CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese application serial no. 2002-102559, filed Apr. 4, 2002. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a key switch structure for use in computer keyboards. More particularly, the present invention pertains to a thin-form key switch structure that is suitable for use in thin notebook-type personal computers or other low profile computers. 
     2. Description of the Prior Art 
     With the advent and popularity of thin, lightweight notebook-type personal computers, manufacturers have continuously sought to further reduce the size, weight and thickness of such computers. The size, height and weight of the key switches that are positioned under each keytop contribute significantly to the size and weight of notebook computers. Thus, manufacturers have attempted to achieve such reductions by developing thin-forming or low profile key switches. However, there are several constraints that make further reduction of the height and weight of the key switch very difficult. For example, for optimal operability a minimum keystroke length of 2.6 to 2.7 is required as well as a certain click sensation. Under such constraints, any significant additional height or weight reduction of key switches cannot be achieved using the conventional switches. 
     The key switch disclosed in JP Patent 2001-14083 is an example of a key switch which retains a fixed keystroke and click sensation. The main portions of this key switch are as shown, for example, in the vertical cross sections depicted in FIGS. 5 and 6 as well as in the stabilizer section of a conventional key switch as depicted in FIG.  7 . FIG. 5 shows the key switch of Section A—A of FIG. 7 in the OFF state. FIG. 6 shows the same key switch but from a perspective of Section B—B of FIG.  7  and in the ON state. 
     Referring to the key switch of depicted in FIG. 5, the key switch structure is comprised of reinforcing base plate  1 , membrane switch  2  disposed on reinforcing base plate  1 , frame  3  on membrane switch  2 , stabilizer  4 , key top  5 , and click rubber  6  sandwiched between the membrane switch  2  surface and the key top  5  bottom surface. Membrane switch  2  includes two (flexible film) layers  8  and  9 . Membrane switch  2  further includes spacer  7  that is positioned between membrane layers  8  and  9 . Electrical contact  10  and electrical circuits are provided on the opposing surfaces of membrane layers  8  and  9 . Hole  11  is an opening in spacer  7  at the location of the electrical contact  10 . When membrane layer  8  is pushed downward in the area above electrical contact  10  membrane layer  8  is distorted downward placing electrical contact  10  in the ON state. 
     As shown in FIG. 7, stabilizer  4  is affixed on each of the left and right sides of the click rubber  6  (left and right as seen from the key switch operating position). Two long and narrow flat pieces  15  and  16  are arranged to form X-shaped part  18 . Flat pieces  15  and  16  may be made from a rigid material such as metal, hard plastic or the like. Stabilizer  4  formed as X-shaped part  18  is linked at an intermediary point by a support axis  17  allowing stabilizer  4  to have a variable crossing angle α. Slider  20  is circular and perpendicularly protrudes from the bottom end of each X-shaped part  18 . Four sliding portions or sliders  20   a - 20   d  are contained in a free-fitting state within the square-shaped free motion range limiter  13  respectively located at four points around the hole  12  in frame  3 . 
     The width W of the free motion range limiter  13  described above is slightly greater than the width of the slider  20 . The length of free motion range limiter  13  must be relatively substantial in order to allow the bottom of key top  5  to make contact with the top surface of frame  3 . Thus, slider  20  is able to move freely while sliding on the surface of membrane layer  8  within free motion range limiter  13 . A stopper (not shown) projects over free motion range limiter  13  that contains slider  20 , such that the slider  20  does not jump out of free motion range limiter  13 . 
     Frame  3  identified above is affixed to the top of membrane switch  2  and has a flat shape. Hole  12  is an opening in frame  3  that is positioned above electrical contract  10  of membrane switch  2 . Moreover, in addition to free motion range limiter  13 , there is a slit shaped storage hole  21  for the X-shaped part that connects two free motion range limiters  13  on each side of click rubber  6 . More specifically, X-shaped part storage hole  21   a  connects the two free motion range limiters  13  on one side of click rubber  6  and X-shaped part storage hole  21   b  connects the two free motion range limiters  13  on the other side of click rubber  6 , as shown in FIG.  7 . There is also a notch in storage hole  21  at the position at which stabilizers  4  are linked. 
