BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a capacitive touch switch to be used on control panels of domestic appliance such as cooking ovens, cooking hobs, microwave ovens or the like. The invention particularly refers to a capacitive touch switch comprising a control board and capacitive detecting means provided on a surface of the control board. 
         [0003]    2. Description of the Related Art 
         [0004]    With the term “switch” we mean every kind of touch sensors which usually replace the traditional buttons in modern flush user interfaces. 
         [0005]    Visual feedback activation of a touch switch control is simply provided by a light source, for example a light emitted diode (LED). Usually several touch switches are arranged, together with control board thereof, in a control panel. 
         [0006]    To avoid optical interference between adjacent touch switches, light flux shall be conveyed from a light source to the related touch sensitive switch surfaces. 
         [0007]    Opaque walls or light guide means are normally provided for this object. The use of opaque walls increases physical dimensions and renders the mounting quite complex. 
         [0008]    Light guides can be used as mechanical support for the electrodes. Electrodes can also be made of transparent conductive material (such as indium tin oxide), located between the light guide and the lower surface of the transparent cover under which the touch switch is placed, with an increase of the overall cost of the control panel. 
         [0009]    On top of the above illumination problems, there is the need of assuring a good mechanical connection between the touch switch and the transparent cover (usually a glass or plastic plate) under which the switch is placed. Up to now, for assuring such mechanical contact springs or conductive rubber elements have been used, which increase the overall cost and complexity of the single touch switch. Moreover technical solutions are known (for instance from US-A-2006/0243575) in which a support member, carrying electronic components, is separated from the transparent cover by a series of opaque septum-like partitions for creating physically delimited illumination channels. Even if such solution does not require light guides (light emitting diodes are placed on the support member directly), nevertheless it is quite bulky and of complex manufacturing. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an aspect of the present invention to propose a different structural arrangement of a touch switch with a somehow embedded light guide, in order to allow a reduction of physical dimensions of the switch, the use of normal metal electrodes, easy mounting process, and improvement on versatility for different symbols as painted objects. 
         [0011]    Another aspect of the present invention is to provide a touch switch that has an improved sensibility. 
         [0012]    A further aspect of the present invention is to provide a touch switch that can give a direct feedback to the user when a command has been duly received. 
         [0013]    A further aspect of the present invention is to provide a touch switch that is substantially invisible by the user, its presence being detected only when the user&#39;s finger touches an enabling switch. 
         [0014]    These and further aspects are obtained thanks to the features listed in the appended claims. 
         [0015]    The capacitive touch switch according to the present invention is adapted to be used in association with a glass ceramic surface used in cooking appliances. Due to its limited thickness, the touch switch according to the invention does not increase the overall thickness of glass ceramic cooking hobs. Moreover, it is adapted to be used on glass door for cooking ovens and/or microwave ovens where the user interface is integrated in the door itself (and not on a “fixed” user interface usually above the door, as in traditional ovens). This allows the designer to get a very clean and modern design, where the “buttons” are substantially invisible up to when the user decides to switch on the appliance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Further features and advantages of the present invention will be clearer from the following description given by way of non-limiting example, with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a cross section of the touch switch according to a first embodiment of the invention; 
           [0018]      FIG. 2  is a cross section of the touch switch according to a second embodiment of the invention; 
           [0019]      FIG. 3  is a cross section of the touch switch according to a third embodiment of the invention; 
           [0020]      FIG. 4  is a cross section of the touch switch according to a fourth embodiment of the invention; 
           [0021]      FIG. 5  is a view of a control panel using the touch switches according to the present invention, and showing the possibility to change configuration of the panel; and 
           [0022]      FIG. 6  is a view of four different configurations that can be obtained by starting from the same control panel. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    With reference to  FIG. 1 , a structural section for the arrangement of the touch switch and the light guide inside is shown. A planar light guide  5  is located under to the lower surface of the transparent cover  3 . 
         [0024]    Between light guide  5  and the transparent cover  3  there is a non-conductive layer of transparent paint  4 . 
         [0025]    A single side printed circuit board (PCB)  7  is attached to the lower side of planar light guide  5  by a no-conductive adhesive layer  6 . In this embodiment the PCB is single side, and a capacitive electrode  8  is shown on the lower surface of the PCB. A visual feedback to the user, when his finger  1  touches the sensitive area  2 , is given by switching on a led  9  (i.e. when the control board detects valid touch activation). A buzzer (not shown) can generate an acoustic feedback. The led  9  is mechanically and optically coupled to the light guide  5  on an L-shaped edge  5   a  thereof. Light generated by led  9  is conveyed inside the light guide (indicated by arrows in  FIG. 1 ). 
