Patent Document

TECHNICAL FIELD 
     The present invention relates to a key switch structure of a keyboard that is used as an input device in an information processing device, a measurement device, a medical device, and the like, and particularly, to a key switch structure having an illuminating function. 
     BACKGROUND ART 
     Recently, a keyboard having an illuminating function has been disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-260478 (refer to  FIG. 7 ) and the like. In the key structure disclosed in this Patent Document, an elastic member and a linking mechanism are disposed below a key top, and a membrane circuit board, a light guiding plate, and a base plate are disposed therebelow. In addition to the configuration in which a light emitting diode (LED) as a light emitting element is provided on the membrane circuit board, there is also disclosed a configuration in which an LED is provided on an exclusive circuit board. When the exclusive circuit board is provided, the circuit board is disposed below the base plate. 
     In the key switch structure disclosed in this Patent Document, when light is emitted from the LED as the light emitting element, the light is guided by the light guiding plate, and is emitted upward from the light guiding plate at a position below the key top. Accordingly, a character, a symbol, and the like on the key top are brightly illuminated. 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the key switch structure disclosed in this Patent Document, the plate and the light guiding plate are formed as separate members. Accordingly, the height of the entire key switch is high, which is an obstacle in attaining a thin key switch. Further, when the height of the key switch is high, it is difficult to mount the key switch on a personal computer (PC) or to replace a key switch without an illuminating function with a key switch with an illuminating function. Further, since many components are used, manufacturing costs become high. Furthermore, since a process of bonding the light guiding plate and the base plate to each other is required in the manufacturing process, manufacturing costs become high also due to this reason. 
     Therefore, the invention provides a key switch structure in which the height of the key switch is reduced and the number of components is reduced, so that the manufacturing costs are lowered. 
     Solution to Problem 
     According to an aspect of the invention, there is provided a key switch structure including: a key top; a linking mechanism adapted to support and allow elevation of the key top; a membrane sheet having a contact point portion; a back plate disposed below the membrane sheet, supporting the linking mechanism, and having a light guiding function; and a sheet member having a light emitting element disposed thereon, wherein the light emitting element is disposed to be adjacent to the back plate. 
     Advantageous Effects of Invention 
     According to the invention, since the back plate is endowed with a function to guide light, the height of the key switch may be reduced, the number of components may be reduced, and the manufacturing costs may be lowered. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view illustrating a key switch structure of a first embodiment. 
         FIG. 2  is a cross-sectional view illustrating the key switch structure of the first embodiment. 
         FIG. 3  is an enlarged view illustrating a welded state of a welding pin of the first embodiment. 
         FIG. 4  is an enlarged view specifically illustrating an LED housing of the first embodiment. 
         FIG. 5  is an exploded perspective view illustrating a key switch structure of a second embodiment. 
         FIG. 6  is a cross-sectional view illustrating the key switch structure of the second embodiment. 
         FIG. 7  is an enlarged view specifically illustrating an LED housing of the second embodiment. 
         FIG. 8  is an enlarged view specifically illustrating the LED housing of the second embodiment. 
         FIG. 9  is an exploded perspective view illustrating a key switch structure of a third embodiment. 
         FIG. 10  is a cross-sectional view illustrating the key switch structure of the third embodiment. 
         FIG. 11  is en exploded perspective view illustrating a key switch structure of a fourth embodiment. 
         FIG. 12  is a cross-sectional view illustrating the key switch structure of the fourth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described by referring to the drawings. The same reference numerals are given to the same components in the drawings.  FIG. 1  is an exploded perspective view illustrating a key switch structure of a first embodiment.  FIG. 2  is a cross-sectional view illustrating the key switch structure of the first embodiment. 
     In  FIGS. 1 and 2 , a key switch  10  of a first embodiment includes a key top  11 , a first linking member  12  which is adapted to be slidable with respect to the key top  11 , a second linking member  13  which is adapted to be rotatable with respect to the key top  11 , a rubber dome (elastic member)  14  which is bent when the key top  11  is pressed down and returns the key top  11  to the original position when the pressing force is released, a holder  15  which holds the first and second linking members  12  and  13 , a membrane sheet  16  which has a contact point portion directly beneath the rubber dome  14 , a back plate  17  which supports the holder  15  and has a light guiding function, and an LED sheet  18  on which a light emitting diode (LED) is disposed. 
