Abstract:
A magnetically coupled pushbutton switch having hard electrical conductors is discrete and may be used in place of a dome tact switch. The hard electrical conductors of the magnetically coupled pushbutton switch are uniquely arranged and may be soldered to a circuit board or surface mounted. Additionally, modifications and improvements made to the switch allow it to maintain good tactile response even though the switch may be as compact as a smaller tactile dome switch. A further benefit of the switch is its ability to be normally open, normally closed, or both. This capability stems from the unique arrangement of the hard electrical conductors that, in one preferred embodiment, extend over the top of a magnetically coupled switch armature. All of the hard electrical conductors are arranged within the switch so that the pushbutton armature of the switch is movable into and out of shorting relationship with the electrical conductors to change the circuit logic for a circuit incorporating the switch.

Description:
BACKGROUND OF THE INVENTION 
     Dome tact switches are commonly used with short travel keyboards. They give a tactile feedback to a user, are compact, and are discrete. These switches have hard electrical conductors, such as stamped beryllium copper, that are soldered to a circuit board or other substrate material. Unfortunately, dome switches fracture over time and are not normally sealed offered as a normally closed switch. Magnetically coupled pushbutton switches, on the other hand, have a long life and are normally sealed, but the electrical conductors of a magnetically coupled pushbutton switch are printed or painted onto the surface of a substrate. Additionally, a magnetically coupled switch, though thin, has a larger surface area than smaller dome tact switches. There is currently no magnetically coupled pushbutton switch that is a suitable replacement for a dome tact switch primarily because of the differences in electrical conductors and size. 
     Magnetically coupled pushbutton switches have a metal armature that is normally held spaced from switch contacts by bonded sheet magnet. The switch contacts are usually painted or printed onto the surface of a non-conductive substrate. A non-conductive spacer layer is fixed to the substrate, with an opening in the spacer layer exposing the switch contacts. The sheet magnet overlies the spacer layer. A user-provided actuating force applied to the armature causes it to snap free of the sheet magnet and close the switch contacts by electrically connecting them. Release of the actuating force allows the sheet magnet to attract the armature back to a normal position, in coupled engagement with the sheet magnet so that the armature is spaced from the switch contacts, to reopen the switch. Preferably, the armature has a crown that protrudes through an aperture in the magnet layer. Most often, a polyester membrane layer with suitable graphics overlies the sheet magnet to direct a user of the switch as to location and function of the switch. The benefits of magnetically coupled pushbutton switches have been demonstrated in U.S. Pat. Nos. 5,523,730, 5,666,096, 5,990,772 and 6,069,552, incorporated herein by reference, but not intended to limit the scope of the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a method of making a magnetically coupled pushbutton switch that is discrete and may be used in place of a dome tact switch. The method of the current invention includes hard electrical conductors that are uniquely arranged and may be soldered to a circuit board, surface mounted or insert molded. Additionally, the method of the current invention includes many modifications and improvements to a magnetically coupled pushbutton switch that allow the switch to maintain good tactile response even though the switch may be as compact as a smaller tactile dome switch. 
     A further benefit of the present invention is the ability of the switch to be normally open, normally closed, or both. This capability stems from the unique arrangement of the hard electrical conductors that, in one preferred embodiment, extend over the top of a magnetically coupled switch armature of the present invention. All of the hard electrical conductors are arranged within the switch so that the pushbutton armature of the switch is movable into and out of shorting relationship with the electrical conductors to change the circuit logic for a circuit incorporating the switch. An alternative construction for a normally closed switch of the present invention uses the magnetic attraction of the armature against a magnet to compress spring-loaded normally closed hard electrical conductors against a conductive surface. As used herein, the term “top” refers to that surface of any part in a cross sectional figure of the drawings that faces the top edge of the page, while “bottom” refers to that surface of any part in a cross sectional figure of the drawings that faces the bottom edge of the page. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a switch according to the present invention. 
     FIG. 2 is a cross-section of the switch of FIG.  1 . 
     FIG. 3 is a plan view showing the bottom of a molded magnet for a switch according to the present invention that has a normally closed set of hard electrical conductors. 