     As shown in FIGS. 5-7, slider  22  is circular and protrudes perpendicularly from the both top ends of each X-shaped part  18 . As shown in FIG.  5 , slider  22  loosely fits within the horizontal part of L-shaped engagement piece  23  that is disposed at the four corners of the bottom surface of key top  5 . The distance between hanging portion  24  on the side of key top  5  and the vertical portion of engagement piece  23  is sufficiently greater than the diameter of slider  22 . This distance allows slider  22  to move freely between hanging portion  24  and vertical portion of engagement piece  23  while sliding on the bottom surface of key top  5  or on the horizontal portion of engagement piece  23 . 
     X-shaped part  18  is linked to frame  3  and key top  5  so as to be able to move freely within a fixed range on the surface that contains it. The arrangement, dimensions, and shape of each of the parts which control the range of play are selected so that sliders  20  and  22  have a predetermined positional relationship to free movement range limiters  13  when key top  5  is in the OFF or normal state and in the ON or compressed state. 
     This prior art key switch design, however, has several disadvantages. As shown in FIG. 6, key top  5  may be depressed so that X-shaped part  18  is compressed until the crossing angle α is 180 degrees and key top  5  cannot be depressed further. At this point, lower end slider  20  of X-shaped part  18  is in a position whereby it contacts free motion range limiter  13  outside wall surface  13   b,  and top end slider  22  is in a position whereby it contacts key top  5  side hanging portion  24 . As a result, further displacement is prevented and key top  5  is stopped at that position. 
     While the height reduction technique for the prior art key switch described above has affected the size and weight reduction of personal computers, frame  3  in such switches overlaps and tightly adheres to the upper portion of the membrane switch  2 . As a result, even when free motion range limiter  13  outside wall surface  13   b  is extended and key top  5  bottom surface contacts the upper surface of frame  3 , the overall key switch thickness cannot be brought below the sum of key top  5  height (including the side hanging portion  24 ) and the respective thickness of frame  3 , membrane switch  2 , and reinforcing base plate  1 , thus limiting the ability to reduce key switch thickness or height further. 
     Another disadvantage is that although the spreading width W of free motion range limiter  13  is only slightly larger than the width of slider  20 . However, a substantial amount of lengthwise space is required so as to allow key top  5  to make contact with the top surface of frame  3  when key top  5  is fully depressed. As a result of this substantial lengthwise space in free motion limiter  13 , when key top  5  is depressed the direction of movement for key top  5  differs depending on whether the force applied to key top  5  is exerted towards a the keyboard operator or away from the keyboard operator. In other words, key top  5  makes undesirable movements in the frontward or backward direction depending on the degree of force exerted by the keyboard operator when pressing down on a key top. 
     SUMMARY OF THE PRESENT INVENTION 
     Accordingly, the object of the present invention is to eliminate some of the disadvantages found in the prior art and to provide a low profile key switch structure that is substantially reduced in height so as to reduce the thickness of personal notebook computer or the like. Another object of the present invention is to provide a key switch structure that minimizes undesirable key top movement that occurs when the key is depressed and which varies depending on the direction of the force that is exerted on the key top at that time. 
     In general, the first aspect of the present invention features a key switch structure that includes, a key top, a click rubber having a resilient force that pushes the key top upward when the key top is depressed and a membrane switch which disposed under the click rubber. This key top switch structure also includes a reinforcing base plate disposed on the bottom of the key switch structure, a pair of stabilizers disposed between the reinforcing base plate and the bottom surface of the key top wherein the pair of stabilizers are arranged so that there is one stabilizer on each side of the click rubber and a pair of stabilizer holders are formed at opposing ends of the bottom of the key top so that each stabilizer holder can hold one top end of each stabilizer. 