         [0026]    Cuts on light guide or different light conductivity avoids the possibility of light interference between adjacent touch switches. It is worth noting that the planar light guide  5  works also as adjunctive dielectric substrate. 
         [0027]    When the function related to the touch switch is OFF, the paint  4  impressed on the non-conductive layer does not result visible to the user through the transparent touch sensitive area. 
         [0028]    When the function related to the touch switch is ON, the paint  4  impressed on the non-conductive layer is lighted up by light diffused inside the light guide  5 . In this condition, the paint  4  results visible to the user through the transparent touch sensitive area. This is advantageous when the control panel is a unique monochromatic surface when the domestic appliance is switched off. The user interface and the display will light up when the user switches on the appliance or starts using it. 
         [0029]    Another possibility with a touch switch according to the present invention is to have a different degree of illumination for each touch switch depending on the actual status thereof. For instance, it is possible to light up certain touch switches with a low level of illumination, while some of them may be lighted up at an higher level of illumination in order to indicate to the user which are the switches which can be actually used. An higher level of illumination can be used also for giving the user an immediate feedback when he touches a switch. 
         [0030]      FIG. 2 , in which similar components have been indicated with the same reference numerals of  FIG. 1 , relates to a second embodiment of the invention in which an icon serigraphy  2   a  is placed on the touch sensitive area. A reverse mounted light emitting diode (LED)  9   a  is placed on an opening or cutout  12  provided in the single side PCB  7 , so that light can enter the light guide  5  from the bottom (with reference to an horizontal mounting of the touch switch). When the LED  9   a  is switched on, the user can detect the shape of the icon  2   a  (which for instance can indicate a specific function of the appliance). 
         [0031]    A third embodiment is shown in  FIG. 3 , which differs from the previous embodiment by a different placing of the icon serigraphy  2   a  that, in this case, is sandwiched between the transparent ceramic glass or plastic cover  3  and the light guide  5  (with the interposition of the adhesive layer  6 ). Also in this embodiment, as in the one of  FIG. 1 , a dark effect on the external transparent touch sensitive area is obtained when the switch is off (i.e. not illuminated). 
         [0032]    In the embodiment of  FIG. 4  the light emitting diode  9   b  is fixed to a side of a double side PCB  7   a  facing the light guide  5 . In this embodiment the light from LED  9   b  is entering an edge of the light guide  5 . 
         [0033]      FIG. 5  relates to a plurality of capacitive touch switches associated to a general printed circuit board  7   b  where the small pads or electrodes can be connected with soldered bridges or jumpers  14  in order to create a bigger pad from smaller pads. According to traditional technology, several available capacitive touch switches consist of a microcontroller unit having dedicated pins connected to related touch pads. Each pad is in most of the cases just a conductive area shaped as the desired activation area. The most common use is to build the touch system on a PCB where the pad is a copper area. Button position differentiation usually requires different dedicated PCB layouts. 
         [0034]    According to this further embodiment of the present invention it is possible to obtain, in a very easy way, a differentiation of the user interface in designing a new product family of domestic appliances. 
         [0035]    Core of this embodiment is to use a general PCB layout and by populating or not populating a number of 0 ohm resistors (jumpers) so that the board can be configured to obtain different key layouts. 
         [0036]    The general board is a matrix of smaller pads, and larger pads are accomplished by connecting two, or more, smaller pads. This is shown for instance in  FIG. 6 , where on the right side thereof it is shown how the single pads are connected together from a general configuration of PCB and, on the left side thereof, it is shown the overall result in term of four different user interface layouts that, according to the traditional technology, would need four different printed circuit boards. 
         [0037]    According to the features described above, with the touch switches according to the invention it is possible to obtain a physical reduction of the dimensions for the backside of the touch switch. Moreover the sensibility of the sensor, since the light guide works also as adjunctive dielectric substrate, is increased. The mounting process is simpler. There is an improvement on versatility for different symbols as painted objects. The use of transparent electrode (for example ITO) is not necessary. No particular arrangement (for example springs, rubber, etc. . . . ) is needed to achieve electrical and mechanical contacts. There is a sharp cost reduction of the overall solution (mechanics, optics and electronics).
A capacitive touch switch having a printed circuit board and capacitive electrode provided on a surface of the printed circuit board is disclosed. The printed circuit board is interposed between a transparent planar light guide and the electrode, the planar light guide being attached to a first face of a transparent cover whose second face is adapted to be touched by the user, a light source being connected to the printed circuit board and being able to convey light to the planar light guide.