     FIG. 4 is a plan view of a stamped armature for a switch according to the present invention. 
     FIG. 5 is a cross section of a machined armature. 
     FIG. 6 is a perspective view of an armature nestled in a base housing for a switch according to the present invention having normally closed hard electrical conductors. 
     FIG. 7 is a perspective view of the bottom of a spring-loaded normally closed electrical conductor arrangement according to the present invention. 
     FIG.  8 . is a cross-sectional elevation, similar to FIG. 2, but the button includes a tappet that depends through the magnet aperture. 
     FIG. 9 is a cross-sectional elevation of a base housing, with magnet and armature, having a cavity with a top and bottom that are sloped with respect to each other so that the heel end of the armature has very little range of motion. 
     FIG. 10 is a perspective view of a magnet having painted electrical conductor pads that are in electrical contact with short prongs of normally closed hard electrical conductors. 
     FIG. 11 is a perspective view of an armature having a hard electrical conductor formed as an extension of the heel end of the armature. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIGS. 1 and 2, the magnetically coupled pushbutton switch of the present invention, shown generally at  2 , requires, from the top down, a top cover  4 , a button  6 , a magnet  8 , an armature  10 , and a base housing  12  that accepts hard electrical conductors. There are several additional features shown and described in the foregoing description that, though preferred, are not necessary and may be excluded where cost or preference dictates otherwise. FIGS. 1 and 2 show how a magnetically coupled pushbutton switch would appear if the most preferred embodiment of the present invention were used. Preferred materials, shapes, methods of attachment and methods of assembly will be discussed, but these preferences are not intended to exclude suitable or functionally equivalent alternatives. 
     The top cover  4  has a substantially square top surface  14  with a cover aperture  16  that is centrally located. There are four sides that extend downwardly from the four sides of the top surface  14 . Ideally, the top cover  4  is molded from a material such as nylon or acetal, but there are numerous other rigid materials, such as steel, that may also be used to make the top cover  4 . Also, where appropriate, the top cover  4  may be stamped, machined, or otherwise formed. For quality control purposes, two of the sides of the top cover  4  have raised alignment tracks  18  that are used to align a button  6 . 
     The button  6  includes a support structure  20  that aligns with the top cover  4  so that a central pad portion  22  of the button  6  extends through the central cover aperture  16  of the top cover  4 . The button  6  is made from an elastic and flexible material, such as silicone rubber or an elastomer. The support structure  20  of the button  6  includes alignment notches  24  that align with the raised alignment tracks  18  of the top cover  4 . The top of the support structure  20  includes a seal ridge  26  that completely contacts the top cover  4  after assembly. The seal ridge  26  prevents contaminants from entering the switch. The support structure  20  additionally includes concentric deformable ridges  28  centered around the central pad portion  22  of the button  6  so the central pad portion can be easily depressed when a user provided actuation force  30  is applied, causing the central pad portion to travel down through the cover aperture  16  in the top cover  4 , and return up through the cover aperture when the user provided actuation force is removed. 
     There is a magnet  8  below the button  6 , the magnet having a magnet aperture  32  that is substantially centered under the central pad portion  22  of the button  6 . The magnet  8  is preferably extruded, calendered or molded magnet that has a substantially flat bottom surface. Neodymium Iron Boron (NdFeB) or Samarium Cobalt (SmCo5) should be used for more compact switch designs because those materials have a stronger magnetic holding force than the more commonly used barium ferrite sheet magnet material. Extruded or calendered sheet magnet may be machined or blade cut with a magnet aperture  32  and, for alignment purposes, a trimmed corner  34 . Extruded or calendered sheet magnet is appropriate for a norm ally open switch or a normally closed switch that utilizes printed electrical conductor pads on the bottom surface of the magnet  8 . Injection molded magnet  8 , like the one shown in FIGS. 1 and 3, is appropriate for a normally closed switch that has hard electrical conductor prongs  36 , as will be described later. There are channels  38  formed in the injection molded magnet  8  that accept the prongs  36  of a normally closed switch. 
