Patent Publication Number: US-2020302129-A1

Title: Card reader

Description:
TECHNICAL FIELD 
     The present invention relates to a card reader including a pre-head configured to detect a change in magnetic field by a magnetoresistive element. 
     BACKGROUND ART 
     Patent Literature 1 discloses a card reader that reads magnetic information from a magnetic card including a magnetic track. The card reader disclosed in Patent Literature 1 includes a card insertion slot, a magnetic head for reading magnetic information, a card conveying passage extending from the card insertion slot via the read position where the magnetic head performs reading, and a conveying mechanism that conveys a magnetic card along the card conveying passage. The card reader also includes a pre-head that detects whether magnetic information is recorded on a magnetic stripe of a magnetic card inserted into the card insertion slot. The detection position where the pre-head performs detection is set to be closer to the card insertion slot than the read position. 
     The pre-head disclosed in Patent Literature 1 includes a core having a gap and a coil wound around the core. When a magnetic card having magnetic information recorded thereon passes through the gap, voltage signals with an amplitude corresponding to a change in magnetic field are output from the two ends of the coil. The card reader determines whether the magnetic information is stored in the magnetic card based on the voltage signals from the pre-head. When it is determined that the magnetic information is stored in the magnetic card, the card reader drives the conveying mechanism to convey the magnetic card along the card conveying passage. The card reader then obtains the magnetic information from the magnetic card passing through the read position. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Publication No. 2010-205187 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       FIG. 4  is a schematic block diagram of a control system of a pre-head  50  in a card reader. The control system is configured to determine whether magnetic information is stored in a magnetic card, and the card reader includes the pre-head  50  having a core  51  with a gap  51   a  and a coil  52  wound around the core  51 . 
     As shown in  FIG. 4 , the control system of the pre-head  50  includes the pre-head  50 , a signal processing circuit  24  that outputs a detection signal based on a voltage signal output from the pre-head  50 , and a control unit  25  that determines whether magnetic information is recorded on a magnetic card inserted into a card insertion slot based on a detection signal. The signal processing circuit  24  includes an amplifier circuit  31  that amplifies voltage signals output from a first terminal  53  connected to one end of the coil  52  and a second terminal  54  connected to the other end of the coil  52 , a comparator circuit  32  that compares an amplified signal having been amplified by the amplifier circuit  31  to a predetermined threshold and outputs a detection signal when the amplified signal is equal to or larger than the threshold, and a reference voltage circuit  30  that sets the reference potential of the amplitude of a voltage signal input to the amplifier circuit  31  to a reference voltage V 0  when a magnetic card is not inserted into the card insertion slot (when the pre-head  50  does not detect a change in magnetic field). The reference voltage circuit  30  superimposes the reference voltage V 0  on a voltage signal from the pre-head  50 . The control unit  25  determines whether magnetic information is recorded on a magnetic card based on the number of times that the detection signal is output from the signal processing circuit  24  or the like when the magnetic card is inserted into the card insertion slot. 
     It is not necessary to supply a power to a pre-head that includes a core and a coil. Consequently, when a magnetic card passes through a gap in the core, a voltage signal corresponding to a change in magnetic field is always output from the pre-head. If a malicious user moves the magnetic card to pass through the gap in the pre-head for the purpose of obtaining a voltage signal output from the pre-head when a card reader is not switched on or the like, the malicious user may illegally obtain magnetic information having been recorded on the magnetic card. 
     The present invention has been achieved in view of the above problems, and an object of the invention is to provide a card reader in which magnetic information recorded on a magnetic card cannot be illegally obtained through a pre-head when the power supply is not switched on. 
     Means for Solving the Problem 
     In order to solve the above problems, a card reader according to the present invention includes a pre-head that detects a change in magnetic field by a magnetoresistive element, a voltage applying circuit that applies an applied voltage set in advance to the pre-head, a signal processing circuit that includes a reference voltage circuit configured to superimpose a reference voltage on a voltage signal from the pre-head and outputs a detection signal based on a difference between the voltage signal and the reference voltage, a capacitor inserted between the pre-head and the reference voltage circuit, a switching circuit that switches a state of the capacitor and the reference voltage circuit from a disconnected state to a connected state simultaneously with application of the applied voltage from the voltage applying circuit to the pre-head, and a control unit that determines whether magnetic information is recorded on a magnetic stripe of a magnetic card inserted into a card insertion slot based on the detection signal. 
