Patent Publication Number: US-2018043861-A1

Title: Detection device and detection control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2016-157625, filed on Aug. 10, 2016, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a detection device configured to output, through wireless communication, detection information indicating the occurrence of a predetermined state detected by a detector. 
     A detection device is used as, for example, a seatbelt reminder for a vehicle (refer to Japanese Laid-Open Patent Publication No. 2005-75123). The seatbelt reminder includes a buckling detection switch located in a seatbelt buckle. Engagement of a seatbelt tongue with the seatbelt buckle activates the buckling detection switch. Disengagement of the seatbelt tongue from the seatbelt buckle deactivates the buckling detection switch. A seatbelt reminder controller is configured to visually and acoustically prompt a vehicle occupant to fasten a seatbelt when the buckling detection switch is off. 
     SUMMARY 
     The electric power source of a conventional detection device is a battery incorporated in the detection device. The battery of the detection device often needs to be changed and is burdensome for a user. Further, a detection device controller needs to always acknowledge whether the detection switch is on or off. 
     It is an object of the present disclosure to provide a detection device that eliminates the need for a user to often change the battery while allowing for constant detection acknowledgement. 
     A first aspect of the present disclosure is a detection device for use with a master device. The detection device includes an electric power source that is intermittently driven to generate electric power, a detector switched between a number of states by occurrence of a predetermined event or a predetermined state, an electric power accumulator that is charged or discharged by switching the state of the detector, and a controller powered by the electric power source. The controller is configured to determine the state of the detector and notify the master device of detection information corresponding to the state of the detector. Activation of the controller powered by the electric power source is followed by acknowledgment of the state of the detector by the controller based on a state of the electric power accumulator. 
     In the first aspect of the present disclosure, the detection device includes the electric power source that is intermittently driven to generate electric power. This eliminates the need for a user to often change the battery. It is desirable that the controller be in the standby state to reduce power consumption during a period when the electric power source of the detection device does not generate electric power or during a period when the output voltage of the electric power source is less than a predetermined value. However, the detector may detect the occurrence of a predetermined state during the period when the electric power source does not generate electric power or during the period when the output voltage of the electric power source is less than the predetermined value. In this regard, in the first aspect of the present disclosure, the detection device includes the electric power accumulator that is charged or discharged by switching the state of the detector. The fact the detection unit detected the occurrence of the predetermined state while the controller was in the standby state can be acknowledged by the controller based on the state of the electric power accumulator when the controller is switched to the standby state. This allows the controller  10  to always acknowledge the occurrence of the predetermined state. 
     In some implementations, the detection device includes a passive tag capable of performing wireless communication with the master device. The controller transmits the detection information to the master device via the passive tag. 
     This configuration allows the detection device to perform wireless communication with the master device and thus increases the degree of freedom for the location of the detection device. 
     In some implementations, in the detection device, the controller operates in accordance with control information written to the passive tag by the master device through wireless communication. 
     This configuration allows the detection device to be operated remotely from the master device. 
     In some implementations, in the detection device, the electric power source is an energy harvesting unit configured to convert environmental energy into electric power, accumulate the electric power, and supply the electric power to the controller. 
     This configuration eliminates the need to change the battery in the detection device. 
     In some implementations, in the detection device, the energy harvesting unit is configured to convert radio waves, used to transmit the control information output by the master device, into electric power and accumulate the electric power. 
     This configuration allows the harvest energy unit to be charged using the radio waves that are regularly transmitted from the master device. 
     In some implementations, the detection device further includes a diode that limits flow of current from the electric power accumulator to the electric power source. 
     This configuration limits voltage drop of the electric power accumulator and is advantageous for prolonging the time in which the voltage at the electric power accumulator is maintained at a high value. 
     In some implementations, the detection device further includes a discharger that discharges the electric power accumulator after the controller completes determination of the state of the detector. 
     In this configuration, when the controller is switched from the standby state to the activated state, the electric power accumulator is discharged by the discharger. This avoids situations in which electric charges remain maintained by the electric power accumulator when the controller normally operates. 
     In some implementations, in the detection device, the controller has a threshold voltage, and the controller compares detector voltage output from the detector with the threshold voltage to acknowledge whether the detector is on or off as the state of the detector. 
