Patent Publication Number: US-11639841-B2

Title: Weapon detector with user interface

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/994,066, filed Aug. 14, 2020, which is a continuation of U.S. patent application Ser. No. 16/218,086, filed Dec. 12, 2018, now issued as U.S. Pat. No. 10,775,131, which is a continuation of U.S. patent application Ser. No. 15/842,149, filed Dec. 14, 2017, now issued as U.S. Pat. No. 10,190,846, which claims the benefit of U.S. Provisional Application No. 62/458,941, filed Feb. 14, 2017, each of which are incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the present invention relate to a detector that detects a weapon. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       Embodiments of the present invention will be described with reference to the drawing, wherein like designations denote like elements, and: 
         FIG.  1    is a block diagram of a system for providing a notice according to various aspects of the present disclosure; 
         FIG.  2    is a diagram of an implementation of the coil of  FIG.  1   ; 
         FIG.  3    is a cross-section view of the coil of  FIG.  2    along  3 - 3 ; 
         FIG.  4    is a diagram of an electromagnetic field from the coil of  FIG.  1    or  FIG.  2    in the absence of a weapon; 
         FIG.  5    is a diagram of the electromagnetic field from the coil of  FIG.  1    or  FIG.  2    in the presence of a weapon; 
         FIG.  6    is a view of an implementation of the system for providing a notice of  FIG.  1    with a first implementation for mounting the detector proximate to the holster; 
         FIG.  7    is an exploded view of the implementation of the system for providing a notice of  FIG.  6   ; 
         FIG.  8    is an exploded view of the implementation of detector of  FIGS.  6  and  7   ; 
         FIG.  9    is a front view of the implementation of the detector of  FIGS.  6 - 8   ; 
         FIG.  10    is a view of an implementation of the system for providing a notice of  FIG.  1    with a second implementation for mounting the detector proximate to the holster; 
         FIG.  11    is an exploded view of the second implementation for mounting the detector of  FIG.  10   ; 
         FIG.  12    is a state diagram of the operating modes of the detectors of  FIGS.  1  and  6 - 11   ; and 
         FIG.  13    is a state diagram of a method for monitoring the presence or absence of a firearm. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Body cameras, vehicle cameras, wireless microphones and/or other recording systems are used by many security forces to record the events of an incident. Security forces include police departments, investigative and enforcement departments of a government (e.g., DOJ, FBI, CIA, ATF, CPB), and military forces. A recording, if properly handled, may serve as evidence in a subsequent proceeding. The operation of a recording system may be initiated manually or by a trigger. A trigger may include a signal sent to the recording system. A signal may be sent by a wired or wireless circuit. A signal sent wirelessly may include sending a message (e.g., information, data packet) using any conventional wireless communication protocol. 
     A trigger may be obligatory or permissive. A recording system (e.g., recording device, camera, microphone, video recorder) must initiate recording upon receipt of an obligatory trigger. A recording system that receives a permissive trigger is not required to initiate recording. A permissive trigger may include a message transmitted and/or received wirelessly that reports the status and/or the identity of the device sending the message. A recording system may initiate recording depending on the value of the status. A recording system may initiate recording depending on the value identity of the sending device. A recording system may decline to initiate recording from some values. 
     A situation for which it is desirable to initiate recording by recording systems is when a security officer draws a weapon (e.g., firearm). In many instances, personnel of security forces carry conventional firearms. In many instances, such firearms are carried bodily in a holster for transport and immediate access in case of need. Withdrawing a weapon from a holster may indicate that events of an incident are about to occur or that have just occurred should be recorded. A system for providing a notice, as discussed herein, may detect withdrawal of a weapon from a holster and provide a notice (e.g., trigger, message, permissive trigger, obligatory trigger) that the weapon has been withdrawn. A notice system may provide a notice in the form of a message transmitted wirelessly. Recording systems may receive the wireless notice. A recording system, depending on the type of notice (e.g., obligatory, permissive) may start recording. 
     A system for providing a notice may also provide a notice that the weapon has been inserted into the holster. 
     A system for providing a notice may provide a permissive trigger. A system for providing a notice may provide information that a recording system may use to determine whether or not to start recording. A recording system may use information provided by a system for providing a notice to perform other functions. 
     Information may include a unique identifier (e.g., alphanumerical, serial number) of a detector, a state of operation of the detector, and/or an identity of the user of the system that provides the notice. 
     For example, system for providing a notice system  100  of  FIG.  1    includes detector  110  and firearm system  140 . Firearm system  140  includes firearm  142  and holster  150 . Firearm  142  may include a conventional handheld firearm. Firearm  142  includes barrel  144 . At least a portion of barrel  144  is formed of metal that is susceptible to detection using inductive sensing. 
     Detector  110  includes processing circuit  112 , memory  114 , user interface  116 , communication circuit  122 , real-time clock  124 , authentication circuit  126 , sensor circuit  128 , and coil  130 . User interface  116  includes control  118  and indicator  120 . Detector  110  may further include NFC tag  860 . 
     A detector detects whether a firearm is positioned in a holster. A detector detects whether a firearm is positioned out of (e.g., removed from) the holster. Depending on the implementation of the sensor circuit, the coil, and their sensitivity, a detector may detect the position of a firearm in a holster as opposed to solely whether the firearm is in or out of the holster. 
     A detector may wirelessly transmit a notice (e.g., message, data packet, data, signal, trigger) in response to detecting a change in the status of a firearm with respect to a holster. A change in status includes withdrawing of the firearm from the holster and inserting the firearm into the holster. The notice may include information to describe the detector transmitting the data, the user of the detector, the user of the firearm, the date and time of detecting, date and time of transmission of the notice, the status of the holster (e.g., firearm withdrawn, firearm inserted, firearm partially withdrawn, firearm partially inserted), previously transmitted dates and/or times, and/or previously transmitted status. The notice may further include information for authenticating the detector to one or more recording systems. 
     One or more recording systems may receive a notice from notice system  100 . A recording system may use the information transmitted with the notice to determine whether or not (e.g., permissive trigger) to perform an operation (e.g., perform a function). An operation of a recording system may include starting (e.g., initiating) recording. 
     A detector may store information in a memory (e.g., a log). Information stored by a detector may include information related to the operation and/or status of the detector. Information stored by a detector may be stored as an entry in the log. Each entry in a log may include the date and time of recording the entry. Information stored in a log may include detecting withdraw of a weapon from a holster, detecting insertion of a weapon into a holster, activation (e.g., starting, operation of) a mute operation of the recorder, resetting of the detector, setting of the time of the circuit used to generate timestamps or to record actions in the log, executing a software (e.g., firmware) upgrade, updates to user settings, reverting to an earlier version of software, and/or detecting a system fault. The mute operation, discussed in more detail below, alters the information transmitted in one or more notices for a period of time. 
     A detector may receive information (e.g., data) such as information to upgrade the software of the detector. A detector may receive information from a user via a user interface. A detector may provide information to a user via the user interface. 
     A processing circuit includes any circuitry, component, and/or electrical/electronic subsystem for performing a function. A processing circuit may include circuitry that performs (e.g., executes) a stored program. A processing circuit may include a digital signal processor, a microcontroller, a microprocessor, an application specific integrated circuit, a programmable logic device, logic circuitry, state machines, MEMS devices, signal conditioning circuitry, communication circuitry, a conventional computer, a conventional radio, a network appliance, data busses, address busses, and/or a combination thereof in any quantity suitable for performing a function and/or executing one or more stored programs. 