     According to the present invention, the pre-head detects a change in magnetic field by the magnetoresistive element. The magnetoresistive element changes its resistance value based on a change in magnetic field and outputs the change in magnetic field as a change in voltage when a voltage is applied thereto. Consequently, when no voltage is applied to the pre-head, the pre-head does not output a voltage signal even if the magnetic card passes through the detection position where the pre-head performs detection. When a power supply for the card reader is not switched on (when no voltage is applied to the pre-head), it is thus difficult to obtain magnetic information on the magnetic card through the pre-head. 
     If a control system of a pre-head including a core and a coil can be used as a control system of the pre-head that performs a determination operation whether magnetic information is stored in the magnetic card, changes in the conventional card reader can be reduced. It is thus possible to reduce risk when the pre-head is replaced by the pre-head that detects a change in magnetic field by the magnetoresistive element. 
     However, when the pre-head that detects a change in magnetic field by the magnetoresistive element is used while the configuration of the control system of the pre-head is the same as that of the conventional one, in order to set the reference potential of the amplitude of a voltage signal output from the pre-head to a reference voltage, the capacitor must be inserted between the pre-head and the reference voltage circuit for the purpose of compensating for an error between the voltage signal output from the pre-head and the reference voltage. 
     In addition, when the capacitor is inserted between the pre-head and the reference voltage circuit, the capacity of the capacitor must be set to be relatively large for the purpose of preventing a frequency component necessary for the determination operation of whether or not the magnetic information is recorded from a voltage signal input from the pre-head to the signal processing circuit from being cut off by a filter effect due to insertion of the capacitor. Herein, when the capacity of the capacitor is increased, the time from a time point when a voltage is applied to the pre-head to a time point when a voltage signal from the pre-head is input via the capacitor to the signal processing circuit (a start-up time of the pre-head) is extended, and thus it takes a long time to perform the determination operation of whether or not the magnetic information is recorded on the magnetic card inserted into the card insertion slot. 
     In order to solve such problems, the present invention includes the switching circuit that connects the capacitor to the reference voltage circuit simultaneously with application of the applied voltage from the voltage applying circuit to the pre-head. It is possible to reduce, simultaneously with the application of the applied voltage to the pre-head, the difference between the potential of the capacitor on a side of the pre-head and the potential of the capacitor on the opposite side to the pre-head and thus it is possible to reduce the saturation time of the capacitor. As a result, it is possible to reduce the time from the time point when a voltage is applied to the pre-head to the time point when a voltage signal from the pre-head is input via the capacitor to the signal processing circuit and thus it is possible to quickly perform the determination operation of whether or not the magnetic information is recorded on the magnetic card inserted into the card insertion slot. 
     According to the present invention, the switching circuit includes a first switching element that connects and disconnects the voltage applying circuit to and from the pre-head and a second switching element that connects and disconnects the capacitor to and from the signal processing circuit. The first switch and the second switch are simultaneously switched from a disconnected state to a connected state. 
     In this case, a detector that mechanically or optically detects the magnetic card inserted into the card insertion slot is further included. When the detector detects the magnetic card, the first switch and the second switch are simultaneously switched from a disconnected state to a connected state. Consequently, when the magnetic card is inserted into the insertion slot, the voltage applying circuit is connected to the pre-head to apply the applied voltage to the pre-head. In addition, the capacitor is connected to the signal processing circuit simultaneously with the application of the applied voltage to the pre-head, and thus the potential of the capacitor on the opposite side to the pre-head is the reference voltage. 
     In this case, a human sensor that detects a movement of a user is further included. When the human sensor detects a movement of the user, it is preferable that the first switch and the second switch are simultaneously switched from a disconnected state to a connected state. Consequently, when the user of the card reader is detected, the voltage applying circuit is connected to the pre-head to apply the applied voltage to the pre-head. In addition, the capacitor is connected to the signal processing circuit simultaneously with the application of the applied voltage to the pre-head, and thus the potential of the capacitor on the opposite side to the pre-head is the reference voltage. 
     According to the present invention, each of the first switching element and the second switching element is preferably an analog switch. As the analog switch is used for the switching elements, the resistance of the switching element when switched on is reduced. It is thus possible to prevent the switching elements from affecting characteristics of the magnetoresistive element. 