     This invention further includes a pair of frames that are arranged so that each said frame supports one stabilizer, a pair of top free motion range limiters are each formed between the stabilizer holder and the key top for each stabilizer wherein the top free range motion limiter is further formed to enclose a first top end and a second top end of each stabilizer therein in a free fitting manner. Similarly, the key switch structure of the present invention further includes a pair of bottom free motion range limiters are formed on each frame so that the bottom of each free motion range limiter engages with and is affixed to a free range limiter engagement section wherein the free motion range limiting engagement section is formed within the reinforcement base plate. The pair of bottom free-range motion limiters is further formed to enclose a first bottom end and a second bottom end of each stabilizer therein in a free fitting manner. The frame and the membrane switch are affixed on the reinforcing base plate so that the frame and the membrane switch do not overlap. 
     Embodiments of the invention may include one or more of the following features: a key switch structure wherein the first and second top ends and the first and second bottom ends of each of the stabilizers have sliders that projects perpendicularly therefrom so as to be enclosed in the pair of top free motion range limiters and the pair of bottom free motion range limiter. 
     Another embodiment may include a key switch structure, wherein each stabilizer is formed by two flat pieces that are joined at an intermediary point using an intermediary support axis so as to provide each stabilizer with a variable crossing angle that varies with the vertical movement of the key top. 
     Another embodiment may include a key switch structure, wherein a pair of top free motion range limiters and a bottom pair of free motion range limiters include a front and a rear free motion range limiter. 
     Another embodiment may include a key switch structure, wherein the slider of the first top end and the first bottom end of each stabilizer are allowed to rotate within each the front free motion range limiters and wherein the slider of each second top end and the second bottom end of each stabilizer is formed so as to allow the slider to rotate and slide within each rear free motion range limiters. 
     Another embodiment may include a key switch structure, wherein the key top further includes a convex section that protrudes downward from the bottom side of the key top and that is formed to fit that top portion of the click rubber. 
     Another embodiment may include a key switch structure, wherein the key top further includes a pair of engagement parts that project downward from the bottom side of opposing ends of the key top. Each engagement part has a protuberance that projects from its bottom that supports the stabilizer holder. 
     Another embodiment may include a key switch structure, wherein the free motion range engagement limiting section further includes a set of three vertical walls that limit the range of sliders on the bottom of each stabilizer. 
     Another embodiment may include a key switch structure, wherein the free motion range engagement limiting section further includes a pair of support columns that engage each frame. 
     Another embodiment may include a key switch structure, further includes notches that are formed in each frame to enclose sliders that engage each the frame. 
     Another embodiment may include a key switch structure, further includes notches that are formed on the reinforcement base plate to enclose the bottom end of the flat piece of each stabilizer. 
     Another embodiment may include a key switch structure, further includes a concave section that is formed to engage the engagement parts when the key top is depressed so that the engagement parts do not make contact with the membrane switch or frame. 
     The above advantages and features are of representative embodiments only. It should be understood that they are not to be considered limitations on the invention as defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent in the following detailed description and accompanying drawings in which like references denote like or corresponding parts, and in which: 
     FIG. 1 shows a side view of a key switch structure of the present invention wherein the key switch is in the OFF state. 
     FIG. 2 shows a side view of a key switch structure of the present invention wherein the key switch is in the ON state. 
     FIG. 3 shows a partial top surface diagram of a keyboard having multiple key switch structures that are configured as disclosed in the present invention. 
     FIG. 4 shows an expanded top surface diagram of one key switch structure as shown in FIG.  3 . 
     FIG. 5 shows a vertical cross sectional view through A—A in FIG.  7 . FIG. 7 depicts the structure of a convention key switch. 
     FIG. 6 shows a vertical cross sectional view through B—B in the FIG.  7 . FIG. 7 depicts a conventional key switch. 
     FIG. 7 specifically shows the stabilizer portion of a conventional key switch. 
    
    
     DETAILED DESCRIPTION 
     The objective of the present invention is to provide an improved low profile key switch that utilizes major portions of conventional key switches that will provide a reduction in height and superior operability. In order to achieve this objective, the free motion range limiters that enclose the stabilizer ends are made integral to the frame such that the free motion range limiters are formed in the frame. Moreover, the frame is made such that it can fit into or embedded in the reinforcing base plate so as reduce the thickness of the key switch structure. 