     In this configuration, a simple configuration that monitors the voltage output from the detector allows the switched state of the detector to be detected. 
     A second aspect of the present disclosure is a detection control method used when a detection device outputs detection information to a master device. The detection device includes an electric power source that is intermittently driven to generate electric power, a detector configured to switch states when detecting a predetermined event or a predetermined state, an electric power accumulator charged or discharged by switching the state of the detector, and a controller powered by the electric power source. The controller is configured to determine the state of the detector and notify the master device of detection information corresponding to the state of the detector. The detection control method includes acknowledging the state of the detector based on an electric power accumulation state of the electric power accumulator following activation of the controller being powered by the electric power source. 
     Other aspects and advantages of the present disclosure will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing one embodiment of a detection device; 
         FIG. 2  is a timing chart when a detector detects a state when a controller is in an activated state; 
         FIG. 3  is a timing chart when the detector detects a state when a controller is in a standby state; 
         FIG. 4  is a diagram showing another example of a detection device; and 
         FIG. 5  is a timing chart when the detector detects a state when a controller of the other example is in the standby state. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A detection device according to one embodiment will now be described with reference to  FIGS. 1 to 3 . 
     Referring to  FIG. 1 , a detection information transmission system  1  is arranged in, for example, a vehicle. The detection information transmission system  1  includes a master device  2  and a detection device  3  serving as a slave device. The master device  2  and the detection device  3  can communicate with each other. The communication between the master device  2  and the detection device  3  may be, for example, near-field wireless communication that uses a radio frequency identification (RFID) tag. It is preferred that the radio wave frequency used for the wireless communication be, for example, a 433 MHz band, a 920 MHz band, or a 2.45 GHz band. 
     The master device  2  includes a communication controller  6 , which may be an ECU, and a communication unit  7 . The communication controller  6  controls transmission and reception of radio waves that are performed by the communication unit  7 . The communication controller  6  may control various operations of the master device  2  other than communication. The communication controller  6  transmits control information Sa that controls the detection device  3  through wireless communication (RFID communication) through the communication unit  7  to the detection device  3  and receives, at the communication unit  7 , detection information Sb transmitted from the detection device  3  as, for example, a reflected wave. The communication unit  7  performs communication in compliance with, for example, RFID to transmit and receive radio waves of a 433 MHz band, a 920 MHz band, or a 2.45 GHz band. The master device  2 , the communication unit  7 , and/or the communication controller  6  may function as an RFID reader or an RFID reader writer. 
     The detection device  3  includes a controller  10 , a tag  11 , which may be a passive tag, a detector  12  configured to detect the occurrence of a predetermine event or a predetermined state of a measured object (not shown) in a vehicle, and an electric power source  13  that is intermittently driven to generate electric power. The electric power source  13  is an electric power source for operating the controller  10 . The controller  10  is configured to be powered by the electric power source  13 , determine a state of the detector  12 , and notify the master device  2  of a determination result corresponding to the state of the detector  12 . The controller  10  is configured to access the tag  11 , read data from a memory  14  of the tag  11 , and write data to the memory  14 . The tag  11  is a communication circuit configured to perform wireless communication with the master device  2  (communication unit  7 ) in compliance with, for example, an RFID communication standard. 
     In the present embodiment, the detector  12  is a momentary switch. The momentary switch is configured to maintain an on state only during a period when, for example, the momentary switch is operated by a user and automatically returned to an off state during a non-operation period when, for example, the momentary switch is not operated by the user. For example, when the momentary switch is on, it can indicate the occurrence of a predetermined state or a predetermined event. In another example, the detector  12  may be an alternate action switch. The alternate action switch shifts to an on state when, for example, operated by the user and maintains the on state until the alternate action switch is operated next time. The alternate action switch is switched from the on state to an off state when, for example, operated by the user and maintains the off state until the alternate action switch is operated next time. The switching of off to on of the alternate action switch can indicate, for example, the occurrence of the predetermined state or the predetermined event. The momentary switch and the alternate action switch are known in the art. Thus, the structures of the momentary switch and the alternate action switch will not be described in detail. Although no limitation is intended, the detector  12  may be a seating sensor that detects a seating event of a vehicle occupant and/or a buckling sensor that detects a buckling event of a seatbelt. 