     According to the present invention, a first magnetoresistive element and a second magnetoresistive element serially connected to each other are included as the magnetoresistive element. The applied voltage is applied from the voltage applying circuit to a side of the first magnetoresistive element opposite to the second magnetoresistive element and a side of the second magnetoresistive element opposite to the first magnetoresistive element is grounded. The pre-head outputs a voltage at an intermediate point between the first magnetoresistive element and the second magnetoresistive element as the voltage signal. The pre-head can thus output the voltage signal corresponding to a change in magnetic field when the magnetic card passes. 
     In such a case, the applied voltage may be twice as large as a potential of the reference voltage. 
     It is thus possible to easily determine the reference potential of the amplitude of a voltage signal output from the pre-head as the reference voltage. In addition, when the capacitor is connected to the signal processing circuit simultaneously with application of the applied voltage from the voltage applying circuit to the pre-head, the two ends of the capacitor have substantially the same potential. The saturation time of the capacitor can be reduced significantly. Consequently, it is possible to further reduce the time from the time point when a voltage is applied to the pre-head to the time point when a signal from the pre-head is input via the capacitor to the signal processing circuit (the start-up time of the pre-head). 
     Effect of the Invention 
     According to the present invention, when no applied voltage is applied to the pre-head, it is difficult to illegally obtain magnetic information on the magnetic card through the pre-head. In addition, according to the present invention, even when the pre-head that detects a change in magnetic field by the magnetoresistive element is used, the control system of the pre-head including a core and a coil can be used. Moreover, the present invention can reduce the start-up time of the pre-head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  show explanatory diagrams of a card reader according to an embodiment of the present invention. 
         FIG. 2  is a schematic block diagram of a control system of a pre-head. 
         FIG. 3  show graphs of a start-up time of the pre-head. 
         FIG. 4  is a schematic block diagram of a control system of a pre-head including a core and a coil. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A card reader according to an embodiment of the present invention is described below with reference to the drawings. 
     (Overall Configuration) 
       FIG. 1( a )  is an explanatory diagram of a card reader according to an embodiment of the present invention.  FIG. 1( b )  is an explanatory diagram of a magnetic card. A card reader  1  according to the present embodiment reads a magnetic card  3  having a magnetic stripe  2  to obtain magnetic information recorded on the magnetic stripe  2 . As shown in  FIG. 1( a ) , the card reader  1  includes a card insertion slot  5 , a magnetic head  6  for reading magnetic information, a card conveying passage  7  extending from the card insertion slot  5  via a read position A where the magnetic head  6  performs reading, and a conveying mechanism  8  that conveys the magnetic card  3  inserted into the card insertion slot  5  along the card conveying passage  7 . 
     The card reader  1  also includes a detector  9  that mechanically detects that the magnetic card  3  has been inserted into the card insertion slot  5  and a pre-head  10  that magnetically detects that the magnetic card  3  has been inserted into the card insertion slot  5 . A detection position B where the detector  9  detects the magnetic card  3  corresponds to the card insertion slot  5 . A detection position C where the pre-head  10  detects the magnetic card  3  is between the detection position B where the detector  9  detects the magnetic card  3  and the read position A, and is set to be closer to the card insertion slot  5  than the read position A. The detection position C where the pre-head  10  performs detection is near the detection position B where the detector  9  detects the magnetic card  3 . 
     The magnetic head  6  is disposed in a manner that a sensor surface  6   a  faces the card conveying passage  7 . The magnetic head  6  is a three-channel magnetic head  6  that reads three magnetic tracks  11  formed on the magnetic stripe  2  of the magnetic card  3 . The conveying mechanism  8  includes two pairs of conveying rollers  13  configured to sandwich the magnetic card  3  therebetween and convey the magnetic card  3 . One conveying roller pair  13  is disposed between the detection position C where the pre-head  10  performs detection and the read position A. The other conveying roller pair  13  is disposed to be opposite to the pre-head  10  with the read position A being interposed therebetween. The conveying mechanism  8  also includes a conveying motor  14  functioning as a drive source for the conveying roller pairs  13 . 