     Referring to FIGS. 1-4, we provide detail descriptions of various embodiments of the low profile key switch structure disclosed in the present invention. FIGS. 5-7 refer to the prior art key switch. FIGS. 1 and 2 are diagrams of the right side as seen from the key switch operating position. FIGS. 3 and 4 are top views of membrane switch  2  and frame  3 , and an expanded view of a portion thereof, respectively. In each of the figures, the same reference numerals are used for those parts that function in the same manner as those in FIGS. 5 and 6. In the explanation which follows, stabilizer  4  and key top  5  structures are left-right symmetrical when viewed from the key switch operator&#39;s position. Therefore only one is explained and explanation of the other is omitted. 
     FIG. 1 shows a side view of the key switch structure disclosed in the present invention wherein the key switch is in the OFF state. In this state no electrical contact is made because key top  5  is not depressed. The key switch of FIG. 1 includes reinforcing base plate  1 , membrane switch  2  and frame  3 . Reinforcing base plate  1  is preferably made of a rigid material. Membrane switch  2  is positioned on and caused to adhere to reinforcing base plate  1 . The key switch further includes stabilizer  4 , key top  5  and click rubber  6 . Click rubber  6  is positioned between the top surface of membrane switch  2  and the bottom surface of key top  5 . Membrane switch  2  and frame  3  are disposed on top of reinforcing base plate  1  so as to form approximately the same surface. More specifically, only one of either membrane switch  2  or frame  3  actually contacts reinforcing base plate  1  because membrane switch  2  and frame  3  do not overlap. Using stabilizer  4  and click rubber  6 , a click sensation is obtained while also providing the appropriate keystroke length for optimal operability. 
     Membrane switch  2  is a typical membrane switch and the membrane switch depicted in FIG. 1 is similar to the switch shown in FIG.  5 . The membrane switch in FIG. 5 includes membrane layers  8  and  9 , electrical contact  10  and spacer  7 . Spacer  7  and membrane layers  8  and  9  are omitted from FIGS. 1 and 2. Membrane switch  2  is preferably made of a flexible film material or the like. 
     FIG. 1 also shows key top  5 . Key top  5  includes convex section  31  that is formed to fit the tip of click rubber  6 . Click rubber  6  is concentrically shaped and positioned above the electrical contact (not shown). Hanging portion  24  is formed facing downward around key top  5 , and engaging parts  24   a  and  24   b,  which engage stabilizer holders  23   a  and  23   b,  and which in turn hold sliders  22   a  and  22   b,  are formed on the inside thereof. Structures of the stabilizer holders  23   a  and  23   b,  and of the engaging parts  24   a  and  24   b  are as described below. 
     Sliders  22   a  and  22   b  at the top end of stabilizer  4  project perpendicularly from each side. Sliders  22   a  and  22   b  freely move within free motion range limiters  13   c  and  13   d  in which sliders  22   a  and  22   b  are enclosed by stabilizer holders  23   a  and  23   b  and engaging parts  24   a  and  24   b.  More specifically, slider  22   a  at one end of the stabilizer  4  is enclosed so as to be rotatable within the free motion range limiter  13   c  that is formed between the engaging part  24   a  and the stabilizer holder  23   a  vertical portion  23   a   1 . Similarly, slider  22   b  at the top end of stabilizer  4  is enclosed so as to be able to rotate and slide within free motion range limiter  13   d  that is formed between engaging part  24   b  and stabilizer holder  23   b  vertical portion  23   b   1 . Stabilizer holders  23   a  and  23   b  are such that their horizontal portions  23   a   2  and  23   b   2  are prevented from moving by protuberances  24   a   2  and  24   b   2  provided on the end of engaging parts  24   a  and  24   b.  This movement is specifically prevented when key top  5  and stabilizer  4  are depressed. 
     More specifically, slider  22   a  at one of the top ends of stabilizer  4  is enclosed so that it is allowed to rotate when moving between vertical portion  24   a   1  of engaging part  24   a  and the vertical portion  23   a   1  of the stabilizer holder  23   a.  The gap between vertical portions  23   a   1  and  24   a   1  is formed to be approximately equal to the diameter of slider  22   a  and is designed such that slider  22   a  at one end of the stabilizer  4  is allowed to rotate. Similarly, slider  22   b  at the other top end of stabilizer  4  is enclosed so as to be able to rotate and slide within the range of the engaging part  24   b  vertical component  24   b   1  and the stabilizer holder  23   b  vertical portion  23   b   1 . 