     A character or symbol  11   c  is formed on the surface (front surface) of the key top  11 , where the entire key top is painted, and a character portion is formed thereon by laser or the like. The rear surface of the key top  11  is provided with a sliding support portion  11   a  which slidably supports, in the horizontal direction, a sliding pin  12   a  of the first linking member  12 , and a rotation support portion lib which rotatably supports a rotation pin  13   a  of the second linking member  13 . The first linking member  12  is formed in a substantially frame shape, and includes a pin  12   c  which is inserted into a pivot hole  13   c  (in  FIG. 1 , only one of the pivot holes is shown, but the pivot holes  13   c  are formed at both sides) of the second linking member  13  and a pin  12   b  which is rotatably supported by a pivot portion  15   b  of the holder  15  in addition to the sliding pin  12   a.    
     The second linking member  13  includes a pivot hole  13   c  into which the pin  12   c  of the first linking member  12  is inserted and a pin  13   b  which is slidably guided by a sliding support portion  15   c  of the holder  15  in addition to the rotation pin  13   a  that is rotatably held by the rotation support portion  11   b  of the key top  11 . The rubber dome  14  is disposed below the key top  11 , and is formed of a material such as rubber and in a substantially cup shape, where a contact point pressing portion  14   a  is formed protruding downward from the center of the inner surface of the rubber dome, so that a contact point portion  16   b  of the membrane sheet  16  may be pressed down by the contact point pressing portion. A linking mechanism includes the first linking member  12  and the second linking member  13 . 
     The holder  15  is separately formed in a frame shape for each key. The holder  15  includes a pivot portion  15   b  which rotatably supports the pin  12   b  of the first linking member  12 , a sliding support portion  15   c  which slidably supports the pin  13   b  of the second linking member  13 , and plural welding pins  15   a  which are fitted into holes  17   a  formed in the back plate  17  having a light guiding function. A flange portion  15   d  is formed at the upper portion (base portion) of each welding pin  15   a  as shown in  FIG. 2 . The diameter of the flange portion  15   d  is set to be slightly smaller than the diameter of the hole  16   a  of the membrane sheet  16 . Further, the thickness of the flange portion  15   d  is set to be slightly thicker than the thickness of the membrane sheet  16 . When the key switch is assembled, the welding pin  15   a  is fitted into the hole  17   a  of the back plate  17 , and the tip portion of the welding pin  15   a  is welded as shown in  FIG. 3  to be deformed in a plane shape, so that the pin is fixed to the periphery of the lower portion of the hole  17   a . Accordingly, the holder  15  is strongly fixed to the back plate  17  while the welding pin  15   a  does not protrude downward. Further,  FIG. 3  is an enlarged view illustrating a welded state of the welding pin of the first embodiment. 
     The first linking member  12 , the second linking member  13 , and the holder  15  are formed of a transparent or translucent material. The membrane sheet  16  includes the contact point portion  16   b  which is pressed down by the contact point pressing portion  14   a  of the rubber dome  14  and a hole  16   a  into which the flange portion  15   d  of the holder  15  is inserted. While not shown in the drawings, the membrane sheet  16  includes upper and lower flexible sheets and a spacer sheet interposed therebetween and the spacer sheet is provided with plural through-holes corresponding to the plural keys. The through-hole forms a space between the upper and lower sheets, a fixed contact point is provided at the lower sheet on the side of the back plate  17 , a movable contact point is provided at the upper sheet on the side of the rubber dome  14 , so that these contact points face one another inside the space. The fixed contact point and the movable contact point constitute the contact point portion  16   b.    