     The master device  2  (communication controller  6 ) can transmit the control information Sa from the communication unit  7  to the tag  11  and write the control information Sa to the memory  14 . The controller  10  of the detection device  3  may control various operations of the detection device  3  in accordance with the control information Sa in the memory  14  of the tag  11 . The controller  10  writes the detection information Sb, which is an output of the detector  12 , to the memory  14  of the tag  11  and transmits the detection information Sb in the memory  14  from the tag  11  to the master device  2  through wireless communication. 
     The electric power source  13  may be or include an energy harvesting unit  16  configured to convert environmental energy to electric power, accumulate the electric power, and supply the accumulated electric power to the controller  10 . The controller  10  is connected to the energy harvesting unit  16  by a control line  17  and an electric power source line  18 . The control line  17  transmits, to the controller  10 , an enable signal that switches the controller  10  between a standby state and an activated state. The electric power source line  18  transmits, to the controller  10 , electric power accumulated in the energy harvesting unit  16 . 
     The energy harvesting unit  16  can include a capacitor CO that accumulates electric charges generated based on environmental energy. The energy harvesting unit  16  may be configured as, for example, a dedicated IC. The energy harvesting unit  16  supplies the electric charges accumulated in the capacitor C 0  as output voltage Ve through the electric power source line  18  to the controller  10  and the detector  12 . The energy harvesting unit  16  is configured to convert environmental energy such as vibration, light, radio waves, or pressing of a switch into electric power and accumulate the electric power. The energy harvesting unit  16  may be configured to convert, for example, radio waves that transmit the control information Sa output by the master device  2  into electric power and accumulate the electric power. 
     The energy harvesting unit  16  supplies a high-level enable signal through the control line  17  to the controller  10  when the accumulated electric power (proportional to electric charges of capacitor C 0 ) becomes greater than or equal to a desired value Wk. When the controller  10  receives the high-level enable signal from the energy harvesting unit  16  (or when enable signal shifts to high-level), the controller  10  shifts to the activated state using the output voltage Ve at the energy harvesting unit  16  as an operation electric power source. 
     When the accumulated electric power (proportional to electric charges of capacitor C 0 ) becomes less than or equal to a lower limit value Wmin, the energy harvesting unit  16  shifts the enable signal to a low level. When the enable signal falls (or when high-level enable signal is not received), the controller  10  shifts to the standby state, which may be an electric power source off state. In such a manner, the controller  10  (processor or CPU or the like included in controller  10 ) is switched to an activated state or a standby state in accordance with the charged amount of the energy harvesting unit  16 . 
     In the illustrated example, the detector  12  is arranged on a branching wire  19  that branches from the electric power source line  18  and connects the energy harvesting unit  16  and a control terminal  20  of the controller  10 . The detector  12  may include an input terminal connected to the capacitor C 0  of the energy harvesting unit  16  directly or via a diode  27  and an output terminal connected to the control terminal  20  of the controller  10 . The detector  12  can include, for example, a switch that supplies the output voltage Ve at the energy harvesting unit  16  to the control terminal  20  of the controller  10  when the switch is activated. 
     The detection device  3  includes an electric power accumulator  26  that accumulates electric charges when the state of the detector  12  switches. An electric power accumulation state of the electric power accumulator  26  can indicate the fact that or history in which the detector  12  detected a predetermined state or a predetermined event. For example, when the controller  10  is switched to the activated state, the controller  10  can acknowledge the fact that or history in which the detector  12  detected the predetermined state or the predetermined event during a period when the controller  10  was in the standby state based on the electric power accumulation state of the electric power accumulator  26 . In such a manner, the detection device  3  includes the electric power accumulator  26  connected to the detector  12  so that the state of the detector  12  is always reflected in the state or voltage at the control terminal  20  when the controller  10  is activated even in the activated state in which the controller  10  can read a state or voltage at the control terminal  20  or even in the standby state in which the controller  10  cannot read a state or voltage at the control terminal  20 . In the illustrated example, the electric power accumulator  26  includes a capacitor C 1 . It is preferred that the capacitor C 1  be connected to the detector  12  and a GND. When the detector  12  goes on, the capacitor C 1  accumulates the output voltage Ve (electric charges of capacitor C 0 ) at the energy harvesting unit  16 . The activation of the controller  10  powered by the electric power source  13  is followed by the acknowledgment of the state of the detector  12  based on the state of the electric power accumulator  26 . 