     Concave section  32  is shown in FIG.  4 . Concave section  32  is provided so that the ends of engaging parts  24   a  and  24   b  do not contact membrane switch  2  or frame  3  when key top  5  is depressed to its lowest point. Moreover, the bottom surface of the hanging portion  24  of key top  5  is formed so as not to contact membrane switch  2  and frame  3  when key top  5  is depressed to its lowest point, as shown in FIG.  2 . 
     Stabilizer  4  of the present invention is disposed and affixed on the left and right of click rubber  6  (left and right when viewed from the key switch operating position) and is similar to stabilizer  4  shown in FIG.  5 . X-shaped part  18  of the present invention which form stabilizer  4  is also similar to stabilizer  4  parts shown in FIG.  5 . Moreover, stabilizer  4  of the present invention is formed of X-shaped part  18  in which two long, narrow flat pieces  15  and  16  are linked at an intermediary point by a support axis  17  that provides the stabilizer with a variable crossing angle α. Pieces  15  and  16  may be made of a rigid material such as metal, hard plastic or the like. Sliders  20   a  and  20   b  are circular and protrude perpendicularly from the bottom ends of X-shaped part  18 . Sliders  20   a  and  20   b  are enclosed inside of free motion range limiters  13   a  and  13   b  that are formed on frame  3 . Sliders  20   a  and  20   b  fit loosely within free motion range limiters  13   a  and  13   b.  Free motion range limiters  13   a  and  13   b,  as shown in FIGS. 1-4 are formed such that sliders  20   a  and  20   b  do not jump out of free motion range limiters  13   a  and  13   b.    
     Free motion range limiter  13   b  has sufficient space in the front-to-rear or lengthwise direction over a range described below, even when it encloses slider  20   b.  Slider  20   b  can move freely while rotating and sliding within free motion range limiter  13   b.  In contrast, free motion range limiter  13   a  does not have space to allow sliding in the lengthwise direction when enclosing slider  20   a.  Slider  20   a  is capable of rotating within free motion range limiter  13   a.    
     As described above, in stabilizer  4 , one of the pair of sliders at the front end, namely sliders  20   a  and  22   a  which are enclosed are allowed to rotate, while the other pair at the rear end, namely sliders  20   b  and  22   b  are also enclosed but in addition to rotate these sliders also slide. As a result, when key top  5  is depressed, slider  22   a  and slider  20   a  rotate within a fixed position. Similarly, slider  22   b  and slider  20   b  move while rotating and sliding when stabilizer  4  is compressed or collapsed downward. More specifically, the direction of movement of key top  5  is fixed. For example, when free motion range limiter  13   a  is placed in a position close to the keyboard operator, or when free motion range limiter  13   b  is placed in a position far from the keyboard operator, the direction of movement of key top  5  is fixed in a direction going away from the keyboard operator. Thus, an uncomfortable key operating position can be avoided. 
     Click rubber  6  is made of soft rubber and has an inverted funnel shape. As mentioned above, the top of click rubber  6  fits between convex section  31  on the underside of the key top. The bottom portion of click rubber  6  is positioned on the membrane switch  2  and positioned concentrically with respect to the electrical contact (not shown). Click rubber  6  barely deforms under the weight of key top  5  and stabilizer  4  alone, but when compressed by normal key operation it does in fact deform. At the time of compression click rubber  6  has an elasticity that provides the appropriate click sensation to the finger operating key top  5 . Moreover, a known switch pushing component may be used which will cause membrane switch  2  to distort in a downward position when key top  5  is depressed. 
