitting diode of the indicator light  16  will then be operable, as shown by box  75 . The magnet  10  and the magnetic field sensor  14  are next positioned relative to one another as shown in  FIGS.  7  to  16   . The blue light-emitting diode of the indicator light  16  will turn on when a magnetic field generated by the magnet  10  is sensed by the magnetic field sensor  14  and the blue light-emitting diode of the indicator light  16  will turn off when a magnetic field generated by the magnet  10  is not sensed by the magnetic field sensor  14 . The indicator light  16  accordingly assists an installer in determining proper relative positioning of the magnet  10  and magnetic field sensor  14 . Proper relative positioning of the magnet  10  and magnetic field sensor  14  may allow the window  50  to be opened a certain distance without triggering the alarm. 
     If an installer is unable to position the magnet  10  and magnetic field sensor  14  within the predetermined period of time before the software turns off the indicator light  16  then the lid  19  can be removed to restart the algorithm as shown in  FIG.  18   . Thus the magnetic field sensor determines if the lid thereof is open or still within the predetermined period of time, as shown by box  77 . If the predetermined period of time has expired, the blue LED light is turned off, as shown by box  79  in  FIG.  19   . If the lid is open and then closed, the predetermined period of time begins once more and it is next determined whether the magnetic field sensor senses a magnetic field, as shown by box  81 . If yes, the blue LED light will turn on as shown by box  83 . If no magnetic field is sensed by the magnetic field sensor, the blue LED light remains off, as shown by box  85 , and the positioning of the magnetic field sensor relative to the magnet will need to be adjusted. 
     A red light-emitting diode of the indicator light  16  will flash when the coin cell battery  24  runs down below a predetermined low battery threshold in this example. The frequency of the flashing will increase as the battery continues to run down. 
     The software may run other routines as set out below during operation of the magnetic field sensor  14 . 
     task 1: 
     
         
         
           
             if tamper switch active (lid open) now or within past three minutes:
           if magnet is present:
               blue light-emitting diode ON   
               else
               blue light-emitting diode OFF
 
task 2
   
               
         
             if magnet becomes present or magnet becomes absent:
           send magnet-change message
               if battery is failing
                   blink red light-emitting diode   
                   
               else
               if tamper switch active (lid open) now or within past three minutes:
                   blink green light-emitting diode
 
task 3:
   
                   
               
         
             if approximately one hour has passed with no message sent:
           send supervisory message   
         
           
         
       
    