     The controller  10  includes a comparator  23  configured to detect a state of the detector  12  and changes in a state of the detector  12  based on input voltage Vin at the control terminal  20 . It is preferred that the comparator  23  have a high input impedance. The comparator  23  compares the input voltage Vin at the control terminal  20  with threshold voltage Vth and outputs an output signal Vout in accordance with the comparison result. The controller  10  is configured to determine a state of the detector  12  based on the output signal Vout of the comparator  23 . For example, when the input voltage Vin becomes greater than or equal to the threshold voltage Vth, the controller  10  determines that the state of the detector  12  has changed (or that detector  12  is on) so that the output signal Vout becomes, for example, high-level. When the input voltage Vin becomes less than the threshold voltage Vth, the controller  10  determines that the state of the detector  12  has not changed (or that detector  12  is off) so that the output signal Vout becomes, for example, low-level. It is preferred that the threshold voltage Vth be set to a value of approximately zero volts. 
     The diode  27  that prevents reverse current is located between the detector  12  and the energy harvesting unit  16 . The diode  27  limits the flow of the electric charges accumulated in the capacitor C 1  from the capacitor C 1  into the energy harvesting unit  16 . The diode  27  minimizes and prevents temporal decreases in the voltage at the capacitor C 1  and maintains the voltage (input voltage Vin) at the capacitor C 1 . 
     When the controller  10  is switched from the standby state to the activated state, the comparator  23  compares the input voltage Vin, which is voltage at the capacitor C 1 , with the threshold voltage Vth and outputs the output signal Vout, which is the comparison result. In such a manner, the controller  10  can acknowledge whether or not the state of the detector  12  has been switched when the controller  10  is in the standby state from the voltage comparison that is made by the comparator  23  when the controller  10  is switched from the standby state to the activated state. 
     The detection device  3  includes a discharger  28  that discharges the electric power accumulator  26 . For example, the discharger  28  is or can include a transistor Tr 1 . A collector terminal of the transistor Tr 1  is connected to the capacitor C 1 , an emitter terminal of the transistor Tr 1  is connected to the GND, and a base terminal of the transistor Tr 1  is connected to the controller  10 . The controller  10  determines a state of the detector  12  based on the output signal Vout of the comparator  23  and then discharges the capacitor C 1  through the discharger  28 . 
     The operation and advantages of the detection device  3  will now be described with reference to  FIGS. 2 and 3 . 
     As shown in  FIG. 2 , for example, when the master device  2  (communication controller  6 ) acknowledges that an ignition switch of the vehicle has been switched to the activated state, the master device  2  (communication controller  6 ) activates the communication unit  7  that was in the standby state and switches the near-field wireless communication (for example, RFID communication) to the on state. The activated communication unit  7  starts regular communication of the near-field wireless communication. For example, the master device  2  first transmits a monitor start request Sa 1 , which serves as the control information Sa, from the communication unit  7  to the tag  11  through the RFID communication. The tag  11  receives the monitor start request Sal and writes the monitor start request Sal to the memory  14 . 
     When the capacitor C 0  is charged so that the electric power based on environmental energy becomes greater than or equal to the desired value Wk, the energy harvesting unit  16  supplies a high-level enable signal through the control line  17  to the controller  10 . For example, when the energy harvesting unit  16  is charged sufficiently, the output voltage Ve at the energy harvesting unit  16  activates the controller  10 . In such a manner, when the controller  10  receives the high-level enable signal from the energy harvesting unit  16 , the controller  10  activates the output voltage Ve at the energy harvesting unit  16  as the operation electric power source. 
     When the controller  10  is in the activated state, the controller  10  monitors a written state of the memory  14 . Thus, when the monitor start request Sa 1  is written to the memory  14 , the controller  10  reads the monitor start request Sal. The controller  10  confirms the output of the detector  12  in accordance with the read monitor start request Sa 1 . Based on the input voltage Vin at the control terminal  20 , the controller  10  in the activated state monitors whether the state of the detector  12  has been switched. That is, the controller  10  detects the state of the detector  12  based on the output signal Vout of the comparator  23 . 