     As shown in FIG. 1, free motion range limiters  13   a  and  13   b,  which enclose the stabilizer  4  bottom-end sliders  20   a  and  20   b  in a free-fitting state are formed on the frame  3 . Vertical walls  3   b,    3   c,  and  3   d,  which respectively limit the range of free motion of sliders  20   a  and  20   b  at the bottom end of stabilizer  4  are formed on free motion range limiters  13   a  and  13   b.  Free motion range limiters  13   a  and  13   b  are inserted and secured to free motion range limiting engagement section  30  that is formed in reinforcing base plate  1  by press fitting. Moreover, support columns  3   a  and  3   e  are inserted and secured to free motion limiting section  30  in reinforcing base plate  1  by press fit engagement. Flange portions of support columns  3   a  and  3   e  may be formed using a heat press, fusion or another conventional method. Support columns  3   a  and  3   e  are provided on the each side of support axis  17  of stabilizer  4 . Slider  20   a  on stabilizer  4  is enclosed by vertical walls  3   b  and  3   c  that form free motion range limiter  13   a  in such a way that slider  20   a  contacts the vertical portions  13   a   1  and  13   a   2  and is able to freely rotate. 
     The gap between vertical walls  3   b,    3   c  is formed to be slightly smaller than the diameter of stabilizer  4  bottom-end slider  20   a  and is configured such that slider  20   a  is rotatable when fit into free motion range limiting engagement section  30  described below. Stabilizer  4  bottom-end slider  20   b  is enclosed in such a way as to be able to rotate and slide over an allowable range within free motion range limiter  13   b.  After support columns  3   a,    3   e  and vertical walls  3   b,    3   c,  and  3   d  have been inserted into free motion range limiting engagement section  30  the ends of support columns  3   a,    3   e,  and vertical wall  3   d  are pressed down by a predetermined method and fused to free motion range limiting engagement section  30 . 
     FIG. 3 shows is a partial top view of frame  3  and membrane switch  2  where the key switch of the present invention has been installed on a keyboard having a plurality of such key switch structures. More specifically, FIG. 3 depicts nine key switches that include T 10 , Y 10 , U 10 , G 10 , H 10 , J 10 , B 10 , N 10 , and M 10 . Sliders  20   a  and  20   b  are configured to form a single key switch by means of the respective frames provided at the top and bottom of FIG.  3 . For example, H 10  constitutes a single key switch structure. Frames  3 H 1  and  3 H 2  each support stabilizer  4  and correspond to key switch structure H 10 . Key top  5  (not shown) may be depressed causing stabilizer  4  to collapse and key switch H 10  to be set to the ON state. Each key switch is similarly configured. 
     Membrane switch  2  is formed so as to surround frame  3 . Frame  3  and membrane switch  2  are affixed to and arranged on the reinforcing base plate  1  so as not to overlap with one another. Frames  3 Y 2  and  3 N 2  correspond to key switches Y 10  and N 10  at the top and bottom of FIG. 3, respectively. Frames  3 Y 2  and  3 N 2  are surrounded by membrane switch  2 . Moreover, frames  3 H 1  and  3 Y 2 , and frames  3 H 2  and  3 N 2  are provided within the areas a and b, respectively. In portions of areas a and b where no frame is provided, concave section  32  (see FIG. 4) is formed such that the ends of engaging parts  24   a  and  24   b  do not contact the membrane switch  2  or frame  3  when stabilizer  4  is collapsed to the bottom and it has a crossing angle α of 180 degrees. 
     Because frame  3  and membrane switch  2  are positioned to adhere to reinforcing base plate  1  in such a way that they do not mutually overlap, it is possible to form the top surface of membrane switch  2  and frame  3  at approximately the same height. Moreover, the area surrounding frame  3  may be preferably designed to surround two frames. However, if the requirement that the ends of the engaging parts  24   a  and  24   b  do not make contact membrane switch  2  and frame  3  is satisfied, the surrounding area may also be otherwise designed. For example, an acceptable design alternative is to have membrane switch  2  surrounds a single frame instead of two frames as depicted. 
     FIG. 4 shows is an expanded top surface diagram of frames  3 J 1  and  3 J 2  and membrane switch  2  for a single key top. More specifically, FIG. 4 shows an expansion of the key switch J 10  area in FIG.  3 . Key switch J 10  is configured in a similar manner as the other key switches. Key switch J 10  is formed on the membrane switch  2  by a conventional method. The push component associated with click rubber  6  (not shown) pushes against membrane  2  on an electrical contact such that membrane  2  is deformed and as a result of such deformation the electrical contact is closed. 
