Abstract:
A handset microphone includes: a microphone unit, a first switch for turning on or off the output from the microphone unit, and a second switch for enabling or disabling the output from the microphone unit; the first and second switches have contacts operated by a magnetic field; the contact of the first switch is closed and short-circuits the output terminals while the first switch is being affected by the magnetic field; and the contact of the second switch is opened and disconnects one of the output terminals while the second switch is being affected by the magnetic field.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a handset microphone that is free of noise contamination during a switching operation and malfunction due to an external magnetic field. 
       BACKGROUND ART 
       [0002]    Various types of microphones are known that acquire and output sound. One of the types is called a handset microphone, which can transmit speech when a transmission switch is turned on and cannot transmit speech when the transmission switch is turned off (see Japanese Patent No. 3857591). The transmission switch of a handset microphone is generally implemented by, for example, a push switch. The push switch of the handset microphone is continued to be pressed during speech transmission. The push switch is returned to the original state (i.e., not pressed) to stop speech transmission. The contact of the push switch is mechanically opened or closed. 
         [0003]    Mechanical turning on/off operation of the contact of the switch readily generates noise. In order to prevent such noise contamination, mechanical contact may be replaced with a reed switch openable or closable by the movement of a magnet attached to a switch knob. 
         [0004]    Use of a reed switch as an operation switch of a handset microphone can prevent noise caused at a start or stop operation of speech transmission. A reed switch having contact operated by a magnetic field may however malfunction in an unintentional magnetic field from the exterior. 
         [0005]    Handset microphones are often used for radio communication machines and usually transmit radio waves in cooperation with switch operations. In such a radio communication machine, a malfunction of the switch in the handset microphone is caused by an external magnetic field and then leads to undesirable transmission of radio waves. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    It is an object of the present invention to provide a handset microphone that is free of noise contamination during a switching operation and malfunction due to an external magnetic field. 
       Solution to Problem 
       [0007]    According to an aspect of the present invention, a handset microphone includes a microphone unit, a first switch for turning on or off the output from the microphone unit, and a second switch for enabling or disabling the output from the microphone unit; the first and second switches have contacts operated by a magnetic field; the contact of the first switch is closed and short-circuits the output terminals while the first switch is being affected by the magnetic field; and one of the contacts of the second switch is opened and disconnects one of the output terminals while the second switch is being affected by the magnetic field. 
       Advantageous Effects of Invention 
       [0008]    A handset microphone according to an aspect of the present invention is free of noise contamination during a switching operation and malfunction due to an external magnetic field. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a circuit diagram illustrating a handset microphone an embodiment of the present invention. 
           [0010]      FIG. 2A  is a circuit diagram illustrating a preparative state for the handset microphone. 
           [0011]      FIG. 2B  is a circuit diagram illustrating an operational state for the handset microphone. 
           [0012]      FIG. 2C  is a circuit diagram illustrating an operated state of the handset microphone affected by an external magnetic field. 
           [0013]      FIG. 3A  illustrates an example transmission switch of the handset microphone in which the transmission switch is not operated. 
           [0014]      FIG. 3B  illustrates an operated state of the transmission switch. 
           [0015]      FIG. 4A  illustrates an example hook switch of the handset microphone in which the hook switch is not operated. 
           [0016]      FIG. 4B  illustrates an operated state of the hook switch. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]    A handset microphone according to an embodiment of the present invention will now be described with reference to the accompanying drawings.  FIG. 1  is a circuit diagram illustrating a configuration of the handset microphone  100  according to the embodiment. As illustrated in  FIG. 1 , the handset microphone  100  includes a dynamic microphone unit  30 , a transmission switch (a first switch)  10 , and a hook switch (a second switch)  20 . 
         [0018]    The transmission switch  10  is a reed switch having a mechanism for moving position of a magnet in synchronization with the operation of an operational unit (not illustrated) of the handset microphone  100 . During the quiescent state of the operational unit, the contact of the reed switch is magnetized by a magnetic field from the magnet. The contact of the transmission switch  10  is closed by the magnetic field. If no magnetic field is applied from the magnet, the contact of the transmission switch  10  is opened. That is, the transmission switch is a make-contact reed switch. 
         [0019]    As illustrated in  FIG. 1 , the transmission switch  10  is connected in parallel to output terminals of the microphone unit  30 . If the operational unit of the handset microphone  100  is not operated (for example, not pressed), the transmission switch  10  has a magnetic field generated by the magnet positioned near the contact. At this time, the contact of the transmission switch  10  is closed to short-circuit the output terminals of the microphone unit  30 . 
         [0020]    If the operational unit of the handset microphone  100  is operated (for example, pressed), the magnet recedes from the contact. As a result, the contact is not affected by the magnetic field, then the contact is opened. In other words, if the operational unit is operated, speech signals are output from the output terminals of the microphone unit  30  depending on the state of the hook switch  20  described below. 
         [0021]    More specifically, the output of the speech signals from the handset microphone  100  is switched by the transmission switch  10 . 
         [0022]    The hook switch  20  is a transfer contact reed switch and is switched from a normally closed contact to a normally open contact by the magnetic field.  FIG. 1  illustrates the hook switch  20  affected by the magnetic field, i.e., switched to the normally open contact. The magnetic field affecting the hook switch  20  is generated by, for example, a magnet provided in a hook for holding the handset microphone  100  during nonuse. 
         [0023]    The handset microphone  100  includes a housing (not illustrated) accommodating several components, such as the microphone unit  30 , the transmission switch  10 , and the hook switch  20  in the handset microphone  100 . The housing has a hanging member for hanging the housing on a wall. The wall has a prefixed hook or holding member provided with a magnet. When the handset microphone  100  is not used, the hanging member is put on the hook (the holding member). The hook switch  20  is shifted to the normally open contact by the magnetic field from the magnet and maintains this state. In other words, the handset microphone  100  has output terminals opened or closed by the hook switch  20 . 
