Abstract:
A keyboard has a base plate, a membrane sheet, a housing, a key-top, a link mechanism disposed between the housing and the key-top, and a switch operating member disposed under the key-top and operated by the key-top. The housing, the membrane sheet and the base plate have slits at a position corresponding to a surrounding portion of the key-top. Consequently, the distance between the key-top and the housing can be decreased even if the skirt part of the key-top is long. Furthermore, the slit of the housing is formed as a discontinuous annular shape to surround the link mechanism, and the surrounding portion of the key-top has a slit corresponding to the bridge shaped portion of the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application and is based upon PCT/JP03/07922, filed on Jun. 23, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a keyboard. 
     2. Description of the Related Art 
     There is well-known a keyboard comprising a base plate, a membrane sheet disposed on the base plate, a housing disposed on the membrane sheet, a key-top, a link mechanism disposed between the housing and the key-top, and a switching operation unit (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-213806 and No. 2001-229764). 
     The key-top has a horizontal top portion, and a skirt portion obliquely hanging down from the horizontal top portion. A pivotal support portion of the link mechanism is disposed on an internal surface of the top portion of the key-top and an upper surface of the housing, and the link mechanism guides the key-top to drop down, keeping the horizontal state thereof, when the key-top is pushed. In the case of pushing the key-top, the key-top pushes down the switching operation unit made of a rubber and, thereby, the switching operation unit makes contact a pair of opposite connecting pieces of the membrane sheet. 
     In accordance with the recent requirements, a distance between the key-top and the housing is required to be short so as to realize a thin keyboard. However, when the key-top is pushed and the skirt portion of the key-top is long, a lower end of the skirt portion of the key-top (surrounding portion of the key-top) interferes with the housing. Therefore, the distance between the key-top and the housing cannot be short. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a keyboard which can be made thin. 
     A keyboard in accordance with the present invention comprises a base plate; a membrane sheet disposed on the base plate; a housing disposed on the membrane sheet; a key-top; a link mechanism disposed between the housing and the key-top; and a switching operation unit disposed under the key-top and moved by the key-top, wherein the housing has a slit placed at a surrounding portion of the key-top. 
     In this configuration, the surrounding portion of the key-top (lower end portion of skirt portion of the key-top) can be inserted into the slit of the housing. Therefore, the distance between the key-top and the housing can be short, and a thin keyboard can be realized, when the skirt portion of the key-top is long. 
     Preferably, the slit of the housing may be formed as a discontinuous annular shape and has first and second ends, a bridge shaped portion of the housing placed between the first end and the second end may make contact with a part of the housing placed inside of the slit to a part of the housing placed outside of the slit, and the surrounding portion of the key-top may have a slit at a position corresponding to the portion of the housing placed between the first end and the second end. 
     Preferably, the membrane sheet may have a slit corresponding to the slit of the housing. Further, the base plate may have a slit corresponding to the slit of the housing and the slit of the membrane sheet. In addition, when the key-top is pushed, a bottom surface of the surrounding portion of the key-top may become approximately the same as a bottom surface of the base plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing an embodiment of a keyboard according to the present invention. 
         FIG. 2  is a sectional view showing a state of the keyboard of  FIG. 1  when a key-top is pushed. 
         FIG. 3  is a perspective view showing the keyboard of  FIG. 1  where the key-top is removed. 
         FIG. 4  is a perspective view showing the key-top of  FIG. 1 . 
         FIG. 5  is a perspective view showing a first link of a link mechanism. 
         FIG. 6  is a perspective view showing a second link of the link mechanism. 
         FIG. 7  is a partial sectional view showing a membrane sheet. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Below, an embodiment of a keyboard according to the present invention will be described with reference to the accompanying drawings. 
     A keyboard  10  comprises a base plate  12 , a membrane sheet  14  disposed on the base plate  12 , a housing  16  disposed on the membrane sheet  14 , a key-top  18 , a link mechanism  20  disposed between the housing  16  and the key-top  18  and a switching operation unit  22  disposed between the key-top  18  and the membrane sheet  14 . 
     The base plate  12  is made of metal, and is disposed on a housing of an electronic device such as a computer (not shown). As shown in  FIG. 7 , the membrane sheet  14  comprises a pair of insulation sheets  14   a  and  14   b , a pair of connecting pieces  14   c  and  14   d  disposed on an inner surface of the insulation sheets  14   a  and  14   b . The connecting pieces  14   c  and  14   d  are normally held at a specific distance by a spacer  14   e , and when the switching operation unit  22  drops down, the connecting pieces  14   c  and  14   d  are contacted each other by pushing. Further, the membrane sheet  14  has wiring patterns connected to the connecting pieces  14   c  and  14   d.    
     The switching operation unit  22  is a rubber dome made of an elastic material, e.g., a rubber, and has a downward pushing portion  22   a  disposed at the center of the switching operation unit. 
     The housing  16  is a metal or resin plate, and supports a pivotal support portion of a link mechanism  20 , which will be explained later. The housing  16  has an opening portion  16   a  to expose portions of the connecting pieces  14   c  and  14   d  of the membrane sheet  14 , and the pushing portion  22   a  of the switching operation unit  22  is disposed over the opening portion  16   a  of the housing  16  and the connecting pieces  14   c  and  14   d  of the membrane sheet  14 . 