     A processing circuit may further include conventional passive electronic components (e.g., resistors, capacitors, inductors) and/or active electronic components (op amps, comparators, analog-to-digital converters, digital-to-analog converters, programmable logic). A processing circuit may include conventional data buses, output ports, input ports, timers, memory, and arithmetic units. 
     A processing circuit may provide and/or receive electrical signals whether digital and/or analog in form. A processing circuit may provide and/or receive digital information via a conventional bus using any conventional protocol. A processing circuit may receive information, manipulate the received information, and provide the manipulated information. A processing circuit may store information and retrieve stored information. Information received, stored, and/or manipulated by the processing circuit may be used to perform a function and/or to perform a stored program. 
     A processing circuit may have a low power state in which only a portion of its circuits operate or it performs only certain functions. A processing circuit may be switched (e.g., awoken) from a low power state to a higher power state in which more or all of its circuits operate or it performs additional certain functions or all of its functions. 
     A processing circuit may control the operation and/or function of other circuits and/or components of a system. A processing circuit may receive status information regarding the operation of other components, perform calculations with respect to the status information, and provide commands (e.g., instructions) to one or more other components for the component to start operation, continue operation, alter operation, suspend operation, or cease operation. Commands and/or status may be communicated between a processing circuit and other circuits and/or components via any type of bus including any type of conventional data/address bus. A processing circuit may instruct a circuit or component to enter a low power state. A processing circuit may instruct a circuit or component to exit a low power state. 
     A memory stores information. A memory provides previously stored information. A memory may provide previously stored information responsive to a request for information. A memory may store information in any conventional format. A memory may store electronic digital information. A memory may provide stored data as digital information. 
     A memory includes any semiconductor, magnetic, or optical technology (e.g., device, chip, system), or a combination thereof for storing information. A memory may receive information from a processing circuit for storage. A processing circuit may provide a memory a request for previously stored information. Responsive to the request the memory may provide stored information to the processing circuit. 
     A memory may include any circuitry for storing program instructions and/or data. Storage may be organized in any conventional manner (e.g., program code, buffer, circular buffer, database). Memory may be incorporated in and/or accessible by a transmitter, a receiver, a transceiver, a sensor, a controller, and/or a processing circuit. 
     A communication circuit transmits and/or receives information (e.g., data). A communication circuit may transmit and/or receive (e.g., communicate) information via a wired and/or wireless communication link. A communication circuit may communicate using wireless (e.g., radio, light, sound, vibrations) and/or wired (e.g., electrical, optical) mediums. A communication circuit may communicate using any wireless (e.g., Bluetooth, Zigbee, WAP, WiFi, NFC, IrDA, LTE, BLE, EDGE, EV-DO) and/or wired (e.g., USB, RS-232, Firewire, Ethernet) communication protocols. 
     A communication circuit may receive information from a processing circuit for transmission. A communication circuit may provide received information to a processing circuit. 
     A communication circuit in one device (e.g., detector) may communicate with a communication circuit in another device (e.g., smart phone, tablet, mobile computer, server). Communications between two devices may permit the two devices to cooperate in performing a function of either device. For example, a user interface for a detector may be implemented on a smart phone that includes a touch screen. User interaction with the user interface on the smart phone is communicated to the detector via the communication circuits of the smart phone and detector. The detector performs the function indicated by the message from the smart phone. Any information produced by the detector for the user may be communicated from the detector to the smart phone via the communication circuits for presentation on the display of the smart phone. 
     A user interface enables a human user to interact with an electronic device (e.g., detector). A user may control, at least in part, an electronic device via the user interface. A user may provide information and/or commands to an electronic device via a user interface. A user may receive information (e.g., status) and/or responses from the electronic device via the user interface. 
     A user interface may include one or more controls that permit a user to interact and/or communicate with (e.g., provide information to) an electronic device to control (e.g., influence) the operation (e.g., functions) of the electronic device. 
     As discussed above, a user interface may provide information to a user. A user may receive visual, haptic (e.g., tactile, kinesthetic), and/or audible information from a user interface. A user may receive visual information via devices (e.g., indicators) that visually display information (e.g., LCDs, LEDs, light sources, graphical and/or textual display, display, monitor, touchscreen). A user may receive audible information via devices that provide an audible sound (e.g., speaker, buzzer). A user may receive tactile information via devices that vibrate, move, and/or change resistance against a user&#39;s finger as it is pressed. 
     A user interface may include a communication circuit for transmitting information to an electronic device for presentation to a user, as discussed above. 
     A control includes any electromechanical device suitable for manual manipulation by a user. A control includes any electromechanical device for operation by a user to establish or break an electrical circuit. A control may include a portion of a touch screen. Operation of a control may occur by the selection of a portion of a touch screen. A control may include a switch. A switch includes a pushbutton switch, a rocker switch, a key switch, a detect switch, a rotary switch, a slide switch, a snap action switch, a tactile switch, a thumbwheel switch, a push wheel switch, a toggle switch, a reed switch, and a key lock switch (e.g., switch lock). 
     A control may be operated in different manners by a user to provide different information to a detector. For example, in an implementation in which the control is implemented as a push button, a user may press and release the button; press, hold the button for a period of time, then release the button during which the period of time for which the button is held determines whether the press is a long press or a very long press; press the button, release, press again, release (e.g., double press). 
     The term “control”, in the singular, represents a single electromechanical device for operation by a user to provide information to a device. The term “controls”, in plural, represents a plurality of electromechanical devices for operation by a user to provide information to a device. The term “controls” include at least a first control and a second control. 
     A processing circuit may detect the operation of a control. A processing circuit may perform a function responsive to operation of a control. Responsive to a control, a processor may perform a function, halt a function, resume a function, or suspend a function of the electronic device of which the control and the processor are a part. A control may provide analog or binary information to a processor. 
     The function performed by an electronic device responsive to operation of a control may depend on the current operating state (e.g., present state of operation, present function being performed) of the electronic device of which the control is a part. 
     A user may receive information from an electronic device via an indicator. An indicator may provide information visually, via haptic feedback, and/or audibly as discussed above. In an implementation in which the indicator is implemented as an LED, the indicator may convey information by turning the LED on and off (e.g., blink) or vice versa, the color of light provided by the LED, the rate of turning the LED on and off, the duration of time the LED is on or off, and/or the sequence of colors provided by the LED. 
     A real-time clock tracks (e.g., follows, keeps track of) the current (e.g., present) time. The functions of a real-time clock may be performed by a processing circuit. A dedicated circuit may perform the functions of a real-time clock. A real-time clock may be highly accurate (e.g., 5 seconds-12 minutes lost or gained per year). A real-time clock may further track day, date, and year. A real-time clock may provide the present time to a processing circuit. A real-time clock may track time even when other circuits are powered down. A processing circuit may perform some or all of the functions performed by a real-time clock. 
     Authentication is the act of verifying a claim of identity. Authentication may be used to confirm a user&#39;s identity. For example, a bank may authenticate the identity of a person requesting a withdrawal by asking for and inspecting photo ID. Computers may confirm the identity of a user by the user providing a user name and password. One electronic device may be authenticated to another electronic device. Authentication may also be accomplished by a challenge-response protocol in which one party, or electronic device, issues a challenge and the person, or electronic device, must provide a valid answer to be authenticated. 
