Abstract:
A more transparent capacitance membrane switch in which capacitance elements are offset from their associated switch locations so as to be out of the transparent field of view through the switch.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to capacitance membrane switches of the type in which switch activation (e.g., depression of a flexible layer) causes a capacitance change detectable by external circuitry. 
     RELATED APPLICATIONS 
     This application is related to copending application Ser. No. 323,281, filed Nov. 20, 1981, and application Ser. No. 323,290, filed Nov. 20, 1981. 
     SUMMARY OF THE INVENTION 
     In general the invention features a transparent capacitance membrane switch in which capacitance elements are offset from their associated switch locations so as to be out of the transparent field of view through the switch. Placing the capacitive elements to one side reduces the number of layers of material at the switch location and thus improves transparency. 
     In preferred embodiments, the capacitance elements are formed by vacuum depositing conductive and dielectric films; the contact portions of the membrane switch are also vacuum-deposited films; the contact portions are arranged in rows and columns with each row connected to one capacitance element; and the membrane switch is applied to the face of a visual display by vacuum depositing the capacitance films and lowermost contact portions directly to the face of the display. 
     In such preferred, vacuum-deposited-film embodiments, the invention reduces the area of dielectric film deposited (e.g., only along the side of the switch rather than across its entire area) and thus eases the manufacturing task of depositing a uniformly thick layer. Also, the high dielectric coefficient of vacuum-depositable dielectric materials such as Ta 2  O 5  makes possible high capacitance values despite the restriction placed on the capacitance area as the result of offsetting the capacitance from the switch locations. In an embodiment applied to the face of a visual display, the invention provides the advantages given in my copending U.S. patent application Ser. No. 399,716 entitled &#34;Membrane Switch for Face of Visual Display&#34; with the added advantage of improved transparency. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiment, and from the claims. 
    
    
     PREFERRED EMBODIMENT 
     A preferred embodiment of the invention will now be described, after first briefly describing the drawings. 
     DRAWINGS 
     FIG. 1 is a plan view of said preferred embodiment. 
     FIG. 2 is an enlarged cross-sectional view taken at 2--2 of FIG. 1, with the thicknesses of various layers greatly exaggerated and not to scale. 
     FIG. 3 is a schematic diagram of the electrical elements of said embodiment. 
    
    
     DESCRIPTION 
     Turning now to the drawings, there is shown a membrane switch 10 (i.e., a switch in which a flexible layer is flexed in order to produce a signal output). Glass substrate 12 (FIG. 2) has vacuum deposited on its upper surface (by sputtering through a suitable mask) three horizontal row conductors 14 (indium tin oxide (ITO) film, 10% indium, 2000 Angstroms thick). At the left edge of the glass substrate there are formed three thin-film capacitors C 1  -C 3  of different capacitance values. Pad 16 dielectric material (Ta 2  O 5 , 2000 Angstroms) is vacuum deposited over the left ends of row conductors 14. The pad is, in turn, covered by a vacuum deposited layer 18 of aluminum, which extends (FIG. 1) from the pad along the left edge of the switch to the upper edge where a connection can be made to external circuitry. The areas of aluminum overlapping the Ta 2  O 5  dielectric material and row conductors are each a different size to provide the three different capacitance values C 1  -C 3  (8000, 12000, and 16000 picofarads). 
     On top of glass substrate 12 is a top layer 24 of 5-mil thick transparent polyester film on which has been vacuum deposited (by sputtering through a suitable mask) four column conductors 26 (ITO, 2000 Angstroms). 
     Between top layer 24 and glass substrate 12 there is spacer layer 20 (2 mil transparent polyester with 1 mil transparent adhesive on each side), which has generally circular openings 22 aligned with the twelve switch locations defined by the areas where row conductors 14 cross column conductors 26. 
     Electrical connections are made in a conventional manner to conductive layer 18 and to column conductors 26. 
     In operation, a selected switch is activated by depression of top layer 24 at the desired switch location. That action causes a column conductor 26 on the undersurface of layer 24 to engage a row conductor 14 through an opening 22 in the spacer layer. External detection circuitry then senses the value of the capacitance produced and generates a signal identifying the switch location. Detection circuitry such as that described in my copending U.S. patent application Ser. No. 379,770 (incorporated herein by reference) can be used to detect which switch has been activated. 
     Other embodiments of the invention are within the following claims.