     The output voltage Ve at the energy harvesting unit  16  gradually decreases when the controller  10  is driven. When the electric power that can be supplied by the energy harvesting unit  16  becomes less than or equal to the lower limit value Wmin, the energy harvesting unit  16  switches the enable signal from the high-level to the low-level. When the enable signal falls to the low-level, the controller  10  is switched to the standby state. In such a manner, the controller  10  repeats activation and standby in accordance with the amount of electric charges accumulated in the capacitor C 0  of the energy harvesting unit  16 . 
     If the state of the detector  12  changes (for example, switch is on) while the controller  10  is being activated, the electric power that can be supplied by the energy harvesting unit  16  remains sufficient. Thus, after the capacitor C 1  accumulates electric power, sufficiently high voltage Va′ is applied to the control terminal  20  (comparator  23 ) of the controller  10 . When the controller  10  is in the activated state, the comparator  23  immediately performs voltage comparison and outputs a high-level (on signal) output signal Vout. Then, the controller  10  (for example, processor of controller  10 ) immediately acknowledges that the state of the detector  12  has changed. When the controller  10  acknowledges that the state of the detector  12  has changed, the controller  10  writes to the memory  14  of the tag  11  the detection information Sb indicating that the state of the detector  12  has changed. After determining the state of the detector  12 , the controller  10  discharges the capacitor C 1  through the discharger  28 . 
     When the tag  11  communicates with the master device  2  at an initial communication timing T 1  after the detection information Sb is written to the memory  14 , the tag  11  transmits the detection information Sb, which is written to the memory  14 , to the master device  2 . That is, the tag  11  transmits the detection information Sb indicating that the detector  12  has been switched on to the master device  2  through the RFID communication. In such a manner, the detection device  3  uses the detection information Sb indicating that the detector  12  is on to notify the master device  2  that the detector  12  has been switched to the on state. 
     As shown in  FIG. 3 , when the controller  10  is in the standby state, the detection of the detector  12  may be switched from off to on. When the detector  12  is activated while the controller  10  is in a standby state, the capacitor C 1  accumulates the remaining electric power of the energy harvesting unit  16 . That is, the voltage at the capacitor C 1  is accumulated in the voltage Va corresponding to the remaining electric power of the energy harvesting unit  16 , and the voltage is input to the comparator  23  as the input voltage Vin. 
     The diode  27  is connected to the preceding stage of the detector  12 . Thus, after the detector  12  is activated and the electric power of the capacitor C 1  is accumulated (electric charges of capacitor C 0  are moved to capacitor C 1 ), voltage drop of the capacitor C 1  no longer occurs except slow discharge such as self-discharge of the capacitor C 1 . Thus, the input voltage Vin at the input terminal of the comparator  23  is substantially maintained at a value when starting the electric power of the capacitor C 1 . When electric charges move from the capacitor C 0  to the capacitor C 1 , the voltage at the capacitors C 0  and C 1  changes. However, to facilitate understanding, the capacitor C 0  is set to have an amount of accumulating electric power that is greater than the capacitor C 1 , and voltage drop when electric charges move from the capacitor C 0  to the capacitor C 1  is ignored. 
     In this manner, the capacitor C 1  maintains the input voltage Vin at the voltage Va for a relatively long time. Thus, even when the energy harvesting unit  16  performs recharging based on environmental energy so that the controller  10  is switched to the activated state again, the capacitor C 1  is still not discharged, and the input voltage Vin is maintained at the voltage Va. 
     When the controller  10  is first activated after the detector  12  goes on, the comparator  23  compares the input voltage Vin, which is close to voltage Va, with the threshold voltage Vth. This allows the comparator  23  to output an on signal as the output signal Vout. Based on the on signal of the comparator  23 , the controller  10  acknowledges that the detector  12  has been switched to the on state (or that detector  12  is in on state). The controller  10  writes to the memory  14  the detection information Sb indicating the state of the detector  12  has changed and notifies the master device  2  of the changes in the state of the detector  12  through the subsequent RFID communication.  FIG. 3  does not show the communication of the monitor start request Sa 1 . 