     The detector  9  includes a movement member moving while abutting against the magnetic card  3  inserted into the card insertion slot  5  and a switch turned on and off based on the movement of the movement member. The detector  9  may optically detect that the magnetic card  3  has been inserted into the card insertion slot  5 . 
     The pre-head  10  detects a change in magnetic field when the magnetic card  3  is inserted into the card insertion slot  5  using magnetoresistive elements  21 ,  22  (see  FIG. 2 ). More specifically, the pre-head  10  detects a change in magnetic field when one of three magnetic tracks  11  formed on the magnetic stripe  2  of the magnetic card  3  passes through the detection position C using the magnetoresistive elements  21 ,  22 . The magnetoresistive elements  21 ,  22  are MR elements and are obtained by forming a thin film of a nickel-iron alloy (Ni—Fe) on a surface of a sensor substrate (not shown). The magnetoresistive elements  21 ,  22  change their resistance value based on a change in magnetic field. The pre-head  10  outputs a change in magnetic field as a voltage signal S 1 . 
     When the magnetic card  3  is inserted into the card insertion slot  5 , the detector  9  detects the magnetic card  3 . When the detector  9  detects the magnetic card  3  having been inserted into the card insertion slot  5 , the card reader  1  applies a voltage to the pre-head  10 . The card reader  1  then determines whether magnetic information is recorded on the magnetic stripe  2  of the magnetic card  3  based on the voltage signal S 1  output from the pre-head  10 . 
     When it is determined that the magnetic information is recorded on the magnetic card  3 , the card reader  1  drives the conveying motor  14  to convey the magnetic card  3  along the card conveying passage  7 . When the magnetic card  3  passes through the read position A, the card reader  1  reads the magnetic information recorded on the magnetic stripe  2  using the magnetic head  6 . That is, when the magnetic card  3  passes through the read position A, a read signal corresponding to a change in magnetic field is output from the magnetic head  6 . Consequently, the card reader  1  obtains the magnetic information based on the read signal. 
     (Control System of Pre-Head) 
       FIG. 2  is a schematic block diagram of a control system of the pre-head  10 . The control system of the pre-head  10  performs a determination operation whether magnetic information is recorded on the magnetic stripe  2  of the magnetic card  3 . The control system of the pre-head  10  includes the detector  9 , the pre-head  10 , a voltage applying circuit  23 , a signal processing circuit  24 , and a control unit  25 . The control system of the pre-head  10  also includes a coupling capacitor (capacitor)  26  and a switching circuit  27 . 
     As shown in  FIG. 2 , the pre-head  10  includes a first magnetoresistive element  21  and a second magnetoresistive element  22  as magnetoresistive elements. The first magnetoresistive element  21  and the second magnetoresistive element  22  are disposed on a sensor substrate in a manner that their magnetosensitive directions are aligned with an insertion direction I of the magnetic card  3 . 
     In addition, the first magnetoresistive element  21  and the second magnetoresistive element  22  are disposed in parallel in the insertion direction I. The first magnetoresistive element  21  overlaps the second magnetoresistive element  22  on the sensor substrate as viewed from the insertion direction I of the magnetic card  3 . For this reason, when the magnetic card  3  inserted into the card insertion slot  5  passes through the detection position C, one magnetic track  11 , which is a reading target, on the magnetic stripe  2  of the magnetic card  3  passes through the first magnetoresistive element  21  and the second magnetoresistive element  22 . 
     The first magnetoresistive element  21  is serially connected to the second magnetoresistive element  22 . In the present embodiment, the resistance value of the first magnetoresistive element  21  is set to be equal to the resistance value of the second magnetoresistive element  22 . A predetermined applied voltage V is applied from the voltage applying circuit  23  to the side of the first magnetoresistive element  21  opposite to the side that the second magnetoresistive element  22  is disposed. The side of the second magnetoresistive element  22  opposite to the side that the first magnetoresistive element  21  is disposed is grounded. The pre-head  10  outputs, as the voltage signal S 1 , a voltage at an intermediate point  28  between the first magnetoresistive element  21  and the second magnetoresistive element  22 . 
     The signal processing circuit  24  outputs a detection signal S 4  based on the amplitude of the voltage signal S 1  output from the pre-head  10 . The signal processing circuit  24  includes a reference voltage circuit  30 , an amplifier circuit  31 , and a comparator circuit  32 . 