     The back plate  17  having a light guiding function is formed of a resin having transparency or high transmissivity, and light is transmitted through the back plate while being refracted therein. In the back plate  17 , the hole  17   a  is perforated at plural positions corresponding to the holes  16   a  of the membrane sheet  16  so that the welding pin  15   a  of the holder  15  is fitted thereinto. Further, a through-hole  17   c  is provided at a position corresponding to the position where an LED  18   b  is disposed. The hole  17   a  is formed to have a diameter smaller than the diameter of the hole  16   a  of the membrane sheet  16 . A shielding (reflecting) seal  19  is attached to the upper portion of the through-hole  17   c  so that the light emitted from the LED  18   b  does not leak. 
     The shielding seal  19  has luminance higher than that of the LED  18   b , and is disposed at a position where the light of the LED  18   b  is transmitted from the back plate  17  to the membrane sheet  16 . The back plate  17  is provided with a reflecting portion  17   b . The reflecting portion  17   b  is provided at a position corresponding to the character or symbol  11   c  of the key top  11 . Accordingly, plural reflecting portions  17   b  may be provided for each key switch. The reflecting portion  17   b  may be printed in shapes of dots or shaped by forming convex or concave shapes. 
     In the LED sheet  18 , a welding hole  18   a  is provided at a position corresponding to the welding pin  15   a  of the holder  15 , and the LED  18   b  is disposed at a position corresponding to the through-hole  17   c  of the back plate  17 . The welding hole  18   a  is formed to be sufficiently larger than the hole  17   a  of the back plate  17 . The LED  18   b  may be disposed at an arbitrary position on the LED sheet  18 . The front surface of the LED sheet  18  and the rear surface of the back plate  17  are adhered to each other by adhesive or the like. 
       FIG. 4  is an enlarged view specifically illustrating an LED housing of the first embodiment, where the membrane sheet  16 , the back plate  17 , the LED sheet  18 , and the LED  18   b  are shown. In  FIG. 4 , the LED sheet  18  includes a lower LED sheet portion  18   c  and an upper reflecting sheet portion  18   d . The front surface of the lower LED sheet portion  18   c  and the rear surface of the upper reflecting sheet portion  18   d  are adhered to each other. The upper reflecting sheet portion  18   d  is provided with a hole  18   e  corresponding to the LED  18   b . The LED  18   b  is disposed on the lower LED sheet portion  18   c , and a pattern is printed on the sheet. The LED  18   b  is bonded. The lower LED sheet portion  18   c  and the upper reflecting sheet portion  18   d  are adhered to each other with water-resistant starch or the like so as to have a water-resistant function. Further, the upper reflecting sheet portion  18   d  may have a reflecting function by printing a reflective material on the front surface or the rear surface of the transparent PET (polyethylene terephthalate) sheet. The LED  18   b  may be mounted without forming a hole. 
     Next, the welding operation of the welding pin will be described by referring to  FIG. 3 .  FIG. 3  illustrates a state where the LED sheet  18 , the back plate  17 , and the membrane sheet  16  are laminated, and the holder  15  is disposed on the laminated structure. At this time, as depicted by the solid line, the welding pin  15   a  of the holder  15  protrudes downward through the hole  17   a  of the back plate  17  and the welding hole  18   a  of the LED sheet  18 . Further, since the diameter of the flange portion  15   d  is larger than that of the hole  17   a  of the back plate  17 , the flange portion is disposed on the front surface of the back plate  17 . Since the diameter of the flange portion  15   d  is smaller than that of the hole  16   a  of the membrane sheet  16 , a minute gap is formed between the flange portion  15   d  and the membrane sheet  16 . The membrane sheet  16  may be thermally expanded depending on a temperature, and the thermal expansion may be handled by using the gap. 
     Here, a tip portion  15   aa  of the welding pin  15   a  is heated, the tip portion  15   aa  is deformed in a plane shape as depicted by the two-dotted chain line, and is fixed to the lower outer peripheral portion of the hole  17   a  of the back plate  17 . Accordingly, the holder  15  is fixed to the back plate  17 . The welded tip portion  15   aa  of the welding pin  15   a  does not protrude downward beyond the LED sheet  18 . 