     After detecting that the detector  12  is on, the controller  10  discharges the capacitor C 1  through the discharger  28 . This is because the controller  10  cannot acknowledge that the detector  12  has been operated again when the enable signal changes again to the high level again without the capacitor C 1  being always discharged. Thus, after detecting that the detector  12  is on, the capacitor C 1  is discharged so that the input voltage Vin becomes zero volts. This allows the controller  10  to determine whether or not the state of the detector  12  has changed whenever the enable signal is switched to the high level. 
     The detection device  3  of the present example includes the electric power source  13  (energy harvesting unit  16 ) that is intermittently driven. This eliminates the need for the user to often change the battery. During the period in which the electric power source  13  does not generate electric power, the controller  10  needs to be set to the standby state. Further, in this period, state detection of the detector  12  needs to be always detected. In this regard, the detection device  3  of the present example includes the electric power accumulator  26  that can accumulate electric power in accordance with the state of the detector  12 . When the controller  10  is switched to the activated state by the electric power of the electric power accumulator  26 , the controller  10  is notified that the detector  12  has performed state detection before the controller  10  switched to the activated state. This reduces non-detections during the period of the standby state. 
     The detection device  3  includes the passive tag  11  that is capable of performing wireless communication with the communication unit  7 , which is arranged at the master device  2 . The controller  10  transmits the detection information Sb through the passive tag  11  to the master device  2 . The wireless communication performed between the master device  2  and the detection device  3  increases the degree of freedom for the location of the detection device  3 . 
     The controller  10  operates in accordance with the control information Sa written to the tag  11  by the master device  2  through wireless communication. Thus, the detection device  3  can be remotely operated by the control information Sa, which is transmitted from the master device  2 . 
     The electric power source  13  is the energy harvesting unit  16  that accumulates environmental energy and supplies the electric power to the controller  10 . This eliminates the need to exchange a battery in the detection device  3 . 
     The energy harvesting unit  16  is configured to convert radio waves that transmit the control information Sa output by the master device  2  into electric power and accumulate the electric power. The electric power generated and accumulated by the energy harvesting unit  16  is automatically and regularly charged by the radio waves that are regularly transmitted from the master device  2 . 
     The detection device  3  includes the diode  27  that limits the flow of current from the electric power accumulator  26 , where electric power is accumulated, to the energy harvesting unit  16 . Thus, the voltage drop of the electric power accumulator  26  is minimized. This is advantageous for prolonging the time in which the voltage at the electric power accumulator  26  is maintained at a high value. 
     The detection device  3  includes the discharger  28  that is capable of discharging the voltage accumulated in the electric power accumulator  26 . Thus, if the electric power accumulator  26  is charged when the controller  10  is switched from the standby state to the activated state, the electric power accumulator  26  is discharged by the discharger  28 . This avoids situations in which electric charges remain maintained by the electric power accumulator  26  when the controller  10  operates normally. 
     The controller  10  compares the input voltage Vin from the detector  12  with the threshold voltage Vth to acknowledge whether the detector  12  is on or off as the state of the detector  12 . Thus, a simple configuration that monitors the input voltage Vin from the detector  12  allows the switched state of the detector  12  to be detected. Further, the threshold voltage Vth at the comparator  23  is set to a value of approximately zero volts. Thus, even if the voltage at the electric power accumulator  26  is low, the input voltage Vin exceeds the threshold voltage Vth. This allows the controller  10  to acknowledge that the detector  12  is on. 
     It should be apparent to those skilled in the art that the present disclosure may be embodied in many other specific forms without departing from the scope of the invention. Particularly, it should be understood that the present disclosure may be embodied in the following forms. 
     As shown in  FIG. 4 , the number of detectors  12  does not have to be only one and may be two or more. In such a case, sets of the detector  12 , the electric power accumulator  26 , the diode  27 , and the discharger  28  are used. 
     As shown in  FIG. 5 , state detection can be performed by discharging the electric power accumulator  26  that has been in the electrical power accumulation state when the detector  12  performs state detection. In this manner, changes in the state of the detector  12  can be notified to the controller  10  by discharging the electric power accumulator  26  when the state of the detector  12  changes. 