     Cryptographic techniques may be used to confirm the identity of a user. For example, Public Key Infrastructure permits authentication using public and private keys. One device has a private key and issues a public key. When the device requests communication with another device, such as a computer, the computer generates and sends a random number to the user. The user encrypts the random number using its private key. The device sends the encrypted number back to the computer. The computer uses the device&#39;s public key to decrypt the encrypted number. If the decrypted number matches the originally sent random number, then the identity of the user has been authenticated to the computer. 
     An authentication circuit may be used to authenticate a user and/or an electronic device. An authentication circuit may store keys, generate random numbers, generate guaranteed unique numbers, encrypt, and decrypt. An authentication circuit may perform public key (e.g., PKI) algorithms such as high-speed PKI algorithms and elliptical curve algorithms (e.g., P256, B283, K283). An authentication circuit may perform digital signature algorithms such as Digital Signature Algorithm (e.g., FIPS 186, 186-1, 186-2, 186-3, 186-4) and elliptical curve digital signature algorithms (e.g., FIPA 186-3). 
     An authentication circuit may cooperate with a processing circuit, a user interface, and/or a communication circuit to authenticate a user and/or a device. 
     A sensor circuit detects (e.g., measures, witnesses, discovers, determines) a physical property (e.g., intensive, extensive, isotropic, anisotropic). A physical property may include momentum, capacitance, electric charge, electric impedance, electric reactance, inductance, electric potential (e.g., electromotive force), frequency, luminance, luminescence, magnetic field, magnetic flux, mass, electromagnetic field, pressure, spin, stiffness, temperature, tension, velocity, sound, heat, and time. A sensor circuit may detect a quantity, a magnitude, and/or a change in a physical property. A sensor circuit may detect a physical property and/or a change in a physical property directly and/or indirectly. A sensor circuit may detect a physical property and/or a change in a physical property of an object. 
     A sensor circuit may detect a physical quantity (e.g., extensive, intensive). A physical quantity may be positive, negative, or zero. A sensor circuit may detect a change in a physical quantity directly and/or indirectly. A sensor circuit may detect one or more physical properties and/or physical quantities at the same time (e.g., in parallel), at least partially at the same time, or serially. A sensor circuit may deduce (e.g., infer, determine, calculate) information related to a physical property. A physical quantity may a magnitude of any of the physical properties discussed above. For example, a physical quantity may include an amount of time, an elapse of time, a magnitude of an electric current, an amount of electrical charge, a magnitude of a current density, a magnitude of a voltage, an amount of capacitance, an amount of inductance, a magnitude of impedance, a magnitude of reactance, a magnitude of a magnetic field, and a flux density. 
     A sensor circuit may provide force to detect a physical property and/or a physical quantity. A force may include an electromotive force (e.g., voltage, current). A force may be provided before, coincident with, and/or after detecting. A force may be provided once, periodically, repeatedly, and/or as needed. An electromotive force may include a direct current (“DC”) or an alternating current (“AC”). For example, a sensor circuit may provide a voltage to detect a capacitance. A sensor circuit may provide a current to generate an electromagnetic field and/or to detect a change in an electromagnetic field. A sensor circuit may provide a current to an LC circuit (e.g., LC tank circuit) to cause the LC circuit to oscillate. Providing a force may include providing a current to a coil to produce an electromagnetic field. 
     A sensor circuit may include and/or cooperate with a processing circuit for detecting, transforming, relating, and deducing physical properties and/or physical quantities. A processing circuit that is part of or cooperates with a sensor circuit may include any conventional circuit for detecting, transforming, relating, and deducing physical properties and/or physical quantities. For example, a processing circuit may include a voltage sensor, a current sensor, a charge sensor, an electromagnetic sensor, and/or a frequency sensor. 
     A sensor circuit may provide information. A sensor circuit may provide information regarding a physical property and/or a change in a physical property. A sensor circuit may provide information regarding a physical quantity and/or a change in a physical quantity. A sensor circuit may provide information regarding information determined using a processing circuit. 
     For example, a sensor may drive an LC circuit, a frequency counter may measure the frequency of the oscillation of the LC circuit, the measured frequency may be compared to a reference frequency (e.g., clock), the current drawn by the LC circuit may be measured and from the measured frequency and current the parallel resistance of the LC circuit may be deduced. The magnitude of the parallel resistance may be used to determine whether the LC circuit operates in the presence or absence of a weapon in a holster. 
     In another example, a sensor may be implemented using an inductance-to-digital converter (“LDC”), such as the LDC1101 from Texas Instruments. An LDC performs the functions of a sensor circuit. The LDC couples to a coil, provides an alternating current to the coil to generate an electromagnetic field through the coil, measures the inductance and/or equivalent parallel impedance of the circuit that includes the coil, converts the measure inductance and/or impedance to a digital number and reports the number. The LDC may also detect a change in the measured inductance and/or impedance. A processing circuit may receive the digital numbers reported by an LDC. An LDC may provide a current to an LC tank circuit, measure the frequency of the oscillations of the LC tank circuit, and deduce the inductance of the LC tank circuit, and report the inductance of the LC tank circuit. 
     An LDC may provide current to an LC tank circuit, measure the electrical current or power consumed by the LC tank circuit, and deduce the electrical resistance or impedance of the LC tank circuit, and report the resistance or impedance of the LC tank circuit. 
     Because metal (e.g., barrel of a gun) alters (e.g., changes, interacts with) with the electromagnetic field generated by the LDC via the coil, the LDC, in cooperation with processing circuit  112 , may detect the presence and/or absence of metal. The LDC may measure the inductance of the circuit that includes the coil in the absence of metal, measure the inductance in the presence of metal, and report the values to processing circuit  112 . Information from a user via user interface  116  may include whether the firearm was in the holster or not for each measurement made by the LDC. 
     Further, because metal (e.g., barrel of a gun) alters the value of the inductance in an LC tank circuit and thereby the resonant frequency of the LC tank circuit, an LDC coupled to the LC tank circuit may detect the presence and/or absence of metal. 
     The LDC may measure the resonant frequency of an LC tank circuit that includes the coil in the absence of metal and the presence of metal and reports the values to processing circuit  112 . Processing circuit  112  may receive information from the user via user interface  116  to associate the information provided by the LDC with whether the gun was in the holster or not. Processing circuit  112  may determine whether the gun is in the holster using prior values reported by the LDC and the current values reported by the sensor circuit  128 . 
     A coil is a conductor shaped to form a closed geometric path. A closed geometric path is not a closed conducting path unless the two ends of the coil are electrically coupled together. Coils may have multiple turns. A coil may be wrapped around an iron core or an insulating form, or it may be self-supporting. A coil may be formed in a plane of a printed circuit board. A coil may be formed in one or more planes (e.g., layers) of a printed circuit board. 