     The reference voltage circuit  30  sets the potential of the voltage signal S 1  input to the amplifier circuit  31  to a reference voltage V 0  when the magnetic card  3  is not inserted into the card insertion slot  5  (when the pre-head  10  does not detect a change in magnetic field). That is, the reference voltage circuit  30  is connected between the pre-head  10  and the amplifier circuit  31  and enables the reference voltage V 0  to be superimposed between the pre-head  10  and the amplifier circuit  31 . The reference voltage V 0  is set to be equal to the voltage at the intermediate point  28  between the first magnetoresistive element  21  and the second magnetoresistive element  22 . In the present embodiment, the resistance value of the first magnetoresistive element  21  is equal to the resistance value of the second magnetoresistive element  22  and thus the reference voltage V 0  is half the applied voltage of the voltage applying circuit  23 . 
     A superimposed voltage signal S 2  obtained by superimposing the reference voltage V 0  on the voltage signal S 1  from the pre-head  10  and the reference voltage V 0  from the reference voltage circuit  30  are input to the amplifier circuit  31 . The amplifier circuit  31  includes a differential amplifier and outputs an amplified signal S 3  obtained by amplifying the difference between the superimposed voltage signal S 2  and the reference voltage V 0 . The difference between the superimposed voltage signal S 2  and the reference voltage V 0  is equal to the amplitude of the voltage signal S 1 . 
     The amplified signal S 3  from the amplifier circuit  31  is input to the comparator circuit  32 . The comparator circuit  32  compares the amplified signal S 3  from the amplifier circuit  31  to a predetermined threshold. When the amplitude of the amplified signal S 3  is larger than or equal to the threshold, the comparator circuit  32  outputs the detection signal S 4  to the control unit  25 . 
     The coupling capacitor  26  is inserted between the pre-head  10  and the signal processing circuit  24  (the reference voltage circuit  30 ). 
     It is difficult to cause the resistance value of the first magnetoresistive element  21  in the pre-head  10  to be strictly equal to the resistance value of the second magnetoresistive element  22  in the pre-head  10 . Consequently, it is also difficult to set the potential at the intermediate point  28  between the first magnetoresistive element  21  and the second magnetoresistive element  22  to half the potential of the applied voltage V (the reference voltage V 0 ) when the pre-head  10  does not detect a change in magnetic field. That is to say, it is difficult to cause the resistance value of the first magnetoresistive element  21  to be strictly equal to the resistance value of the second magnetoresistive element  22 , and thus it is difficult to set the reference potential of the voltage signal S 1  output from the intermediate point  28  between the first magnetoresistive element  21  and the second magnetoresistive element  22  to the reference voltage V 0 . When it is impossible to accurately set the reference potential of the voltage signal S 1 , however, it is impossible to accurately determine whether magnetic information is recorded on the magnetic card  3  based on the voltage signal Sl. To solve such a problem, the coupling capacitor  26  is inserted between the pre-head  10  (the intermediate point  28 ) and the signal processing circuit  24  (the reference voltage circuit  30 ) in the present embodiment and thus an error between the potential at the intermediate point  28  and the potential of the reference voltage V 0  is compensated for and the reference potential of the voltage signal S 1  output from the pre-head  10  is set to the reference voltage V 0 . The voltage signal S 1  thus has an amplitude whose center is at the reference potential (the reference voltage V 0 ). The signal processing circuit  24  thus outputs the detection signal S 4  based on the difference between the voltage signal S 1  and the reference voltage V 0 . 
     In addition, when the coupling capacitor  26  is inserted between the pre-head  10  and the signal processing circuit  24 , a frequency band component required for the determination operation of whether or not the magnetic information is recorded may be cut off from the voltage signal S 1  input from the pre-head  10  to the signal processing circuit  24  by a filter effect due to insertion of the coupling capacitor  26 . For example, when the magnetic card  3  with reduced magnetic recording density due to degraded intensity of magnetization passes through the detection position C at a predetermined speed, the frequency of the voltage signal S 1  is lower than that in a case where a normal magnetic card  3  passes through the detection position C at the predetermined speed. In such a case, a low-frequency band component of the voltage signal S 1  may be cut off from the voltage signal S 1  by the filter effect. To prevent such a case, the capacity of the coupling capacitor  26  is increased in the present embodiment to an extent that the low-frequency band component of the voltage signal S 1  is not cut off even in the magnetic card  3  whose intensity of magnetization is reduced to 10%. 