     An armature  10 , made from magnetic material, is normally magnetically coupled to the bottom surface of the magnet  8 . The armature  10  has a crown  40  that is off-center and normally protrudes through the magnet aperture  32  so that the crown  40  nearly or minimally touches the bottom of the central pad portion  22  of the button  6 . Alternatively, the bottom of the central pad portion  22  of the button  6  may have an actuating post, or tappet, integrally formed into the button piece part such that the tappet depends through the magnet aperture  32  and performs the same function as the crown  40  of the armature  10  so that a crown is not necessary. Because crowns are more commonly used, this description will utilize a crown instead of a tappet. The outer perimeter of the armature  10  that is closest to the crown  40  is the heel end  44  of the armature, while the outer perimeter of the armature that is farthest from the crown is the toe end  46  of the armature. If the armature  10  is generally disc shaped, which is preferred but not necessary, the heel end  44 , crown  40  and toe end  46  of the armature are substantially centered along a single diameter of the armature. Using NdFeB magnet, a disc shaped armature can range as small as about one quarter inch in diameter and yet require an actuation force of about ten ounces. 
     There is a base housing  12  below the armature  10 , the base housing typically being machined from a material like the ones already mentioned as appropriate materials for the top cover  4  so long as it is not electrically conductive. There is a cavity  48  in the middle of the base housing  12  that houses the armature  10  such that the armature has enough freedom of movement to allow for proper switch travel. At the top of the base housing  12  there is a platform  50  that is broader than the cavity  48 . The platform  50  is about as deep as the thickness of the magnet  8  and the platform is shaped to accept an aligned magnet  8  having a trimmed corner  34  such that the top of the magnet assembles flush with the top of the base housing  12 . Because the platform  50  is broader than the cavity  48 , the base housing  12  at least supports the outer perimeter of the bottom of the magnet  8 . Grooves  52  that accept hard electrical conductor prongs  36  of the switch are an additional base housing  12  feature. A simple way to assemble the base housing  12  to the top cover  4  would be a snap fit, which is ideal because of the flexible nature of the button&#39;s seal ridge  26 . After assembly, the top of the base housing  12  firmly presses against the support structure  20  of the button  6 , especially against the seal ridge  26 . 
     During switch actuation, movement of the armature  10  is such that the heel end  44  breaks away from the magnet  8  until it meets the bottom of the cavity  48  in the base housing  12 . Subsequently, the armature  10  pivots about the heel end  44  of the armature so that the toe end  46  of the armature breaks away from the magnet  8  and travels to the bottom of the cavity  48 . To prevent a double tactile feedback caused by both the heel end  44  and toe end  46  abruptly contacting the bottom of the cavity  48 , the travel of the heel end should be shortened so that only the longer traveling toe end provides any noticeable tactile feedback. As shown in FIG. 5, this may be accomplished by machining an armature  11  so that it is tapered in thickness such that the heel end  44  of the armature is significantly thicker than the toe end  46  of the armature  11 . Alternatively, for cost savings, the armature may be stamped from sheet metal. A stamped armature  10 , like the one shown in FIGS. 1,  2 ,  4  and  6 , has its heel end  44  bent at a significant angle, roughly ninety degrees. The bent heel end  44  has a flat edge that is normally held slightly spaced from the bottom of the cavity  48 . An alternative construction of the cavity  48  could accomplish the same goal as the above mentioned armature designs. If the bottom of the cavity  48  includes an incline such that the volume of the cavity that accepts the heel end  44  of the armature  10  is shallower than the volume of the cavity that accepts the toe end  46  of the armature, then the resulting sloped, or wedge shaped, cavity eliminates the need for a bent heel end  44  on the armature. 