         [0024]    The hanging member of the housing of the handset microphone  100  is, for example, a hole provided in the housing. The hook (the holding member) is engageable with the hole in the housing to fix the handset microphone  100 . 
         [0025]    The handset microphone  100  can be switched with the hook switch  20  between the outputtable state and the output cutoff state.  FIG. 1  illustrates a quiescent state in which the handset microphone  100  does not transmit speech. At this time, the output terminals of the microphone unit  30  are short-circuited by the transmission switch  10 . The normally closed contact of the hook switch  20  is electrically disconnected from the output terminals of the microphone unit  30 . As a result, no signals are output from the microphone unit  30 . 
         [0026]    A configuration of the transmission switch  10  will now be described in detail.  FIGS. 3A and 3B  illustrate an example reed switch used as the transmission switch  10 .  FIG. 3A  illustrates a quiescent state of the transmission switch  10 .  FIG. 3B  illustrates an operated state of the transmission switch  10 . 
         [0027]    As illustrated in  FIG. 3A , the transmission switch  10  includes two ferromagnetic reeds facing each other at a predetermined distance and enclosed in a glass tube  14 . In the transmission switch  10 , a magnetic field applied from the exterior in the axial direction of the reeds magnetizes a first reed  11  and a second reed  12  to cause their facing free ends to attract and contact each other. This mechanism closes the circuit at a contact portion  13 . If no magnetic field is applied, the contact portion  13  is opened by the resilience of each reed to open the circuit. The glass tube  14  is filled with inert gas  15  to prevent activation of the contact portion  13 . 
         [0028]    In the transmission switch  10 , the magnet is positioned so as to apply a magnetic field to the contact portion  13  when the operational unit is not operated, and so as to apply no magnetic field to the contact portion  13  when the operational unit is operated. 
         [0029]    A configuration of the hook switch  20  will now be described.  FIGS. 4A and 4B  illustrate an example reed switch used as the hook switch  20 .  FIG. 4A  illustrates the hook switch  20  not affected by any magnetic field.  FIG. 4B  illustrates the hook switch  20  affected by a magnetic field. 
         [0030]    The hook switch  20  includes a single reed and two reeds facing the single reed at a predetermined distance, all the reeds being enclosed in a glass tube  24 . Two of the three reeds are ferromagnetic, one is a common (COM) reed  21 , and the other is a normally open (NO) contact reed  23 . The remaining one is a normally closed (NC) contact reed  22  having a nonmagnetic free end. The glass tube  24  is filled with inert gas  25  to prevent activation of the contact portion  26 . 
         [0031]    As illustrated in  FIG. 4A , when no magnetic field is applied to the hook switch  20 , the contact portion  26  at the free end of the COM reed  21  is in contact with the free end of the NC reed  22  due to mechanical biasing force of the COM reed  21 . As illustrated in  FIG. 4B , when an external magnetic field is applied in the axial direction of the reeds of the hook switch  20 , the COM reed  21  and the NO reed  23  are magnetized to mutually attract the facing free ends. The contact portion  26  at the free end of the COM reed  21  comes into contact with the free end of the NO reed  23  to close the circuit. 
         [0032]    The operation of the handset microphone  100  will now be explained.  FIG. 2A  is a circuit diagram illustrating a preparative state for the handset microphone  100 .  FIG. 2B  is a circuit diagram illustrating an operational state for the handset microphone  100 .  FIG. 2C  is a circuit diagram illustrating the operated handset microphone  100  affected by an external magnetic field. 
         [0033]    The handset microphone  100  described in  FIG. 1  is held on the hook (not illustrated). The COM reed  21  of the hook switch  20  is in contact with the NO reed  23  due to attractive force by the magnet in the hook. This state changes to an operational state illustrated in  FIG. 2A  if the handset microphone  100  is lifted from the hook. In other words, the COM reed  21  of the hook switch  20  comes into contact with the NC reed  22 . The output terminals of the microphone unit  30  are thereby connected to the output terminals of the handset microphone  100  through the hook switch  20 . The output terminals of the microphone unit  30  however are short-circuited through the transmission switch  10  to disable the output signals from the microphone unit  30 . 
         [0034]    The transmission switch  10  is then operated and changed to a speech transmission state in  FIG. 213 . More specifically, the operational unit (not illustrated) is pressed to repel a magnetic field applied to the transmission switch  10 , and the contact is then opened between the first and second reeds  11  and  12 . This enables signals to be output from the microphone unit  30  through the hook switch  20 . 
         [0035]    The state illustrated in  FIG. 2B  changes to the state illustrated in  FIG. 2C  if an unexpected external magnetic field is applied to the transmission switch  10  and the hook switch  20 . If an external magnetic field is applied to the transmission switch  10 , the first reed  11  comes into contact with the second reed  12  to short-circuit the output terminals of the microphone unit  30  as illustrated in  FIG. 2C . This disables the output from the microphone unit  30 . If an external magnetic field is applied to the hook switch  20 , the contact destination of the COM reed  21  is switched from the NC reed  22  to the NO reed  23 . This disables the output from the microphone unit  30 . 
         [0036]      FIG. 2C  illustrates both the transmission switch  10  and the hook switch  20  affected by an external magnetic field. In the present invention, if an external magnetic field is applied to either the transmission switch  10  or the hook switch  20 , the output from the microphone unit  30  is interrupted. 
         [0037]    The handset microphone  100  described above includes the hook switch  20  and the transmission switch  10 , which are implemented by the reed switches. This configuration is free of noise contamination during a switching operation and malfunction due to an external magnetic field.