     Frames  3 J 1  and  3 J 2  are formed within the areas a and b, respectively and are surrounded by membrane switch  2 . Within areas a and b convex section  32  is formed in the space where frames  3 J 1  and  3 J 2  are not provided. Concave section  32  is lower then the level of frame  3  by appropriately the same as the combined height of frame  3  and membrane  2 . The frames  3 J 1  and  3 J 2  are symmetrical with respect to the key switch J 10 . Therefore, the structure and operation of both frames are identical, so only frame  3 J 2  will be described below. 
     As shown in FIG. 4, notches  33   a  and  33   b  are provided on frame  3 J 2  and are used to enclose sliders  20   a  and  20   b  of stabilizer  4  in a free-fitting state within free motion range limiters  13   a  and  13   b.  Notch  33   b  is formed on frame  3 J 2  such that when stabilizer  4  is depressed to its lowest position and slider  20   b  moves by rotation and sliding within the free motion range limiter  13   b,  slider  20   b  does not reach the notch  33   b.  The notch  33   a  is formed to be approximately the same or slightly smaller than the vertical walls  3   b  and  3   c  which limits the movement of slider  20   a.    
     Free motion range limiters  13   a  and  13   b  are formed at the bottom of notches  33   a  and  33   b,  respectively. Notch  33   a  is formed such that when slider  20   b  is caused to contact the inner wall surface  13   b   1  of free motion range limiter  13   b  with stabilizer  4  raised to the highest point and slider  20   a  is in the position at the bottom of notch  33   a.    
     Notches  34   a  and  34   b  are formed on the reinforcing base plate  1  so as to penetrate the reinforcing base plate  1 . Moreover, notches  34   a  and  34   b  are rectangular in shape and have a size that is same or larger than the width of the bottom end of the flat piece  15  of stabilizer  4 . The bottom ends of the flat pieces  15  are enclosed in notched  34   a  and  34   b,  respectively. 
     In the present invention, stabilizer  4  may be pushed down to the lowest position, and slider  20   b  is inserted from notch  33   b  into the free motion range limiter  13   b.  As a result, flat piece  15 , which is the lower end portion of stabilizer  4  is enclosed in the notch  34   b.  When, in this position, the slider  20   a  is inserted into the notch  33   a  and enclosed so as to be rotatable, the bottom end of stabilizer  4  flat piece  16  is enclosed in the notch  34   a.  As described above, notch  33   a  is formed such that it lies at the position of slider  20   a  when stabilizer  4  is raised to the highest position and the slider  20   b  is moved to a position at which it contacts inside wall surface  13   b   1  of free motion range limiter  13   b.  As a result, when the stabilizer  4  is collapsed down to its lowest position, and slider  20   b  has moved by rotation or sliding inside free motion range limiter  13   b  slider  20   b  will not reach the notch  33   b.  Accordingly, slider  20   b  will not separate from free motion range limiter  13   b.    
     Free motion range limiting engagement section  30  is formed on the reinforcing base plate  1  as described below. Frame  3 J 2  is caused to tightly adhere to the top surface of the reinforcing base plate  1 . One method of adhesion may be to fuse frame  3 J 2  to reinforcement base plate  1 . Penetrating holes (not shown), are formed in the reinforcing base plate  1  at the position where frame  3 J 2  support columns  3   a  and  3   e  and vertical walls  3   b,    3   c,  and  3   d  are formed. The penetrating holes have approximately the same dimension as the frame  3 J 2  support columns  3   a  and  3   e  and vertical walls  3   b,    3   c,  and  3   d.  These penetrating holes are referred to as free motion range limiting engagement section  30 . Frame  3 J 2  support columns  3   a  and  3   e  and vertical walls  3   b,    3   c,  and  3   d  are inserted into the respective sections of free motion range limiting engagement section  30 . The ends of the support columns  3   a  and  3   e  are then fused by heat or other means such that they have the same height as the surface of the bottom portion of reinforcing base plate  1 . As a result, frame  3 J 2  fuses to the top of the reinforcing base plate  1 , and frame  3  and membrane switch  2  are caused to adhere to the reinforcing base plate  1  in positions such that they do not mutually overlap. Therefore, the top surfaces of the membrane switch  2  and the frame  3  have approximately the same height. 