     As shown in  FIGS. 3 ,  5  and  6 , the link mechanism  20  includes a first link  24  and a second link  26 . The first link  24  is a square frame shaped unit having a pair of parallel arms  24   a ,  24   b , shaft portions  24   c ,  24   d  disposed on ends of the arms  24   a ,  24   b , and pivot portions  24   e ,  24   f  disposed on outer sides of the centers of the arms  24   a ,  24   b . The second link  26  is an H-shaped unit larger than the first link  24  and disposed around the first link  24 . The second link  26  has a pair of parallel arms  26   a ,  26   b , shaft portions  26   c ,  26   d  disposed on ends of the arms  26   a ,  26   b , and holes  26   e  formed in inner sides of the centers of the arms  26   a ,  26   b . The pivot portions  24   e ,  24   f  of the centers of the first link  24  are inserted into the holes  26   e  of the centers of the second link  26 , and the first link  24  and the second link  26  are rotatably connected each other. The first link  24  is positioned within the second link  26  and can be rotated without causing interference therebetween. 
     As shown in  FIGS. 1 to 4 , the key-top  18  has a flat top portion  18   a , and a skirt portion  18   b  obliquely hanging down from the top portion  18   a . A lower end of the skirt portion  18   b  constitutes a “surrounding portion” of the key-top  18 . 
     Bearings  20   a  and  20   b  are disposed on an inner surface of the top portion  18   a  of the key-top  18  as a pivotal support portion of the link mechanism  20 . The shaft portion  24   c  placed on an end of the first link  24  is rotatably supported by the bearing  20   a . The shaft portions  26   c  disposed on corresponding ends of respective arms  2   a  and  26   b  of the second link  26  are rotatably and movably supported in a horizontal orientation by the bearing  20   a . On the other hand, as the pivotal support portion of the link mechanism  20 , bearings  20   c  and  20   d  are disposed on an upper surface of the housing  16 . The shaft portion  26   d  disposed on an end of the second link  26  is rotatably supported by the bearing  20   c . The shaft portions  24   d  disposed on an end of the first link  24  are rotatably and movably supported in a horizontal orientation by the bearing  20   d.    
     As described above, when the key-top  18  is pushed, the height of the link mechanism  20  is reduced so as to guide the key-top  18  downwardly in the horizontal orientation. When the key-top  18  is pushed, the key-top  18  pushes down the switching operation unit  22 , and the downward pushing portion  22   a  of the switching operation unit  22  presses and contacts the pair of opposite connecting pieces  14   c  and  14   d . When the key-top  18  is released, the switching operation unit  22  and the key-top return back to an initial position, by the elastic force of the switching operation unit  22 . 
     Further, the housing  16  has a slit  28  placed at a surrounding portion of the key-top  18 . As shown in  FIG. 2 , when the key-top  18  is pushed, the surrounding portion of the key-top  18  (lower end of the skirt portion  18   b  of the key-top  18 ) can be inserted into the slit  28  of the housing  16  without causing interference with the housing  16 . Therefore, even if the skirt portion  18   b  of the key-top  18  is long, the distance between the key-top  18  and the housing  16  is shorter, so that a thin keyboard  10  can be realized. 
     Further, the membrane sheet  14  has a slit  30  at a position corresponding to the slit  28  of the housing  16 . Further, the base plate  12  has a slit  32  corresponding to the slit  28  of the housing  16  and the slit  30  of the membrane sheet  14 . In the embodiment, the slit  28  of the housing  16 , the slit  30  of the membrane sheet  14 , and the slit  32  of the base plate  12  are formed as the same position and the same shape each other. Consequently, the key-top  18  can be pushed further down, and the distance between the key-top  18  and the housing  16  can be further shortened, so that a much thinner keyboard can be realized. Preferably, a bottom surface of the surrounding portion of the key-top  18  becomes approximately the same as a bottom surface of the base plate  12  when the key-top  18  is pushed. 
     As shown in  FIG. 3 , the slit  28  of the housing  16  is formed as a discontinuous annular shape surrounding the link mechanism  20 . The slit  28  is formed as a square frame shaped, and the slit  28  is continuously formed except for one side of the square frame. The slit  28  has first and second end portions  34  and  36 . 
     A bridge shaped portion  38  placed between the first end portion  34  and the second end portion  36  of the slit  28  connects an inner portion  40  of the housing  16  to an outer portion  42  of the housing  16 , the inner portion  40  being placed internally of the slit  28 , and the outer portion  42  being placed externally of the slit  28 . Similarly, the membrane sheet  14  and the slit  30  thereof, and the base plate  12  and the slit  30  thereof are formed as the same as that of the above description. 
     Consequently, the inner portion  40  of the housing  16  is formed as a land shaped portion placed inside of the slit  28 , but the inner portion  40  is connected to the outer portion  42  of the housing  16  placed at the outside of the slit  28  by the bridge shaped portion  38  placed between the first end portion  34  and the second end portion  36  of the slit  28 , so that a special connecting unit to connect the inner portion  40  to the outer portion  42  is not necessary. Similarly, in the membrane sheet  14 , an inner portion of the slit  30  is connected to an outer portion of the slit  30 , so that wiring patterns can be formed on the bridge shaped portion placed between the first end portion  34  and the second end portion  36  of the slit  28 . 
     As shown in  FIGS. 1 and 4 , the surrounding portion of the key-top  18  has a slit  44  corresponding to the bridge shaped portion  38  placed between the first end portion  34  and the second end portion  36  of the slit  28 . Consequently, the surrounding portion of the key-top  18  can be inserted into the slits  28 ,  39  and  32 , and the bridge shaped portion  38  of the housing  16  can be inserted into the slit  44  of the surrounding portion of the key-top  18 , so that the key-top  18  can drop down to the lowest position in the horizontal state.