     Providing an AC signal to a coil causes the coil to generate an electromagnetic field. Metal may interact (e.g., interfere) with the electromagnetic field generated by a coil, as discussed above. Interaction of the magnetic field of a coil with metal alters the measured inductance of the coil. A sensor circuit may provide an AC signal to a coil to cause the coil to generate an electromagnetic field. A sensor circuit may detect the interaction of metal with the electromagnetic field. A sensor circuit and coil may detect a proximity of metal by detecting the presence or absence of interaction with the electromagnetic field by metal. Detecting interaction with the electromagnetic field may be referred to as inductive sensing. 
     For example, coil  200  is formed of conductor  220  on one or more layers of printed circuit board (“PCB”)  210 . The two end portions of conductor  220 , end portion  250  and end portion  350  are available to couple to a sensor circuit. A forward portion of coil  200 , the portion facing outward in  FIG.  2    and upward in  FIG.  3   , is for positioning toward a holster for detecting the presence or absence of the metal of a firearm in the holster. 
     Coil  200  may include shield  240 , around conductor  220  in  FIG.  2    and to the right and left sides in  FIG.  3   . Shield  240  shields conductor  220  from electromagnetic noise and interaction with metal placed to the side of coil  200  rather than the forward portion of coil  200 . For example, shield  240  shields coil  200  from interaction with the metal of a vehicle door when a user leans against the door while wearing a holster equipped with detector  110 . Shield  240  may also set (e.g., limit, focus, direct, point) the direction of the electromagnetic field generated by coil  200  and thereby the direction of sensing by coil  200 . In an implementation, shield  240  limits coil  200  to detecting metal forward (e.g., outward with respect to  FIG.  2   , and upward with respect to  FIG.  3   ) of conductor  220 . 
     A shield may further extend to behind coil  200  (e.g., behind in  FIG.  2   , below in  FIG.  3   ) (not shown) to provide shielding to a rear portion of coil  200 . 
     A shield may be coupled to an electric potential (e.g., ground, any voltage). A shield may be uncoupled to an electric potential (e.g., floating). 
     In an implementation, coil  200  includes conductor  220  formed on a first layer of PCB  210  and conductor  320  formed on a second layer of PCB  210 . Conductors  220  and  320  are formed of metal (e.g., copper, aluminum, alloy). Conductor  220  is deposited on (e.g., in) layer  310  of PCB  210 . Conductor  320  is deposited on (e.g., in) layer  330  of PCB  210 . Conductor  320  may include the same electrical (e.g., impedance, inductance) and physical (e.g., number of turns, shape, position with respect to PCB  210  and/or conductor  220 ) characteristics as conductor  220 . Conductor  220  may couple serially to conductor  320 . 
     In the implementation of  FIGS.  2  and  3   , conductor  220  is positioned on first layer  310  of PCB  210 . End portion  250  of conductor  220  extends to an edge of PCB  210  for electrical coupling. Conductor  320  is positioned on layer  330  of PCB  210 . Conductor  320  has the same number of turns and shape as conductor  220 . An end portion of conductor  220  couples to an end portion of conductor  320  via conductor  226  so that conductor  220  serially couples to conductor  320 . Because conductor  220  serially couples to conductor  320 , a current flowing into end portion  250  of conductor  220  flows out of end portion  350  of conductor  320 . Applying a voltage between end portion  250  and end portion  350  applies a voltage across conductor  220  and conductor  320 . 
     In another implementation, coil  200  includes only conductor  220  implemented on layer  310  of PCB  210 . 
     Shield  240  may be integral with PCB  210 . Shield  240  may be separate from PCB  210 . PCB  210  may be placed inside shield  240 . Shield  240  may be placed around all or a portion of PCB  210 . A shield may be formed of a metal such as aluminum, nickel, or copper. 
     When coil  130  is excited (e.g., driven, powered) with a signal (e.g., AC, impulse) from the sensor circuit  128 , coil  130  generates electromagnetic field  410  that extends from coil  130 . Coil  130  may be position proximate to wall  420  of a holster so that electromagnetic field  410  extends through wall  420  into the cavity of the holster where the firearm is positioned when the firearm is in the holster. In the absence of the firearm, shown in  FIG.  4   , nothing interacts with electromagnetic field  410 . Sensor circuit  128  may measure the inductance of coil  130  in the absence of the firearm from the holster. 
     When the firearm is inserted into the holster, referring to  FIG.  5   , metal from the firearm, in this case, barrel  510 , interacts with electromagnetic field  410  thereby changing the perceived (e.g., measured) inductance of coil  130 . Sensor circuit  128  can sense the difference in the inductance when the firearm is present in the holster. In accordance with the difference, change, and/or magnitude of the inductance, sensor circuit  128  may deduce (e.g., detect, sense, measure) and report the presence of the firearm in the holster. Sensor circuit  128  may further detect the magnitude of the inductance of the circuit, as discussed above, when the firearm is not present in the holster and report the absence of the gun from the holster. 
     In another implementation, coil  130 / 200  may couple to a capacitor (not shown) to form an LC tank circuit. The signal provided by sensor circuit  128  causes the LC tank circuit to resonate (e.g., oscillate). Sensor circuit  128  may measure the frequency of resonation to determine whether the firearm is positioned in the holster. Sensor circuit  128  and/or processing circuit  112  may use the measured frequency of oscillation to determine the inductance of coil  130 / 200 . 
     The frequency of oscillation of the LC tank circuit is governed by the formula f=1/(2π*√(LC)). The value of the capacitance, C, is known and fixed, so the frequency of oscillation of the LC tank circuit is determined by the value of the inductance of coil  130 . While firearm  142  is absent from holster  150 / 650 , the impedance of the LC tank circuit, L, is a first value L 1 . See  FIG.  4   . While the inductance of the LC tank circuit is L 1 , the tank circuit oscillates at a first frequency f 1 . While firearm  142  is positioned in holster  150 / 650 , see  FIG.  5   , the metal of firearm  142  interacts with the electromagnetic field from coil  130 / 200  and thereby alters the value of the inductance of the LC tank circuit. While firearm  142  is inserted in to holster  650  and proximate to coil  130 / 200  the inductance of the LC tank circuit is a second value L 2 . While the inductance of the LC tank circuit is L 2 , the tank circuit oscillates at a second frequency f 2 . 
     Detector  110 / 610  may detect a difference (e.g., |f 1 −f 2 |, f 1 −f 2 , f 2 −f 1 ) in frequency and/or a difference in parallel resistance of the LC tank circuit to determine whether firearm  142  is present in holster  150 / 650 . Or, detector  110 / 610  may use the measured frequency and/or resistance of the LC tank circuit (e.g., f 1 , f 2 ) and the known value of the capacitance of the LC tank circuit to determine the measured inductance (e.g., L 1 , L 2 ) and whether firearm  142  is in or out of holster  150 / 650 . 
     A firearm (e.g., gun) is a weapon that launches a projectile (e.g., bullet, shell) to deliver a force of impact to an object via the projectile. Conventional firearms include pistols (e.g., handguns) and rifles (e.g., long arms, shotguns, carbines) whether single shot, semiautomatic, or fully automatic. Many conventional firearms use combustion of a pyrotechnic to launch the projectile. 
     Most conventional firearms are formed, at least partially, of metal. Most conventional firearms include at least a metal barrel. For example, firearm  642 , shown in  FIGS.  6 - 7  and  10   , is a conventional firearm with a metal barrel. The metal of a firearm may interact with the electromagnetic field generated by a coil. A sensor circuit may detect the change in the electromagnetic field that results when the metal of a firearm is proximate to the coil that is generating an electromagnetic field. 