     Next, the switch operation will be described. In  FIG. 2 , when the key top  11  is pressed down by an arbitrary load from the upside, the key top  11  moves downward, so that the rubber dome  14  is bent. Then, the contact point pressing portion  14   a  of the rubber dome  14  presses down a contact point portion (not shown) of the membrane sheet  16 . Accordingly, the switch is closed. Further, due to the action of the first linking member  12  and the second linking member  13 , the key top  11  moves downward in a horizontal posture so that the switch is closed even when any part on the upper portion of the key top  11  is pressed down. 
     Next, the illuminating operation will be described. In  FIG. 2 , when the LED  18   b  is turned on by a power supply (not shown), the light emitted from the LED  18   b  passes through the inside of the back plate  17  having a light guiding function. The upper portion of the LED  18   b  is provided with the shielding seal  19 . For this reason, the light emitted from the LED  18   b  is reflected by the shielding seal  19  without leaking upward, and passes through the inside of the back plate  17  as depicted by the arrow. The light is reflected by the reflecting portion  17   b  provided at a position corresponding to the character or symbol  11   c  of the key top  11 , and the reflected light is emitted toward the key top  11  through the membrane sheet  16 . 
     The membrane sheet  16  is formed of an optically-transparent material. The light is transmitted through the membrane sheet  16 , and is transmitted through the holder  15  and the rubber dome  14  so that it arrives at the rear surface of the key top  11 . The character or symbol  11   c  of the key top  11  is illuminated from its rear surface, so that its front surface is visibly lit. 
     As described above, in the first embodiment, one sheet, that is, the back plate  17  having a light guiding function is provided. Therefore, compared to the related art in which two sheets, that is, the light guiding plate and the back plate are provided, the height of the entire key switch is reduced. Further, since the base portion (which includes the membrane sheet  16 , the back plate  17 , and the LED sheet  18 ) is thinned, it is easy to mount the key switch on the PC or replace the key switch without an illuminating function with the key switch with an illuminating function. 
     Further, since the number of components is reduced, manufacturing costs may be reduced. Further, since the light guide function and the function of the back plate are realized by a single member, the keyboard may be manufactured by the same process as that of the keyboard without an illuminating function of the related art. Furthermore, since the LED sheet  18  is provided, the height of the entire key switch may be further reduced. Further, since the LED sheet  18  is provided, a water-resistant function may be obtained. 
     Next, a second embodiment of the invention will be described.  FIG. 5  is an exploded perspective view illustrating a key switch structure of a second embodiment.  FIG. 6  is a cross-sectional view illustrating the key switch structure of the second embodiment. In  FIGS. 5 and 6 , a key switch  20  of the second embodiment includes a key top  21 , a first linking member  22  which is adapted to be slidable with respect to the key top  21 , a second linking member  23  which is adapted to be rotatable with respect to the key top  21 , a rubber dome  24  which is bent when the key top  21  is pressed down and returns the key top  21  to the original position when the pressing force is released, a holder  25  which holds the first and second linking members  22  and  23 , an LED sheet  28  on which an LED is disposed, a membrane sheet  26  which has a contact point portion directly beneath the rubber dome  24 , a back plate  27 , and a reflecting sheet  29 . 
     Among the constituents, the key top  21 , the first linking member  22 , the second linking member  23 , the rubber dome  24 , the holder  25 , and the membrane sheet  26  have the same configurations as those of the first embodiment. Accordingly, the detailed description thereof will be omitted. 
     In the LED sheet  28 , a hole  28   a  is provided at a position corresponding to a welding pin  25   a  of the holder  25 . The diameter of the hole  28   a  is set to be larger than that of a flange portion  25   d  of the welding pin  25   a . Further, an LED  28   b  is disposed at a position corresponding to the through-hole  27   c  of the back plate  27 . The LED  28   b  may be disposed at an arbitrary position on the LED sheet  28 . 