     The switching circuit  27  includes a first analog switch (first switching element)  35  that connects and disconnects the voltage applying circuit  23  to and from the pre-head  10  and a second analog switch (second switching element)  36  that connects and disconnects the coupling capacitor  26  to and from the signal processing circuit  24 . The switching circuit  27  also includes a switching element control unit configured to control on and off of the first analog switch  35  and the second analog switch  36 . In the present embodiment, the switching control unit is the control unit  25 . The control unit  25  switches on the first analog switch  35  and the second analog switch  35  based on signals from the detector  9 . 
     More specifically, the detector  9  is connected to an input side of the control unit  25 . When a signal is input from the detector  9  to the control unit  25 , the control unit  25  outputs a control signal to the first analog switch  35  and the second analog switch  36  to simultaneously switch on the switches  35  and  36 . 
     When the first analog switch  35  is switched on, the first analog switch  35  is switched from a disconnected state to a connected state. The voltage applying circuit  23  is thus connected to the pre-head  10  and the applied voltage V is applied from the voltage applying circuit  23  to the pre-head  10 . The potential of the applied voltage V is twice as large as the reference voltage V 0 . Herein, at a time when the applied voltage V is applied to the pre-head  10 , the potential at the intermediate point  28  between the first magnetoresistive element  21  and the second magnetoresistive element  22  (the potential of the coupling capacitor  26  on a side of the pre-head  10 ) is substantially half the applied voltage V and is substantially equal to the reference voltage V 0 . In addition, when the second analog switch  36  is switched on, the second analog switch  35  is switched from a disconnected state to a connected state. The coupling capacitor  26  is thus connected to the signal processing circuit  24  (the reference voltage circuit  30 ) and the potential of the coupling capacitor  26  on the opposite side to the pre-head  10  is the reference voltage V 0 . When the first analog switch  35  and the second analog switch  36  are simultaneously switched on, the potential of the coupling capacitor  26  on the side of the pre-head  10  and the potential of the coupling capacitor  26  on the opposite side to the pre-head  10  become substantially the same potential (the reference voltage V 0 ) at the same time. That is to say, when the first analog switch  35  and the second analog switch  36  are simultaneously switched on, no potential difference is present between the ends of the coupling capacitor  26 . 
     Herein, the capacity of the coupling capacitor  26  is increased in the present embodiment. For this reason, when the coupling capacitor  26  is directly connected to the signal processing circuit  24 , the time from when the applied voltage V is applied to the pre-head  10  to when the voltage signal S 1  from the pre-head  10  is input via the coupling capacitor  26  to the signal processing circuit  24  (a start-up time of a pre-head) may be extended. In order to handle such a case, the second analog switch  36  is interposed between the coupling capacitor  26  and the signal processing circuit  24 , the second analog switch  36  is switched on simultaneously with the application of the applied voltage V to the pre-head  10 , so that the ends of the coupling capacitor  26  have substantially the same potential at the same time. The saturation time required for saturation of the coupling capacitor  26  can thus be reduced. It is thus possible to reduce the time from when a voltage is applied to the pre-head  10  to when the voltage signal S 1  from the pre-head  10  is input via the coupling capacitor  26  to the signal processing circuit  24  (the start-up time of a pre-head). 
     Next, the control unit  25  determines whether magnetic information is recorded on the magnetic stripe  2  of the magnetic card  3  based on the number of times that the detection signal S 4  is output from the signal processing circuit  24  (the comparator circuit  32 ) or the like. For example, when the number of times that the detection signal S 4  is output from the signal processing circuit  24  exceeds a reference number of times set in advance, the control unit  25  determines that the magnetic information is recorded on the magnetic stripe  2  of the magnetic card  3 . 
     (Determination Operation of Whether or not the Magnetic Information is Recorded on Magnetic Card) 
     When the detector  9  detects the magnetic card  3  inserted into the card insertion slot  5 , a signal is input from the detector  9  to the control unit  25 . The control unit  25  then outputs a control signal to the first analog switch  35  and the second analog switch  36  to simultaneously switch on these switches. 
     As a result, the applied voltage V is applied to the pre-head  10 . In addition, the pre-head  10  is connected via the coupling capacitor  26  to the signal processing circuit  24  and the two ends of the coupling capacitor  26  have substantially the same potential. 