     A holster is a case (e.g., holder) that holds a firearm. Conventional holsters for handguns have an opening to facilitate quick removal and insertion of the handgun out of and into the holster respectively. The shape of the holster generally conforms to the shape of the firearm (e.g., barrel, finger guard). A holster may be formed of any material. Common materials include leather and plastics. The material of many holsters, such as those formed of leather or plastics, permit the passage of an electromagnetic field through the walls of the holster so that a sensor circuit may detect the presence or absence of a metal portion of a firearm inside the holster. 
     For example, holster  650  of firearm system  640 , shown in  FIGS.  6 - 7  and  10   , is suitable for accepting and holding firearm  642  and for mounting holster system  600  to a user&#39;s belt. Firearm system  640  includes holster  650  and belt mount  670 . Holster  650  includes mount  652  for mounting to belt mount  670  to couple holster  650  to belt mount  670 . As discussed in further detail below, holster  650  may be decoupled from belt mount  670 . Plate  660  and spacer  662  may be positioned between mount  652  and belt mount  670 . Mount  652  may be coupled to plate  660  so that plate  660  and spacer  662  are positioned and retained between mount  652  and belt mount  670 . 
     In other implementations, belt mount  670  may be replaced with a thigh mount, a MOLLE mount, and a quick-detach mount. 
     Mount  652  and belt mount  670  may have structures (e.g., holes) that correspond to each other to facilitate mounting belt mount  670  to mount  652  with retaining plate  660  and spacer  662  positioned in between. 
     While belt mount  670  is coupled to holster  650 , holster  650  and belt mount  670  function as a single unit to hold firearm  642  and to mount to a user&#39;s belt. While mounted to the user&#39;s belt, firearm  642  may be withdrawn and inserted into holster  650 . 
     While plate  660  and spacer are mounted between mount  652  and belt mount  670 , holster  650 , mount  652 , plate  660 , spacer  662 , and belt mount  670  function as a single unit to hold firearm  642  and to mount to a user&#39;s belt. Firearm  642  may be withdrawn and inserted into holster  650  while these components are coupled to each other. Plate  660  and/or spacer  662  do not interfere with removing and/or inserting firearm  642  out of or into holster  650 . 
     Plate  660  includes positioner  612 . Detector  610  may mount to positioner  612 . Positioner  612  positions and holds (e.g., maintains) detector  610  proximate to holster  650 , so that the electromagnetic field from detector  610  passes through the wall of holster  650  to determine, via measuring inductance, impedance, and/or frequency, whether firearm  642  is inserted into holster  650  or withdrawn from holster  650 . 
     In another implementation, very high bond (“VHB”) mount  1010  positions detector  610  with respect to holster  650  so that the electromagnetic field from detector  610  passes through the wall of holster  650  to determine, via measuring inductance, impedance, and/or frequency, whether firearm  642  is inserted into holster  650  or withdrawn from holster  650 . 
     VHB mount  1010  includes mount  1120  and VHB tape  1130 . Detector  610  mechanically mounts to mount  1120 . Mount  1120  includes flexible tabs  1124 . Tabs  1124  may flex to conform to the exterior surface of holster  650 . Base  1126  is formed of a rigid material to maintain detector  610  positioned with respect to mount  1120  and opening  1122 . VHB tape  1130  is a very high bond tape that provides a strong adhering force between mount  1120  and holster  650 . The shape of VHB tape  1130  is similar to the shape of mount  1120  including tabs  1124 . VHB tape  1130  includes opening  1132  that substantially aligns with opening  1122  when VHB tape  1130  adheres to mount  1120  and holster  650 . Opening  1122  and  1132  may reduce interference with the electromagnetic field that issues from coil wall  834  to detect the presence or absence of firearm  642  in holster  650 . Opening  1122  and  1132  may further operate as a window to permit the serial number of detector  610  to be viewed prior to mounting. 
     After mount  1120  and VHB tape  1130  are coupled to holster  650 , detector  610  may be decoupled from mount  1120  to be service or replaced. Mount  1120  and VHB tape  1130  remain coupled to the exterior of holster  650 . With effort, mount  1120  and VHB tape may be decoupled from holster  650 . 
     Processing circuit  112 , memory  114 , user interface  116 , communication circuit  122 , real-time clock  124 , authentication circuit  126 , sensor circuit  128 , coil  130 , control  118 , indicator  120 , firearm system  140 , and holster  150  may perform the functions and include the structures of a processing circuit, a memory, a user interface, a communication circuit, a real-time clock, an authentication circuit, a sensor circuit, a coil, a control, an indicator, a firearm system, a firearm, and a holster respectively as discussed above. 
     In operation, processing circuit  112  controls, performs, or directs most or all of the operations of detector  110 . Processing circuit executes a program stored in memory  114  to perform or control the functions of detector  110 . In an implementation, memory  114  is implemented as a flash memory. Processing circuit  112  responsive to the program enters various states of operation (e.g., modes) to perform particular functions. Processing circuit  112  may perform method  1000  and/or  1100  in whole or part as discussed below. In each mode, processing circuit  112  performs or controls the performance of specific operations. The modes and the operations performed in the various modes are discussed below. 
     Processing circuit  112  receives information for a user via user interface  116 . In particular, as the user operates or controls control  118 , control  118  sends signals to processing circuit  112 . Responsive to the signals, processing circuit  112  performs functions, controls the performance of a function, and/or changes from one mode to another mode. In an implementation, control  118  is a switch (e.g., electromechanical) that is manually operated by a user. 
     Processing circuit  112  also provides information to the user responsive to the operation and/or modes of detector  110 . Processing circuit  112  provides signals to indicator  120  responsive to performance of an operation, entering a state of operation, exiting a state of operation, and/or occurrence of a change in a state of operation. In an implementation, indicator  120  is an LED. Processing circuit  112  may provide a signal that turns the LED on and off to provide information to the user. Processing circuit  112  may turn the LED on and off at a frequency of operation or in accordance with a pattern to provide information to the user and/or to indicate a state of operation of detector  110 . A pattern and/or color of light provided by the LED may indicate particular information to a user. 
     In another implementation, indicator  120  includes or exclusively provides an audible sound or tactile feedback to provide the user information. Processing circuit  112  may control a sound producing indicator (e.g., buzzer) or tactile producing indicator (e.g., vibrator) in the same manner as the LED including providing sound or vibrations in accordance with patterns. 
     Sensor circuit  128  cooperates with coil  130  to detect the presence or absence of firearm  142  in holster  150 . Sensor circuit  128  may drive coil  130  with a signal (e.g., AC, DC, impulse), as discussed above, so that coil  130  generates an electromagnetic field. Sensor circuit  128  detects (e.g., senses, measures) the inductance, impedance, and/or frequency of oscillation of the circuit that includes coil  130 . When metal from firearm  142  is not proximate to coil  130 , sensor circuit  128  detects a first magnitude of inductance, impedance, and/or frequency. When metal from firearm  142  is proximate to coil  130 , sensor circuit  128  detects a second magnitude of inductance, impedance, and/or frequency. Sensor circuit  128  may report the values (e.g., absolute, actual) of the first magnitude and the second magnitude and/or a change in the magnitude. 