       FIGS.  20  to  21    show a sensor assembly  9 . 1  including a magnet  10 . 1  and magnetic field sensor  14 . 1  according to a second aspect. Like parts have like numbers and functions as the sensor assembly  9 , magnet  10  and magnetic field sensor  14 . 1  shown in  FIGS.  1  to  19    with the addition of decimal extension “0.1”. Assembly  9 . 1  is substantially the same as assembly  9  shown in  FIGS.  1  to  19    with the following exceptions. 
     Magnetic field sensor  14 . 1  has a first indicator light  16 . 1  and a second indicator light  74 . The first indicator light  16 . 1  functions in a manner substantially identical to the indicator light  16  of the magnetic field sensor  14  shown in  FIGS.  1  to  19   . The first indicator light accordingly provides a visual indication as to the presence or absence of a magnetic field. The second indicator light  74  provides a visual indication as to the strength of a magnetic field. The first indicator light  16 . 1  turns on when the magnetic field sensor  14 . 1  is within the magnetic field  12 . 1  generated by the magnet  10 . However, as shown in  FIG.  20   , the second indicator light  74  does not turn on when the magnetic field sensor  14 . 1  is merely near a periphery of the magnetic field  12 . 1 . The second indicator light  74  only turns on when the magnetic field sensor  14 . 1  is well within the magnetic field  12 . 1  as shown in  FIG.  21   . This allows an installer to see the best range for the relative positioning of the magnet  10 . 1  and magnetic field sensor  14 . 1 . The second indicator light  74  is turned on by a microprocessor when a second, parallel device such as a reed switch senses a magnetic field. 
       FIG.  22    shows a sensor assembly  9 . 2  including a magnet  10 . 2  and magnetic field sensor  14 . 2  according to a third aspect. Like parts have like numbers and functions as the sensor assembly  9 , magnet  10  and magnetic field sensor  14  shown in  FIGS.  1  to  19    with the addition of decimal extension “0.2”. Assembly  9 . 2  is substantially the same as assembly  9  shown in  FIGS.  1  to  1 . 9    with the following exceptions. 
     Magnetic field sensor  14 . 2  has a socket  84  for an additional dry contact. This allows another device, such as a reed switch  86 , which senses the presence or absence of the magnetic field  12 . 2  generated by the magnet  10 . 2 , to be coupled to the magnetic field sensor  14 . 2  by a tether  88 . This increases the range of the magnetic field sensor  14 . 2  as the indicator light  16 . 2  will turn on when the reed switch  86  is within the magnetic field  1 . 2 . 2  even if the magnetic field sensor  14 . 2  is not within the magnetic field  12 . 2 . 
     The examples shown in  FIGS.  1  to  22    comprise a magnetic field sensor with an indicator in the form of an indicator light. However, it will be understood by a person skilled in the art that, in other examples, the indicator may be an auditory indicator which produces a sound to indicate the presence or absence of a magnetic field, or a vibratory indicator which vibrates to indicate the presence or absence of a magnetic field, or a combination of indicators selected from a visual indicator, an auditory indicator and a vibratory indicator. A switch may be used to select a desired indicator mode. 
       FIG.  23    shows a sensor assembly  9 . 3  including a magnet  10 . 3  and magnetic field sensor  14 . 3  according to a fourth aspect. Like parts have like numbers and functions as the sensor assembly  9 , magnet  10  and magnetic field sensor  14  shown in  FIGS.  1  to  19    with the addition of decimal extension “0.3”. Assembly  9 . 3  is substantially the same as assembly  9  shown in  FIGS.  1  to  19    with the following exceptions. 
     Magnetic field sensor  14 . 3  includes an indicator in the form of an auditory indictor  16 . 3  that provides an auditory indication as to the presence or absence of a magnetic field. The auditory indicator turns on and emits a sound when the magnetic field sensor  14 . 3  is within the magnetic field  12 . 3  generated by the magnet  10 . 3 . This provides an auditory indication as to the presence of a magnetic field. However, after a predetermined period of time, the auditory indicator  16 . 3  will be turned off even in the presence of the magnetic field  12 . 3 . This conserves power and does away with any auditory annoyance when the magnetic field sensor  14 . 3  is part of a proximity sensor in a security alarm system  31 . 3 . 
       FIG.  24    shows a sensor assembly  9 . 4  including a magnet  10 . 4  and magnetic field sensor  14 . 4  according to a fifth aspect. Like parts have like numbers and functions as the sensor assembly  9 , magnet  10  and magnetic field sensor  14  shown in  FIGS.  1  to  19    with the addition of decimal extension “0.4”. Assembly  9 . 4  is substantially the same as assembly  9  shown in  FIGS.  1  to  19    with the following exceptions. 
     Magnetic field sensor  14 . 4  includes an indicator in the form of a vibratory indictor  16 . 4  that provides a vibratory indication as to the presence or absence of a magnetic field. The vibratory indicator turns on and vibrates when the magnetic field sensor  14 . 4  is within the magnetic field  12 . 4  generated by the magnet  10 . 4 . This provides a vibratory indication as to the presence of a magnetic field. However, after a predetermined period of time, the vibratory indicator  16 . 4  will be turned off even in the presence of the magnetic field  12 . 4 . This conserves power and does away with any vibratory annoyance when the magnetic field sensor  14 . 4  is part of a proximity sensor in a security alarm system  31 . 4 . 
       FIGS.  25  to  28    show a sensor assembly  9 . 5  including a magnet  10 . 5  and magnetic field sensor  14 . 5  according to a sixth aspect. Like parts have like numbers and functions as the sensor assembly  9 , magnet  10  and magnetic field sensor  14  shown in  FIGS.  1  to  19    with the addition of decimal extension “0.5”. Assembly  9 . 5  is substantially the same as assembly  9  shown in  FIGS.  1  to  19    with the following exceptions. 
     As seen in  FIG.  25   , magnetic field sensor  14 . 5  includes an insulating member, in this example a flexible, planer, insulator strip, in this case a pull strip  90 . The pull strip has a proximal end  92  disposed within the housing  18 . 5  and a distal end  94 . The pull strip  90  extends outwards from the rear  96  of the housing in this example, from the distal end thereof towards the proximal end thereof. The proximal end  92  of the pull strip  90  is positioned to inhibit the power source, in this example battery  24 . 5 , from providing power to the rest of the magnetic field sensor  14 . 5 : the pull strip extends between the battery and the rest of the circuitry of the magnified filed sensor and thereby inhibits electrical connection therebetween. In this example the proximal end of the pull strip  90  removably couples to one of the terminals  98  of the battery  24 . 5  via adhesive  95  seen in  FIG.  25   . 