       FIGS. 7 and 8  are enlarged views specifically illustrating the LED housing of the second embodiment. In  FIG. 7 , the LED sheet  28  includes an upper LED sheet portion  28   c  and a lower reflecting sheet portion  28   d . The rear side of the upper LED sheet portion  28   c  and the front side of the lower reflecting sheet portion  28   d  are adhered to each other. In the lower reflecting sheet portion  28   d , a hole  28   e  is provided at a position corresponding to the LED  28   b . The LED  28   b  is disposed downward in the upper LED sheet portion  28   c , and a pattern is printed on the sheet. The LED  28   b  is bonded. The upper LED sheet portion  28   c  and the lower reflecting sheet portion  28   d  are adhered to each other with water-resistant starch or the like so as to have a water-resistant function. Further, the upper reflecting sheet portion  28   d  may have a reflecting function by printing a reflective material on the front surface or the rear surface of the transparent PET (polyethylene terephthalate) sheet. 
     However, the lower reflecting sheet portion  28   d  may not necessarily have the reflecting function. In this case, the membrane sheet  26  is allowed to have the reflecting function. Then, when the LED sheet  28  is not formed to have the reflecting function, the LED sheet  28  includes only the upper LED sheet portion  28   c  as shown in  FIG. 8 . That is, the upper LED sheet portion  28   c  is the LED sheet  28 . The LED sheet  28  is disposed above the membrane sheet  26 . When both of the upper LED sheet portion  28   c  and the lower reflecting sheet portion  28   d  are disposed, the upper LED sheet portion  28   c  and the lower reflecting sheet portion  28   d  are adhered to each other by water-resistant starch or the like so as to have a water-resistant function. When the water-resistant function is not required, only the upper LED sheet portion  28   c  is provided. 
     The membrane sheet  26  includes a contact point portion  26   b  which is pressed down by a contact point pressing portion  24   a  of the rubber dome  24 . Further, the membrane sheet  26  is provided with a hole  26   a  into which the flange portion  25   d  (shown in  FIG. 6 ) of the holder  25  is inserted and a hole  26   c  into which the LED  28   b  is inserted. The back plate  27  having a light guiding function like the first embodiment is formed of a resin having transparency or high transmissivity, and light is transmitted through the back plate while being refracted therein. In the back plate  27 , the hole  27   a  is perforated at plural positions corresponding to the holes  26   a  of the membrane sheet  26  so that the welding pin  25   a  of the holder  25  is fitted thereinto. Further, a through-hole  27   c  is provided at a position corresponding to the position where an LED  28   b  is disposed. The hole  27   a  is formed to have a diameter smaller than the diameter of the hole  26   a  of the membrane sheet  26 . Further, a shielding seal used for blocking the through-hole  27   c  is not provided below the through-hole  27   c . The back plate  27  is provided with a reflecting portion  27   b.    
     In the reflecting sheet  29 , a hole  29   a  is provided at a position corresponding to the welding pin  25   a  of the holder  25 . The reflecting sheet  29  is adhered to the rear side of the back plate  27  by adhesive or the like. A hole is not provided at a position corresponding to the through-hole  27   c  in the reflecting sheet  29 . Through the reflecting sheet  29 , the light emitted from the LED  28   b  is reflected toward the back plate  27 . 
     Next, the welding operation of the welding pin will be described. In  FIG. 6 , the reflecting sheet  29 , the back plate  27 , the membrane sheet  26 , and the LED sheet  28  are laminated, and the holder  25  is disposed on the laminated structure. The welding pin  25   a  of the holder  25  protrudes downward through the hole  27   a  of the back plate  27  and hole  29   a  of the reflecting sheet  29 . Further, the flange portion  25   d  is disposed on the front surface of the back plate  27 . The diameter of the flange portion  25   d  is smaller than the diameter of the hole  26   a  of the membrane sheet  26  and the diameter of the hole  28   a  of the LED sheet  28 . Accordingly, a minute gap is formed between the flange portion  25   d  and the pair of the membrane sheet  26  and the LED sheet  28 . 
     Here, when the tip portion of the welding pin  25   a  is heated, the tip portion is deformed, and is fixed to the lower outer peripheral portion of the hole  27   a  of the back plate  27 . Accordingly, the holder  25  is fixed to the back plate  27 . The welded tip end portion of the welding pin  25   a  does not protrude downward beyond the reflecting sheet  29 . 