     Processing circuit  112  may receive reports from sensor circuit  128 . Processing circuit  112  may use the information that is detected by sensor circuit  128  to determine whether firearm  142  is in holster  150  or whether holster  150  has been withdrawn from holster  150 . 
     Responsive to determining that firearm  142  had been withdrawn from holster  150 , processing circuit  112  may instruct communication circuit  122  to transmit a message. A message transmitted by communication circuit  122  may include information such as an identifier (e.g., serial number) of detector  110  and/or an identity of the user, as discussed above. An identifier of detector  110  may be unique. The message may further include the status of firearm  142  with respect to holster  150  (e.g., withdrawn, inserted), a cryptographic signature, a time-stamp, a state of the battery (e.g., power level) of detector  110 , the mode of detector  110  (e.g., test, calibrate, field, mute), the serial number of firearm  142 , and/or the version of the software of detector  110 . In an implementation, the message transmitted by communication circuit  122  includes all or some of the above information. 
     The message transmitted by communication circuit  122  may be received by any electronic device capable of receiving messages that is in communication with communication circuit  122 . The message may be transmitted wirelessly. The electronic device receiving the message may analyze the information provided in the message. Responsive to the content of the information in the message, an electronic device may perform an operation. 
     In an embodiment, body cameras, vehicle cameras, wireless microphones and/or other recording systems may receive a message from detector  110 . Responsive to determining that the message reports that a weapon has been withdrawn from a holster, the recording system may (e.g., permissive trigger) start capturing and/or recording information. A recording system may use other information from a message to determine whether or not to start recording. For example, if the information in the message shows that detector  110  is in the mute mode (e.g., mute bit set to 1), discussed below, the recording system may decide to not start recording. If the identifier of detector  110  or the user of detector  110  does not match a list of permitted detectors or users, the recording system may elect to not begin recording. 
     Communication circuit  122  may transmit a message upon detecting that firearm  142  has been placed into holster  150 . A recording system may elect (e.g., permissive trigger) to stop recording upon receiving such information. A recording device may elect to continue recording even though firearm  142  has been returned to the holster so that the user must manually terminate recording. 
     Communication circuit  122  may also receive information. For example, communication circuit  122  may receive data for updating the software of detector  110 . Communication circuit  122  may receive information as to the identity of the user of detector  110  (e.g., holster  150 , firearm  142 ). Communication circuit  122  may receive information as to the serial number of firearm  142 . 
     Real-time clock  124  may provide time, day, and/or date information. Information from real-time clock  124  may be included in a message transmitted by communication circuit  122 . Real-time clock  124  may also provide time information for logging information as discussed above. The present time of real-time clock  124  may be changed. Communication circuit  122  may receive a new time and the present time of real-time clock  124  may be set to the new time. Setting real-time clock  124  to a new time may be performed during manufacture, to synchronize the time maintained by two or more detectors  110 , or to correct an error in the time maintained by real-time clock  124 . Real-time clock  124  may track time as universal time coordinated (“UTC”), yet report time in a local format (e.g., UTC-7 for Arizona). Time reported by real-time clock  124  may account for local time zone and/or daylight savings time. Real-time clock  124  may also report time in UTC format and a receiving device may make any adjustments to determine local time. Processing circuit  112  may perform some or all of the functions of real-time clock  124 . 
     The time maintained by real-time clock  124  may be updated in the field when detector  110  communicates with another system that includes a more accurate or more frequently updated clock. For example, the time of real-time clock  124  may be updated to match the time of a body-worn camera when detector  110  communicates with the body-worn camera. In another example, the time of real-time clock  124  may be updated to match the time of a handheld device (e.g., cell phone, smart phone) when detector  110  communicates with the handheld device. 
     Processing circuit  112  may store information regarding the operation and status of detector  110 . As discussed above, stored information may be referred to as a log. Information that is logged may be stored in memory  114 . Log information may be retrieved. Communication circuit  122  may transmit log information to another electronic device, such as a server. Log information may be used to analyze the performance and operation of detector  110 . Log information may be used to detect faults in the operation of detector  110 . Information stored in a log may include events such as removal of firearm  142  from holster  150 , insertion of firearm  142  into holster  150 , activation of mute operation (e.g., mute mode), deactivation of mute operation, reset of detector  110 , setting of time of real-time clock  124 , a change in configuration of detector  110 , receiving and/or installing a software upgrade, reverting to an earlier version of software, occurrence of a system fault either hardware or software, transmission of a message by communication circuit  122 , receipt of a message by communication circuit  122 , battery energy level, report of battery energy level, battery change, magnitude of inductance and/or impedance when firearm  142  is proximate, magnitude of inductance and/or impedance when firearm  142  is not proximate, receipt of user identity, successful authentication, unsuccessful authentication, state of operation, and/or receipt of serial number of firearm  142 . 
     Information stored in a log may be referred to as an entry. A single type of information and/or information related to a single event or occurrence may be stored in an entry. Each entry may include a time-stamp of when the entry was recorded. Real-time clock  124  may provide the time-stamp. The time-stamp may include time, day, and/or date as discussed above. 
     Authentication circuit  126  may store keys used for encryption. Authentication circuit  126  may encrypt and/or decrypt data. Authentication circuit  126  may receive data from communication circuit  122  for decrypting. Authentication circuit  126  may provide encrypted data to communication circuit  122  for transmission. Authentication circuit  126  may cryptographically sign data prior to transmission. 
     Authentication circuit  126  may provide information for authenticating (e.g., confirming) the identity of detector  110 . Authentication circuit  126  may request information for authenticating the identity of another system (e.g., server, recording system). A server may request that detector  110  authenticate its identity before the server communicates with detector  110 . For example, a server may request that detector  110  authenticate its identity prior to providing detector  110  with sensitive data, such as a software update. Detector  110  may request that a server authenticate its identity prior to providing log entries to the server. 
     In an implementation of detector  110 , referring to  FIGS.  6 - 11   , detector  610  includes front housing  810 , screws  812 , holes  960 , circuitry  840 , battery holder  820 , battery  822 , back housing  816 , coil  830 , shield  814 , NFC tag  860 , and shield  862 . Front housing  810  may include user interface  850 . User interface  850  may include indicator  854  (e.g., LED) and control  852  (e.g., user-operated switch). Back housing  816  may include coil cavity  832 , and coil wall  834 . 
     Circuitry  840  may include processing circuit  112 , memory  114 , communication circuit  122 , real-time clock  124 , authentication circuit  126 , and sensor circuit  128 . Battery holder  820  holds battery  822 . Battery  822  provides power to operate circuitry  840 , coil  830 , indicator  854 , and control  852 . Screws  812  coupled front housing  810  to back housing  816  to enclose circuitry  840 , battery holder  820 , battery  822 , coil  830 , shield  814 , NFC tag  860 , and shield  862 . 
     Shield  814  is positioned around a perimeter of PCB  210  and therefore around an edge of coil  830 . Coil  830  is positioned in coil cavity  832  to position coil  830  proximate to coil wall  834 . Shield  814  remains positioned around an edge of coil  830  while coil  830  is positioned in coil cavity  832 . While detector  610  is in use with a holster, coil wall  834  is position proximate to holster  650  so that the electromagnetic waves from coil  830  pass through the wall of holster  650  to detect the presence or absence of metal. Detector  610  couples to positioner  612  to position coil wall  834  proximate to holster  650 . 