     Referring to  FIG.  26   , an installer pulling on distal end  94  of the pull strip, as shown by arrow  100 , causes the pull strip to be removed and thereby causes the magnetic field sensor  14 . 5  to be powered up by the battery  24 . 4  seen in  FIG.  25   . Still referring to  FIG.  25   , the indicator  1 . 6 . 5  is initially operable following the pull strip  90  being removed from the power source and the magnetic field sensor  14 . 5  being powered up, and the microprocessor  22 . 5  renders the indicator inoperable a predetermined period of time after the pull strip has been removed from the power source and the magnetic field sensor is powered up. The indicator remains inoperable until the lid  19 . 5  of the housing  18 . 5  is removed and the lid of the housing is closed once more at which point the indicator  16 . 5  is operable until the microprocessor  22 . 5  renders the indicator inoperable a predetermined period of time after the lid of the housing is closed. 
       FIGS.  29  to  32    show a sensor assembly  9 . 6  according to a seventh aspect. Like parts have like numbers and functions as the sensor assembly  9  shown in  FIGS.  1  to  19    with the addition of decimal extension “0.6”. Assembly  9 . 6  is substantially the same as assembly  9  shown in  FIGS.  1  to  19    with the following exceptions. 
     As seen in  FIG.  29   , sensor assembly  9 . 6  includes a first subassembly in the form of an RFID tag  10 . 6  in this example. 
     Sensor  14 . 6  includes an RFID reader  101  mounted on circuit board  20 . 6 . Radio  28 . 6  and antenna  30 . 6  allow the sensor to communicate with a control panel  102  as part of a wireless security alarm system  31 . 6 . There is a wire  104  in this example which may be electrically and releasably connected to the sensor  14 . 6 . The wire allows the sensor to communicate with the control panel  102  as part of a wired security alarm system. The sensor  14 . 6  communicates with the control panel to selectively trigger an alarm. 
     The RFID tag  10 . 6  and the sensor  14 . 6  are used as a window sensor for window  50 . 6  seen in  FIGS.  30  to  32    in this example in a first configuration of security alarm system  31 . 6 ; however this is not strictly required and the sensor assembly  9 . 6  may be used in other manners in other examples. In a wireless security alarm system  31 . 6  configuration the sensor  14 . 6  is mounted on stile  52 . 6  of window  50 . 6  and RFID tag  10 . 6  is mounted on side jamb  54 . 6  of the window near sill  56 . 6  thereof. In a wired configuration the RFID tag is mounted on the stile of window and the sensor is mounted on the side jamb of the window near sill thereof. 
     Referring to  FIG.  30   , the window is fully closed with bottom rail  58 . 6  of the window abutting the sill thereof. The sensor  14 . 6  seen in  FIG.  29    is configured to read the RFID tag  10 . 6  when the window  50 . 6  is fully closed, and signals that the window  50 . 6  is closed. A signal is thus sent by the RFID reader  101  to the microprocessor  22 . 6  when the RFID reader is in communication range with the RFID tag in one example. Indicator  16 . 6  indicates when data from the RFID tag  10 . 6  has been transmitted to the RFID reader  101  in response to the RFID tag being triggered by electromagnetic interrogation from the RFID reader. The indicator thus indicates when the RFID reader  101  in communication range with the RFID tag  10 . 6  and data from the RFID tag is received by the RFID reader, and turns off when the RFID tag is not detected and not in communication range in one example. 
     In addition or alternatively and still referring to  FIG.  29   , the microprocessor is configured to analyze changes in signals from the RFID tag to determine when a distance between the RFID tag and the RFID reader is within a predetermined threshold. This may be determined by the microprocessor  22 . 6  analyzing and comparing changes in time and amplitude characteristics of the signals, for example. In this case the indicator  16 . 6  indicates when the distance between the RFID tag  10 . 6  and the RFID reader  101  is within a predetermined threshold. In this case the indicator turns on when the distance between the RFID tag and the RFID reader is within said predetermined threshold and turns off when the distance between the RFID tag and the RFID reader is outside of said predetermined threshold. 
     Likewise, as shown in  FIG.  31   , the sensor  14 . 6  is also able to read the RFID tag  10 . 6  when the window  50 . 6  is open up to a threshold distance D1.6. It is desirable to allow the window  50 . 6  to be partially opened for ventilation but not opened enough to allow an intruder to enter through the window  50 . 6 . The sensor  14 . 6  will accordingly not trigger an alarm when the sensor  14 . 6  is able to read the RFID tag  10 . 6 . 
     However, and with reference to  FIG.  32   , when the window  50 . 6  is open to a distance D2.6, which is greater than the threshold distance D1.6, the sensor  14 . 6  is no longer able to read the RFID tag  10 . 6  and/or the microprocessor determines that the RFID tag is at a distance greater than the threshold distance, and an alarm is triggered. The sensor may be mounted on bottom rail  58 . 6  of the window in other configurations, for example. 
       FIG.  33    shows a sensor assembly  9 . 7  according to an eighth aspect. The sensor assembly is substantially similar to the sensor assembly  9 . 6  of  FIGS.  29  to  32   , with like parts having like numbers and functions with decimal extension “0.7” replacing “0.6” and decimal extension “0.7” being added for numbers not previously having decimal extensions, with the following exception. Sensor assembly  9 . 7  includes a pull strip  90 . 7  to selectively enable a power source thereof to power the sensor  14 . 7  thereof in a manner substantially similar to that described for sensor assembly  9 . 5  seen in  FIGS.  25  to  28   . 
     It will be understood by a person skilled in the art that many of the details provided above are by way of example only, and are not intended to limit the scope of the invention which is to be determined with reference to the following claims.