     Next, the illuminating operation will be described. In  FIG. 6 , when the LED  28   b  is turned on by a power supply (not shown), the light emitted from the LED  28   b  passes through the inside of the back plate  27  having a light guiding function. The reflecting sheet  29  is provided below the LED  28   b . Accordingly, the light emitted from the LED  28   b  is reflected by the reflecting sheet  29  without leaking downward, and transmitted inside the back plate  27 . Further, the membrane sheet  26  or the LED sheet  28  above the back plate  27  is allowed to have a reflecting function. Accordingly, the light emitted from the LED  28   b  transmitted inside the back plate  27  without leaking upward. The light is reflected by the reflecting portion  27   b  provided at a position corresponding to the character or symbol of the key top  21 . The reflected light is emitted toward the key top  21  through the membrane sheet  26 . 
     Hereinafter, as in the first embodiment, the light is transmitted through the membrane sheet  26 , and is transmitted through the holder  25  and the rubber dome  24 , so that the light arrives at the rear surface of the key top  21 . The character or symbol of the key top  21  is illuminated from its rear surface, so that its front surface is visibly lit. 
     As described above, in the second embodiment, one sheet, that is, the back plate  17  having a light guiding function is provided as in the first embodiment. Therefore, compared to the related art in which two sheets, that is, the light guiding plate and the back plate are provided, the height of the entire key switch is reduced, and the base portion is thinned. Accordingly, it is easy to mount the key switch on the PC or replace the key switch without an illuminating function with the key switch with an illuminating function. 
     Further, since the number of components is reduced, manufacturing costs may be reduced. Further, since the light guide function and the function of the back plate are realized by one member, the keyboard may be manufactured by the same process as that of the keyboard without an illuminating function of the related art. Further, in the second embodiment, since the LED  28   b  is formed to protrude downward, the front surface of the keyboard may be less influenced by the brightness of the LED  28   b . Furthermore, it is not necessary to attach the shielding seal used for blocking the through-hole  27   c  of the back plate  27  on which the LED  28   b  is disposed. 
     Next, a third embodiment of the invention will be described.  FIG. 9  is an exploded perspective view illustrating a key switch structure of the third embodiment.  FIG. 10  is a cross-sectional view illustrating the key switch structure of the third embodiment. In  FIGS. 9 and 10 , a key switch  40  of the third embodiment includes a key top  41 , a first linking member  42  which is adapted to be slidable with respect to the key top  41 , a second linking member  43  which is adapted to be rotatable with respect to the key top  41 , a rubber dome  44  which is bent when the key top  41  is pressed down and returns the key top  41  to the original position when the pressing force is released, a holder  45  which holds the first and second linking members  42  and  43 , a membrane sheet  46  which has a contact point portion directly beneath the rubber dome  44 , a back plate  47 , and an LED sheet  48 . 
     Among the constituents, the key top  41 , the first linking member  42 , the second linking member  43 , the rubber dome  44 , the holder  45 , and the membrane sheet  46  have the same configurations as those of the first embodiment. Accordingly, the detailed description thereof will be omitted. Further, a flange portion is not formed at a welding pin  45   a  of the holder  45 . 
     In the back plate  47  having a light guiding function, a hole  47   a  is perforated in plural positions corresponding to holes  46   a  of the membrane sheet  46  so that the welding pin  45   a  of the holder  45  is fitted thereinto, and a through-hole  47   c  is provided at a position corresponding to the disposition position of the LED  48   b . Further, the back plate  47  is provided with a striking-out portion  47   d . The hole  47   a  is formed at the striking-out portion  47   d . The diameter of the hole  47   a  is smaller than that of the hole  46   a  of the membrane sheet  46 . A shielding (reflecting) seal  49  is attached to the upper portion of the through-hole  47   c  so that the light emitted from the LED  48   b  does not leak therefrom. 
     The shielding seal  49  has brightness stronger than that of the LED  48   b , and is disposed at a position where the light of the LED  48   b  is transmitted from the back plate  47  to the membrane sheet  46 . The back plate  47  is provided with a reflecting portion  47   b . The reflecting portion  47   b  is provided at a position corresponding to the character or symbol of the key top  41 . The reflecting portion  47   b  may be printed as a dot shape. 