     After that, when the magnetic card  3  is then fully inserted into the card insertion slot  5  and passes through the detection position C, the pre-head  10  detects a change in magnetic field due to the movement of the magnetic card  3  and outputs the voltage signal S 1 . The voltage signal S 1  from the pre-head  10  is input via the coupling capacitor  26  to the signal processing circuit  24 . 
     The signal processing circuit  24  outputs the detection signal S 4  based on the amplitude of the voltage signal S 1  output from the pre-head  10  (the difference between the voltage signal S 1  and the reference voltage V 0 ). The control unit  25  determines whether magnetic information is recorded on the magnetic card  3  based on the detection signal S 4  output from the signal processing circuit  24 . Herein, when the control unit  25  determines that the magnetic information is recorded on the magnetic card  3 , the conveying motor  14  is driven to convey the magnetic card  3  through the read position A. 
     (Operations and Effects) 
     The pre-head  10  detects a change in magnetic field using the magnetoresistive elements  21 ,  22  in the present embodiment. 
     Consequently, when no voltage is applied to the pre-head  10 , the pre-head  10  does not output the voltage signal S 1  even if the magnetic card  3  passes through the detection position C where the pre-head  10  performs detection. When the power supply for the card reader  1  is not switched on (when no voltage is applied to the pre-head  10 ), it is thus difficult to obtain magnetic information on the magnetic card  3  through the pre-head  10 . 
     In addition, the coupling capacitor  26  is inserted between the pre-head  10  and the signal processing circuit  24  in the present embodiment. The reference potential, which is the reference for the amplitude of the voltage signal S 1  from the pre-head  10 , is thus the reference voltage V 0 . It is thus possible to precisely determine whether magnetic information is recorded on the magnetic stripe  2  of the magnetic card  3  inserted into the card insertion slot  5  based on the voltage signal S 1  from the pre-head  10 . 
     Herein, when the coupling capacitor  26  is inserted between the pre-head  10  and the signal processing circuit  24 , a frequency band component required for a determination operation of whether or not the magnetic information is recorded may be cut off from the voltage signal S 1  input from the pre-head  10  to the signal processing circuit  24  by a filter effect due to insertion of the coupling capacitor  26 . 
     In order to handle such a case, the capacity of the coupling capacitor  26  is set to be relatively large in the present embodiment so that a low-frequency component of the voltage signal S 1  is not cut off. 
     In addition, when the capacity of the coupling capacitor  26  is increased, the time from when a voltage is applied to the pre-head  10  to when a signal from the pre-head  10  is input via the coupling capacitor  26  to the signal processing circuit  24  (a start-up time of the pre-head  10 ) may be extended due to the saturation time of the coupling capacitor  26 . In order to handle such a case, when the detector  9  detects the magnetic card  3 , in the present embodiment, ends of the coupling capacitor  26  are made to have substantially the same potential simultaneously with application of the applied voltage V to the pre-head  10 , so that the saturation time of the coupling capacitor  26  is reduced. 
       FIG. 3  show graphs of a start-up time of the pre-head  10 .  FIG. 3( a )  shows a case of the present embodiment, that is, a case where when the detector  9  detects the magnetic card  3 , the first analog switch  35  and the second analog switch  36  are simultaneously switched on and thus ends of the coupling capacitor  26  have the same potential at the same time.  FIG. 3( b )  shows a case where a control system of the pre-head  10  does not include the second analog switch  36  and the pre-head  10  is directly connected to the signal processing circuit  24 . In the present embodiment shown in  FIG. 3( a ) , a time Δt from a time point t 0  when the detector  9  detects the magnetic card  3  (a time point when a voltage is applied to the pre-head  10 ) to a time point t 1  when a signal from the pre-head  10  is input via the coupling capacitor  26  to the signal processing circuit  24  (the start-up time of the pre-head  10 ) is shorter than that in the case of  FIG. 3( b ) . It is thus possible to quickly perform the determination operation of whether or not the magnetic information is recorded on the magnetic card  3  inserted into the card insertion slot  5 . 