     Sensor circuit  128  and/or a processing circuit of sensor circuit  128 , or processing circuit  112 , may include a temperature sensor. Information regarding temperature may be used to correct (e.g., adjust, compensate for) operation of sensor circuit  128  or other components that varies with temperature. For example, coil  200  may be formed on a PCB as discussed above. The electrical properties of coil  200  change over temperature. Further, when sensor circuit  128  is implemented using an LDC, the LDC receives a clock from an oscillator. The oscillator may be temperature sensitive thereby affecting the operation and measurements made by the LDC. A coil and an oscillator used as a clock may be characterized to determine how they vary over temperature. Processing circuit  112  may use the current temperature and the characterization data to adjust operation to compensate for temperature. 
     Near-field-communication (“NFC”) tag  860  may communicate with a reader via wireless near-field communication. NFC tag  860  may be passive or active. NFC tag  860  may provide information to a reader. In an implementation, NFC tag  860  provides the serial number of detector  610  to the reader via NFC communication. NFC tag may operate independent of processing circuit  112 , memory  114 , communication circuit  122 , user interface  116 , sensor circuit  128 , coil  130 , real-time clock  124 , and authentication circuit  126 . Any device (e.g., smartphone, tablet, mobile computer, recharging station) may perform the function of an NFC reader. Shield  862  may shield NFC tag  860  from the electrical and electromagnetic noise produced by circuitry  840 . 
     A user may operate control  852  to provide information to detector  610 . A user may operate control  852  by pressing the releasing control  852 . Pressing control  852  may provide a signal to processing circuit  112  as discussed above. Detector  610  may provide information to a user via indicator  854 . Indicator  854  may provide information via illumination of a light. Indicator  854  may operate to provide a pattern of on-off flashes (e.g., blinks) of light to convey information. The pattern provided by indicator  854  may depend on the operating state of detector  610 . 
     Processing circuit  112  may execute a program to perform the functions of an operating state. As discussed above, an operating state may be referred to as a mode of operation or simply a mode. In an implementation, the operating states of detector  110  and/or  610  may include sleep  1020 , test  1030 , field  1040 , mute  1050 , calibrate  1060 , reset  1070 , rollback  1080 . 
     Processing circuit  112  may cooperate with or received signals from control  120 / 852  to exit or enter a state of operation. Processing circuit  112  may provide information to a user via indicator  120 / 854  upon entering or while operating in a state. Processing circuit  112  may use a timer or may measure an elapse of time using the time provided by real-time clock  124  to enter and/or leave a state of operation. 
     In state diagram of method  1000  of  FIG.  10   , sleep  1020  is a low power state in which most of the circuits of detector  160 / 610  are powered down to save battery power. In an implementation, the only circuitry that is active is the portion of processing circuit  112  that monitors control  118  to detect when control  118  has been pressed. Operation transitions into sleep  1020  from test  1030  after the expiration of a period of time. In an implementation, the period of time is 30 seconds. Operation moves from sleep  1020  to test  1030  responsive to activation of control  118 . In an implementation, indicator  120 / 854  (e.g., LED) blinks three times with a green light to confirm the transition from sleep  1020  to test  1030 . 
     In test  1030 , the circuits of detector  160 / 610  are powered up so that a user may verify the proper operation of detector  160 / 610 . In test  1030 , a user may verify that detector  160 / 610  detects the insertion of firearm  142  into holster  150 / 650  and removal of firearm  142  from holster  150 / 650 . When firearm  142  is inserted into holster  150 / 650 , indicator  120 / 854  (e.g., LED) provides light that is visible to the user. When firearm  142  is removed from holster  150 / 650 , indicator  120 / 854  ceases to provide light. 
     As stated above, operation stays in test  1030  for a duration of time before returning to sleep  1020 . In an implementation, operation moves from test  1030  to sleep  1020  after a 30 second period of time. Thirty seconds is enough time for a user to insert and remove firearm  142  from holster  150 / 650  several times. If indicator  120  indicates proper detection of insertion and removal, a user may press control  118  to move operation from test  1030  to field  1040 . In an implementation, control  118  must be pressed with a long press of 5 seconds or greater for operation to move from test  1030  to field  1040 . 
     While in test  1030 , if indicator  120 / 854  shows that detector  160 / 610  is not properly detecting the insertion and removal of firearm  142 , a user may elect to calibrate detector  160 / 610 . While in test  1030 , a user may move into calibrate  1060  by pressing control  118  with a double press. In an implementation, a double press is one press on control  118 , a pause, then a second press on control  118 . The length of the pause may have an upper boundary. 
     In calibrate  1060 , detector  110 / 610  determines the magnitude of the inductance or impedance that indicates that the weapon is in and/or out of the holster. In one implementation, detector  110 / 610  performs several readings of the inductance and/or impedance while the firearm  142  is withdrawn from holster  150 / 650 . Detector  110 / 610  averages the value of the measured inductance and/or impedance to determine a base-line value of inductance and/or impedance. The base-line value is subtracted from readings performed in field  1040 . A change from the base-line value, either more or less, indicates that firearm  142  is in holster  150 / 650 . 
     In another implementation, detector  110 / 610  performs several readings while firearm  142  is in holster  150 / 650  and averages the values to create a base-line value for when firearm  142  is in holster  150 / 650 . Detector  110 / 610  also performs several readings while firearm  142  is out of holster  150 / 650  and averages the values to create a base-line value for when firearm  142  is out of holster  150 / 650 . While in the operating state field  1040 , detector  110 / 610  uses the base-line values for firearm  142  being in and out of holster  150 / 650  to determine when firearm  142  is in or out of holster  150 / 650 . 
     In an implementation, indicator  120 / 854  blinks three times with a green light after the double press on control  118  to confirm that operation has moved from test  1030  to calibrate  1060 . Once calibration is complete, indicator  120 / 854  blinks three times with a green light to show the transition from calibrate  1060  back to test  1030 . 
     While in operating state test  1030 , a user may use control  118  to move from operating state test  1030  to operating state field  1040 . A user may press control  118  with a long-press to initiate the change from test  1030  to field  1040 . In an implementation, a long press is a press that is greater than 5 seconds, but less than 25 seconds. In an implementation, indicator  120 / 854  blinks 3 times with a green light to indicate the transition from test  1030  to field  1040 . 
     In the operating state field  1040 , detector  110 / 610  performs method  1100  to monitor the presence or absence of firearm  142  in holster  150 / 650 . Method  1100  includes operating states wait  1102 , activate  1104 , measure  1106 , present  1108 , transmit  1110 , and count  1112 . 
     In wait  1102 , detector  110 / 610  waits for a period of time. While in wait  1102 , the circuits of detector  110 / 610  are in a low power state to save energy to prolong the life of the battery. At the end of the period of time, detector  110 / 610  transitions to operating state activate  1104 . 
     In activate  1104 , detector  110 / 610  activates the coil and measures inductance, impedance and/or frequency of oscillation to detect the presence or absence of firearm  142  in holster  150 / 650 . 
     In another implementation, the Texas Instruments inductance-to-digital integrated circuit LCD1101 controls the operation of coil  130 / 200 , measures the frequency, inductance, and/or impedance, and reports a digital value to processing circuit  112 . 