     The height of the striking-out portion  47   d  of the back plate  47  is slightly higher than the thickness of the membrane sheet  46 . A space is formed below the striking-out portion  47   d , and the tip portion of the welding pin  45   a  of the holder  45  is welded in the space. Accordingly, even after the welding operation is performed, as shown in  FIG. 10 , the bottom surface is horizontally maintained without forming a hole in the LED sheet  48 . For this reason, a hole is not formed in the LED sheet  48  like the first embodiment. 
     The assembling operation and the illuminating operation of the third embodiment are the same as those of the first embodiment. According to the third embodiment, the striking-out portion  47   d  is provided in the back plate  47  having a light guiding function in addition to the first embodiment. Accordingly, since the welded tip portion of the welding pin  45   a  is inserted into the striking-out portion  47   d , the welded portion does not protrude downward beyond the back plate  47 . Since it is not necessary to provide the hole in the LED sheet  48 , the water-resistant property may be also ensured. 
     Next, a fourth embodiment of the invention will be described.  FIG. 11  is an exploded perspective view illustrating a key switch structure of the fourth embodiment.  FIG. 12  is a cross-sectional view illustrating the key switch structure of the fourth embodiment. In  FIGS. 11 and 12 , a key switch  50  of the fourth embodiment includes a key top  51 , a first linking member  52  which is adapted to be slidable with respect to the key top  51 , a second linking member  53  which is adapted to be rotatable with respect to the key top  51 , a rubber dome  54  which is bent when the key top  51  is pressed down and returns the key top  51  to the original position when the pressing force is released, a holder  55  which holds the first and second linking members  52  and  53 , an LED sheet  58 , a membrane sheet  56  which has a contact point portion directly beneath the rubber dome  54 , a back plate  57 , and a reflecting sheet  59 . 
     Among the constituents, the key top  51 , the first linking member  52 , the second linking member  53 , the rubber dome  54 , the holder  55 , the LED sheet  58 , and the membrane sheet  56  have the same configurations as those of the second embodiment. Accordingly, the detailed description thereof will be omitted. Further, a flange portion is not formed at a welding pin  55   a  of the holder  55 . 
     In the back plate  57  having a light guiding function, a hole  57   a  is perforated in plural positions corresponding to holes  56   a  of the membrane sheet  56  so that the welding pin  55   a  of the holder  55  is fitted thereinto, and a through-hole  57   c  is provided at a position corresponding to the position at which the LED  58   b  is disposed. Further, the back plate  57  is provided with a striking-out portion  57   d . The hole  57   a  is formed at the striking-out portion  57   d . The diameter of the hole  57   a  is smaller than that of the hole  56   a  of the membrane sheet  56 . A shielding seal is not attached to the lower portion of the through-hole  57   c . Further, the back plate  57  is provided with a reflecting portion  57   b  printed in a dot shape. The reflecting sheet  59  provided below the back plate  57  is not provided with a hole like the second embodiment. 
     As in the third embodiment, the height of the striking-out portion  57   d  of the back plate  57  is slightly higher than the thickness of the membrane sheet  56 . A space is formed below the striking-out portion  57   d , and the tip portion of the welding pin  55   a  of the holder  55  is welded in the space. Accordingly, even after the welding operation is performed, as shown in  FIG. 12 , the bottom surface is horizontally maintained without forming a hole in the LED sheet  58 . For this reason, a hole is not formed in the LED sheet  58  like the second embodiment. 
     The assembling operation and the illuminating operation of the fourth embodiment are the same as those of the second embodiment. According to the fourth embodiment, a striking-out portion  57   d  is provided in the back plate  57  in addition to the second embodiment. Accordingly, since the welded tip portion of the welding pin  55   a  is disposed inside the striking-out portion  57   d , the welded portion does not protrude downward beyond the back plate  57 . Since it is not necessary to provide the hole in the reflecting sheet  59 , the water-resistant property may be also ensured.

Technology Category: 5