     The hard electrical conductors of the switch may be arranged so that the switch is normally open, normally closed, or both, and they may be plated with silver, gold or the like. Hard electrical conductors may made from any electrically conductive material that may be stamped or otherwise formed into a piece part, as distinguished from painted or printed electrical conductors. The hard electrical conductors may be insert molded into the base housing  12 , or otherwise secured. Normally open hard electrical conductors  54  may be formed as pins with broad heads that poke through the bottom of the cavity  48 , with the broad heads usually sitting on the bottom of the cavity so that they may be electrically contacted by the armature  10  during switch actuation. Alternatively, the normally open hard electrical conductors  54  are stamped from electrically conductive sheet metal, such as beryllium copper, and then pre-bent and placed in the channels  38  in the base housing  12  designed to accept and hold the normally open hard electrical conductors  54 . There are usually two hard electrical conductors that are electrically connected by the toe end  46  of the armature  10  when a user provided force causes the armature to travel toward the bottom of the cavity  48  in the base housing  12 . The prongs  36  of the normally open hard electrical conductors  54  may be slightly spring-loaded so that the prongs, extensions of the hard electrical conductors that are normally touched by the toe end  46  of the armature  10  during switch actuation, are slightly spaced from the bottom of the cavity  48 , but the prongs  36  are also spaced from the armature when the switch is in an un-actuated position. By spring loading the prongs  36  of the normally open hard electrical conductors  54 , if the armature  10  touches one of the prongs before the other, then the armature is able to continue to travel until there is positive switch contact with the other prong. At the end of switch travel the user provided actuation force  30  resists the spring force of the two prongs  36  until they reach the bottom of the cavity  48  in the base housing  12 . 
     Where the switch includes normally closed hard electrical conductors  56 , electrical contact is made when the armature  10  is magnetically coupled to the magnet  8 . In one preferred embodiment, the normally closed hard electrical conductors  56  are stamped, similar to the normally open hard electrical conductors  54  above, and then place in grooves  52  in the base housing  12  designed to accept and hold the normally closed hard electrical conductors. Again, insert molding would be a suitable method of securing the hard electrical conductors to the base housing. The prongs  36  of the normally closed hard electrical conductors  56  may extend over the heel end  44  and or toe end  46  of the armature  10 . Molded magnet  8  is used in this embodiment so that the prongs  36  fit into the channels  38  formed in the molded magnet. The channels  38  are deep enough so that the prongs  36  do not significantly interfere with the coupled engagement of the armature  10  to the magnet  8 , but the prongs definitely touch the top of the armature when the switch is in the un-actuated position. To assemble the armature  10  between the base housing  12  and prongs  36 , it may be necessary to bend the prongs after the armature is positioned in the cavity  48  of the base housing. Alternatively, the grooves  52  in the base housing  12  could allow for the prongs  36  to be placed in a pre-bent state and then the top cover  4  to secure the assembly. Yet another possible assembly method would be to slip the armature  10  into place. 
     FIG. 7 shows an alternative embodiment very similar to the one just described, with the normally closed electrical conductors  56  slightly spring-loaded so that, in the absence of an armature, the prongs  36  are at least partially spaced from the channels  38  formed in the molded magnet  8 . An electrically conductive material, such as a copper bar that is molded into the magnet or a silver paint line  58  applied to the bottom of the magnet, electrical connects the channels  38 . When an actuation force is holding the armature spaced from the magnet, the prongs are spaced from the electrically conductive material that connects the channels. When the actuation force is removed so that the armature is magnetically attracted to the magnet, the magnetic attractive force overcomes the spring force of the prongs so that the prongs are pressed into the channels. In the normally closed position, the prongs are electrically connected by the electrically conductive material that connects the channels. 
     In another preferred embodiment, the normally closed hard electrical conductors  56  are formed as above, except the prongs are short and do not extend over the armature  10 . The bottom surface of the magnet  8 , which may be calendered or extruded sheet magnet, has printed or painted electrical conductor pads. The short prongs are in constant electrical contact with the electrical conductor pads on the magnet  8 . One drawback to this design is that painted or printed electrical conductors are not capable of carrying higher currents, which was one of the drawbacks of the prior art. Yet another embodiment, for use with any of the hard electrical conductor arrangements, has a common hard electrical conductor that may be formed by including an extension off the bent heel end  44  of a stamped armature  10 , the extension protruding to an appropriate location external to the base housing  12  where the extension may be used as the common hard electrical conductor of either a set of normally open or normally closed hard electrical conductors  56 , or both. The extension may be a pin or a long and narrow piece of armature material that is similar, in size and shape, to one of the normally closed hard electrical conductors  56 . 
     While a preferred form of the invention has been shown and described, it will be realized that alterations and modifications may be made thereto without departing from the scope of the following claims.