     As shown in FIG. 1, key top  5  is in the OFF, non-depressed state. In this state key top  5  is extended upward by the force of click rubber  6  causing the electrical contact push piece to be separated from the membrane switch  2 . As result of this separation the electrical contact cannot conduct electricity and is therefore sets the switch to the OFF state. Moreover, in the OFF state X-shaped part  18  of stabilizer  4  is also in an extended position. At this point, slider  20   b  of X-shaped part  18  is in a position at which it contacts inside wall surface  13   b   1  of free motion range limiter  13   b  and slider  22   b  is in a position at which it contacts stabilizer holder  23   b  vertical portion  23   b   1 . The pair-forming slider  22   b  and slider  20   b  come into contact with the vertical portions, and no further upward deflection of stabilizer  4  and the attached key top  5  may occur. 
     When key top  5  is depressed by a key operation in this state, the downward pushing force compresses and distorts click rubber  6 , while at the same time exerting a downward force on stabilizer  4 . By this means, X-shaped part  18  of stabilizer  4  opens such that the crossing angle a increases, leaving stabilizer  4  in a collapsed or compressed state. Finally, stabilizer  4  reaches a folded state in which crossing angle α reaches approximately 180 degrees, as shown in FIG.  2 . At this point, slider  20   b  of X-shaped part  18  is closer in position to the inside wall surface  13   b   1  than notch  33   b,  and slider  22   b  is in a position at which it contacts key top  5  engaging part  24   b  side surface  24   b   1 , so that further displacement is prevented. Even when key top  5  is at a midpoint position in the process of depressing key top  5 , stabilizer  4 , by its rigidity, prevents sideways deflection of key top  5  and stabilizes its posture. 
     Before slider  20   b  and slider  22   b  reach the free motion limit position, the push section associated with click rubber  6  pushes on and deforms membrane switch  2  downward over the electrical contact causing the contact to close. When a finger operating the key leaves key top  5  removing downward force, key top  5  rises due to click rubber  6  resilient forces. Stabilizer  4  which is connected to key top  5  is then extended upward causing and the whole key switch structure to return to the OFF state, as shown in FIG.  1 . 
     As described above, membrane switch  2  and frame  3  are provided on approximately the same surface, so that when stabilizer  4  reaches a folded state with a final crossing angle α of 180 degrees, the bottom surface of key top  5  hanging portion  24  will not contact either membrane switch  2  or frame  3 . 
     In accordance with one embodiment of the present invention the height of a key switch may be reduced further when the frame and membrane switch structures that are provided on a reinforcing base plate are arranged and affixed to the reinforcing base plate in such a way as not to mutually overlap. A free motion range limiter that encloses slider  22  in a free-fitting state is formed on the stabilizer holding piece under the key top. Moreover, the free motion range limiter formed on the frame is inserted in and affixed to the free motion range limiting engagement section that is formed on the reinforcing base plate. Thus, allowing a reduction in the overall key switch height. 
     Further, in accordance with one embodiment of the prevent invention, the undesirable key top movement caused by the force and direction in which the key top is pressed is substantially eliminated. In the foregoing description, the apparatus of the present invention has been described with reference to specific examples. It should be understood and expected that variations in the principles of the apparatus herein disclosed may be made by one skilled in the art and it is intended that such modifications, changes, and substitutions are to be included within the scope of the present invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense.
This low profile key switch structure provides a substantial reduction in the height or thickness of a key switch as compared to conventional and other low profile switches. Substantial height reduction in key switch structures allows the size and thickness of notebook-type personal computers and the like to be similarly reduced. One feature of this low profile key switch structure is that unlike in conventional key switch structures the membrane switch and frame structures do not over lap to cause unnecessary height in the structure. Another feature of this key switch structure is that the frames are formed to include the free motion range limiters. The frames are further formed so that they may be embedded in the reinforcing base plate in such a manner that allows the height of the key switch to be further reduced. This low profile key switch structure is also designed to eliminate unwanted key top movement that often occurs in conventional and other switches when the key top is depressed.