     In addition, the signal processing circuit  24  of the pre-head  50  including the core  51  and the coil  52  (see  FIG. 4 ) can be used in the present embodiment as the control system of the pre-head  10  that performs the determination operation of whether or not the magnetic information is stored in the magnetic card  3 . Changes in the control system of the conventional card reader  1  including the pre-head  50  having the core  51  and the coil  52  can thus be reduced. It is thus possible to reduce risk when the pre-head  50  is replaced by the pre-head  10  that detects a change in magnetic field using the magnetoresistive elements  21 ,  22 . 
     In addition, the pre-head  10  is connected and disconnected to and from circuits on input and output sides of the pre-head  10  using the first analog switch  35  and the second analog switch  36  in the present embodiment. 
     An analog switch has low resistance when connected to a circuit. Consequently, when the first analog switch  35  and the second analog switch  36  are simultaneously switched on, it is possible to prevent the first analog switch  35  and the second analog switch  36  from affecting characteristics of the magnetoresistive elements  21 ,  22 . 
     Other Embodiments 
     The magnetoresistive elements  21 ,  22  may be obtained by, for example, forming a thin film of a nickel-cobalt alloy (Ni—Co) on a surface of a sensor substrate. 
     In addition, instead of the first analog switch  35  and the second analog switch  36 , a switching element such as a relay may be used. 
     Furthermore, in the switching circuit  27 , a signal from the detector  9  may be used as a control signal for switching on the first analog switch  35  and the second analog switch  36 . 
     That is, the switching circuit  27  includes a control unit that outputs a control signal for switching on the first analog switch  35  and the second analog switch  36  based on a signal from the detector  9  in the above embodiment. The switching circuit  27  may use a signal from the detector  9  as a control signal for switching on the first analog switch  35  and the second analog switch  36 . 
     In addition, as shown in  FIGS. 2 and 4 , the signal processing circuit  24  of the present embodiment is the same as the one in a case of using the pre-head  50  including the core  51  and the coil  52 . The pre-head  10  of the present embodiment that detects a change in magnetic field using the magnetoresistive elements  21 ,  22  and the pre-head  50  including the core  51  and the coil  52  can be connected to the signal processing circuit  24 . Two pre-heads  10 ,  50  may be connected to the signal processing circuit  24  for exclusive use. 
     Further, the card reader  1  includes the detector  9  configured to detect that the magnetic card  3  has been inserted into the card insertion slot  5  in the embodiment above described. The card reader  1  may include a human sensor configured to detect movements of a user of the card reader  1  instead of or in addition to the detector  9 . 
     In a case where the card reader  1  includes a human sensor, when the human sensor detects movements of a user, the first analog switch  35  and the second analog switch  36  may be simultaneously switched from a disconnected state to a connected state. Specifically, the human sensor is connected to an input side of the control unit  25 . When a signal is input from the human sensor to the control unit  25 , the control unit  25  outputs a control signal to the first analog switch  35  and the second analog switch  36  to switch on these switches at the same time. Alternatively, the first analog switch  35  and the second analog switch  36  may be switched on at the same time by using a signal from the human sensor as a control signal. 
     In this case, assuming that the side of the card reader  1  on which the card insertion slot  5  is disposed in the insertion direction I of the magnetic card  3  is the front side, the human sensor is disposed near the card insertion slot  5  in a manner that its detection direction is directed to the front side of the card reader  1 . The human sensor is, for example, an infrared sensor. With such a configuration, when a user of the card reader  1  is detected, the voltage applying circuit  23  is connected to the pre-head  10  and an applied voltage is applied to the pre-head  10 . In addition, the capacitor  26  is connected to the signal processing circuit  24  simultaneously with application of the applied voltage to the pre-head  10 , and the potential of the capacitor  26  on the opposite side to the pre-head  10  is the reference voltage V 0 . 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1 : Card reader 
           2 : Magnetic stripe 
           3 : Magnetic card 
           5 : Card insertion slot 
           9 : Detector 
           10 : Pre-head 
           21 : First magnetoresistive element 
           22 : Second magnetoresistive element 
           23 : Voltage applying circuit 
           24 : Signal processing circuit 
           25 : Control unit 
           26 : Coupling capacitor (capacitor) 
           27 : Switching circuit 
           28 : Intermediate point 
           33 : Reference voltage circuit 
           35 : First analog switch (first switching element) 
           36 : Second analog switch (second switching element) 
         V 0 : Reference voltage