     The capacity of the capacitor C 1  does not need to be so large since electrical charges only need to be maintained while the energy harvesting unit  16  is being recharged. For example, when the energy harvesting unit  16  is charged with radio waves transmitted from the master device  2 , the radio waves are regularly transmitted. This may allow the energy harvesting unit  16  to be recharged for a short time. Thus, it is assumed that the capacity of the capacitor C 1  does not have to be so large. 
     In the embodiment, the capacitor C 0  has a larger capacity than the capacitor C 1 . Instead, the capacitors C 0  and C 1  may have substantially the same capacity. 
     The detector  12  is not limited to a switch and may be changed to various sensors such as a sensor. 
     The detector  12  does not have to detect two states, namely, on and off states, and may detect the amount of movement. 
     The detection information Sb is not limited to on/off information of a switch. Instead, for example, when the detector  12  is a sensor, the detection information Sb may be sensor information in accordance with the amount of movement. 
     The master device  2  may be installed in, for example, any position of the vehicle. 
     The control information Sa may be information that instructs a function or an operation mode of the controller  10 . 
     The electric power accumulator  26  is not limited to the capacitor C 1 . Instead, the electric power accumulator  26  may be an electric power accumulation element or an electric power accumulation circuit configured to maintain the input voltage Vin at the controller  10  at a constant value when the controller  10  is at least in the standby state. 
     The discharger  28  is not limited to the transistor Tr 1  and may be a switch element or a switch circuit configured to discharge the electric power accumulator  26  at zero volts. 
     The detector  12  may detect an event or a state associated with a certain member in a vehicle instead of or in addition to an event or a state associated with a seatbelt reminder. 
     The electric power source  13  is not limited to the energy harvesting unit  16 . Instead, the electric power source  13  may be an electric power source configured to be intermittently driven and generate electric power in order to generate and/or accumulate electric power. 
     The master device  2  and the detection device  3  may be configured to perform wired communication instead of or in addition to wireless communication. 
     The detection information transmission system  1  does not have to be used for a vehicle and may be applied to a non-vehicle device. 
     The present disclosure encompasses the following implementations. 
     [Implementation 1] A detection device configured to detect occurrence of a predetermined event or a predetermined state, the detection device including: an energy harvesting unit; a switch configured to be switched from an on state to an off state when the predetermined event or the predetermined state occurs; a capacitor connected to the energy harvesting unit via the switch and charged by the energy harvesting unit when the switch is switched to the on state; and a controller powered by the energy harvesting unit, wherein the controller is in an activated state when output voltage at the energy harvesting unit is greater than or equal to a predetermined value, and the controller is in a standby state when the output voltage at the energy harvesting unit is less than the predetermined value, wherein if the switch is switched to on state when the controller is in the activated state, the controller acknowledges a fact that the predetermined event or the predetermined state is currently occurring based on the on state of the switch, and wherein the controller acknowledges the predetermined event or the predetermined state as a past history that occurred when the controller was in the standby state based on an electric power accumulation state of the capacitor immediately after returning from the standby state to the activated state. 
     [Implementation 2] The detection device according to implementation 1, wherein the controller is configured to transmit a wireless signal indicating the occurrence of the predetermined event or the predetermined state via a communication circuit when the controller acknowledges the occurrence of the predetermined event or the predetermined state. 
     [Implementation 3] The detection device according to implementation 1 or 2, wherein the switch is arranged on a branching wire that connects the energy harvesting unit and the controller. 
     [Implementation 4] The detection device according to implementation 3, wherein the switch is arranged on the branching wire between the energy harvesting unit and the capacitor. 
     [Implementation 5] The detection device according to implementation 3 or 4, wherein the capacitor is connected to a node of the branching wire between the switch and the controller. 
     [Implementation 6] The detection device according to any one of implementations 3 to 5, further comprising a discharger connected to a node of the branching wire between the capacitor and the controller. 
     [Implementation 7] The detection device according to implementation 6, wherein the discharger includes a transistor switched by the controller between a discharge state in which the transistor discharges the capacitor and a non-discharge state in which the transistor does not discharge the capacitor. 
     The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims. For example, one or more of the components may be omitted from the components described in the embodiments (or one or more aspects thereof). Components in different embodiments may be appropriately combined.