     After activate  1104  has activated coil  130 / 200  and returns a value of the frequency of the LC tank circuit, the inductance of coil  130 / 200 , and the impedance of coil  130 / 200  and/or LC tank circuit, operation moves to present  1108 . 
     In operating state present  1108 , detector  110 / 610  uses the information determined in activate  1104  to determine whether firearm  142  is present in holster  150 / 650  or whether firearm  142  is absent from holster  150 / 650 . In an implementation, processing circuit  112  compares the value provided by the LDC1101 integrated circuit or the value of the measured frequency of the LC tank circuit, or the measured value of the inductance of the LC tank circuit to one or more base-line values to determine whether firearm  142  is in or out of holster  150 / 650 . If firearm  142  is present in holster  150 / 650 , execution of method  1100  moves to operating state wait  1102 . If firearm  142  is not present in holster  150 / 650 , execution moves to transmit  1110 . 
     In transmit  1110 , detector  110 / 610  transmits a message that contains some or all of the information discussed above including that firearm  142 / 642  has been removed from holster  150 / 650 . Any recording system that receives the message may determine that the information provided by the message indicates that firearm  142 / 642  has been removed from holster  150 / 650 . A recording system may (e.g., permissive trigger) start to capture and/or record information responsive to such information in the message. After transmitting the message, execution moves to operating state count  1112 . 
     In operating state count  1112 , detector  110 / 610  determines whether the message has been transmitted a certain number of times. In an implementation, the message is transmitted once per second for 30 seconds. If the message has been transmitted the predetermined number of times, execution moves to operating state wait  1102 . If the message has not been transmitted the predetermined number of times, execution returns to operating state transmit  1110 . 
     During the time that detector  110 / 610  remains in field  1040 , indicator  120 / 854  may be shut off to conserve energy and to not blink when dark thereby disclosing the position of the user. 
     In the event that a user wishes to remove firearm  142  from holster  150 / 650  without requesting that any cameras or other recording devices start recording, the user may put detector  110 / 610  in the mute  1050  operating state. For example, an officer may need to remove firearm  142  from holster  150 / 650  temporarily upon entering a court house. A user may request a transition from field  1040  to mute  1050  by pressing control  120 / 852  with a long press. 
     Detector  110 / 610  remains in the mute mode for a period of time (e.g., 30 seconds). At the expiration of the period of time, operation returns to field  1040 . While in mute  1050 , detector  110 / 610  performs method  1100 ; however, irrespective of whether firearm  142  is in or out of holster  150 / 650 , the transmitted messages include a mute bit whose value is set to a one. Setting the mute bit to a one indicates that the user as requested the mute mode and the state of operation is presently in mute  1050 . Cameras or other recording devices ignore messages that include a mute bit with the value set to a one. Operation remains in mute  1050  for the predetermined amount of time, thereby possibly repeatedly executing method  1100  several times. 
     While in mute  1050 , indicator  120 / 854  blinks to indicate that operation is in mute  1050 . However, additional information may be provided by indicator  120 / 854  while in mute  1050 . For example, indicator  120 / 854  blinks with a red color if the battery is low and with a green color if the battery level is not low (e.g., above a pre-determined level). 
     At any time and from any state of operation, a user may press control  120 / 852  for an extended long press (e.g., 25 seconds) to transition from whatever the present state of operation is (e.g., sleep  1020 , test  1030 , calibrate  1060 , field  1040 , mute  1050 ) to reset  1070  operating set. Indicator  120 / 854  confirms the transition to reset  1070  by blinking with a blue color. 
     While in reset  1070 , detector  110 / 610  performs operations to reset all components of indicator  120 / 854  to a known state. After setting all components to a known state, detector  110 / 610  restarts operation and enters operating state test  1030 . 
     At any time and from any state of operation, a user may press control  120 / 852  for an even longer extended long press (e.g., 40 seconds) to transition from whatever the present state of operation is to rollback  1080  operating set. Indicator  120 / 854  confirms the transition to rollback  1080  by alternately blinking with a blue and green color. 
     While in rollback  1080 , detector  110 / 610  selects for execution a previous version of software. Processing circuit  112  executes a stored program (e.g., software, firmware) stored in memory  114  to perform the functions of detector  110 / 610 . The stored program may be updated by receiving new software via wired or wireless communication. Detector  110 / 610  may wirelessly communicate with a hand-held device (e.g., smartphone, tablet) or a server to receive updated software. Detector  110 / 610  may store two or more version of software including the factory version loaded into detector  110 / 610  at manufacture. In the event that the present version of software does not operate properly (e.g., bug, corruption of memory  114 ), a user may activate operating state rollback  1080  to return to a prior version of the software. 
     Once the prior version of the software has been selected as the present version of software, operation moves to reset  1070  to reset operation executing the different version of software. 
     In an implementation, field  1040  may perform a further method, in addition to method  1100 , to periodically transmit a status message that is different and separate from the message transmitted in transmit  1110 . A status message may include information regarding the status of detector  110 / 610 . A status message may exclude information regarding the status of firearm  142 / 642  (e.g., withdrawn, inserted). In an implementation, the status message is transmitted every 40 seconds. The status message includes information regarding the status of the battery (e.g., charge level). 
     Other implementations include the implementations provided below. 
     A system for positioning a detector to detect removal of a provided firearm from a provided holster, the holster includes a mount and a belt mount, the mount coupled to the holster, the mount configured to couple to the belt mount, the system comprising: a plate; the detector; and a positioner, the positioner configured to couple to the detector and to the plate; wherein the plate is configured to be positioned between the mount and the belt mount prior to coupling the mount to the belt mount; coupling the mount to the belt mount retains the plate between the holster and the belt mount; coupling the positioner to the detector and to the plate while the plate is retained between the mount and the belt mount positions the detector proximate to a wall of the holster; and while the detector is proximate to the wall of the holster, an electromagnetic field of the detector passes through the wall to detect removal of the firearm from the holster. 
     The foregoing description discusses preferred embodiments of the present invention, which may be changed or modified without departing from the scope of the present invention as defined in the claims. Examples listed in parentheses may be used in the alternative or in any practical combination. As used in the specification and claims, the words ‘comprising’, ‘comprises’, ‘including’, ‘includes’, ‘having’, and ‘has’ introduce an open-ended statement of component structures and/or functions. In the specification and claims, the words ‘a’ and ‘an’ are used as indefinite articles meaning ‘one or more’. While for the sake of clarity of description, several specific embodiments of the invention have been described, the scope of the invention is intended to be measured by the claims as set forth below. In the claims, the term “provided” is used to definitively identify an object that not a claimed element of the invention but an object that performs the function of a workpiece that cooperates with the claimed invention. For example, in the claim “an apparatus for aiming a provided barrel, the apparatus comprising: a housing, the barrel positioned in the housing”, the barrel is not a claimed element of the apparatus, but an object that cooperates with the “housing” of the “apparatus” by being positioned in the “housing”. The invention includes any practical combination of the structures and methods disclosed. While for the sake of clarity of description several specifics embodiments of the invention have been described, the scope of the invention is intended to be measured by the claims as set forth below. 
     The words “herein”, “hereunder”, “above”, “below”, and other word that refer to a location, whether specific or general, in the specification shall refer to any location in the specification.