Push button contact mechanism for use at printed circuit cards
BACKGROUND OF THE INVENTION 
     The present invention relates to a push button contact mechanism intended for use at so called printed circuits provided on thin boards. Such push button contact mechanisms are used, for example, in push button sets as key senders for telephone sets. These push button sets, which are intended to replace the conventional dial, must not be bulky and the construction of the contact mechanism, therefore, presents a problem, especially as the space available for the contact springs is limited. 
     In a known construction according to the Swedish Pat. No. 223 147, contact springs are used which are located on supports projecting from the circuit board and which have movable contact ends arranged substantially perpendicular to the board. This construction of the contact springs has the advantage that the flexible length of the spring when actuated can be made greater without the spring occupying substantially greater space. The drawback with this construction of the contact springs is, however, that the manufacture and the mounting can be detailed and unnecessarily complicated since the contact springs are formed as coil springs. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a push button contact mechanism in which the contact springs upon action show a greater flexible length, whereby space can be saved together with a simplification of the manufacture of the spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described more fully with reference to the accompanying drawings, in which 
     FIG. 1 shows an exploded view of a switching portion of the push button contact mechanism according to the invention; 
     FIG. 2 shows in a top view part of the contact mechanism according to the invention in which the contact spring is in the mounted position; 
     FIG. 3 shows the same part as in FIG. 2 seen from the side, 
     FIG. 4 shows the same part as in FIG. 2 seen from the front and in addition a push button; 
     FIG. 5a-c show the positions of the contact springs at three different phases of the push button movement; 
     FIG. 6 shows a set of contact springs according to FIG. 1; separately, 
     FIG. 7 shows a set of contact sheets separately included in the contact mechanism according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the exploded view according to FIG. 1, a base plate 1 of insulating material is shown, for example plastic which carries a push button contact mechanism. The base plate can be common for a number of push button contact mechanisms, for example a line of four. A push button 10 is provided at its central axis with a hollow cylindrical part 12 to be displaced downwards around the broader part 6b of cylindrical post 6 which is moulded on a depressed plane part of the base plate 1. A guide 5 is moulded closely to the post 6. A helical spring 13 is so dimensioned that it can be threaded into the hollow cylindrical part 12 and bear against the plane part between the narrower part 6a and the broader part 6b of the post 6. Also at the central axis of the button 10, an enlarged part 11 for actuating the contact springs is provided. The enlarged part is preferably formed as a curve, whereby insensitivity against turning of the push button is obtained. In its non-actuated position, the push button 10 is guided laterally by the cover plate of the key set (not shown). When actuated, the position of the button 10 is, at the beginning fixed by means of the guide 5 and upon continued actuation, the guiding is taken over by the post 6 in such a way that the inner envelope surface 12 slides against the broader part 6b of the post 6. When the button 10 has been completely pushed down, the part 12 will abut against the depressed part of the base plate. The helical spring 13 is then in its compressed position by its bearing against the plane part of the post 6 and, therefore, when the pushing actuation ceases, the button 10 will, by spring action, return to its starting position. 
     The contact springs 7 are, as it appears from FIG. 1, manufactured as leaf springs, a contact spring 7 being connected with adjacently situated contact spring by means of a strip 8. Each of the contact springs consists of two legs 7a, 7b and a central tongue part 7c connected between the two legs. The middle portion of the tongue part is formed as an inwardly curved bent part 7d and the lower portion of the part consists of two smaller tongues 7e, which each at their lower portion carries a contact 7f intended to cooperate with a fixed contact 9a on a contact sheet 9 provided in the notch 4 on the base plate 1. 
     As it is shown by the arrows, each of the contact springs 7 is pushed down into the base plate 1 in such a way that the strip 8 between two adjacent contact springs falls along the groove 3 and the lower portion of the legs are jammed between two guide elements 2a, 2b, the tongues 7e being situated right in front of the notch 4. The contact springs are placed towards each other so that one bent part 7d faces the other. The contact sheet 9 consists of a broader portion on which a contact 9a is provided and a long and narrow portion which constitutes a soldering tag when the base plate is to be soldered on an underlying circuit card. 
     FIGS. 2 and 3 show in a top view and from the side the base plate 1 when the contact springs, combined by means of the strips 8, and the contact sheets 9 are mounted in a longitudinal groove 3 of the base plate and in notches 4, respectively. 
     In FIG. 4, in a front view, the base plate 1 is shown with mounted contact springs and contact sheets together with the push button 10 and helical spring 13. The inner outline of the hollow cylindrical part 12 is indicated by the dotted lines which define the space for the spring 13. The bent part 7d of the tongue part 7c of the contact spring serves as an actuation surface for the actuation element (push button) and the part 7c as well as the legs 7a, 7b will when infuenced, be forced outwards, the contacts 7f on the smaller tongues 7e thereby making contact with the contact sheet 9 in the notch 4. The actuation means is constituted by the curved portion of the enlarged part 11. The process at actuation will now be described more fully according to FIGS. 5a-5c. 
     FIGS. 5a-5c show three different phases when pushing down a button. In the phase according to FIG. 5a, the button 10 has been pushed down a bit so that the curved portion of the enlarged part 11 of the button is situated close to the bent part 7d of the tongue 7c of the contact spring, but no force on the spring 7 has yet occurred. In the phase according to FIG. 5b, the curved portion has touched the bent part 7d and thereby has started to force the tongue 7c to spring a bit outwards, but the influence of the contact spring is not yet such that the two legs 7a, 7b have started to spring outwards. The contact profiles 7f of the contact spring 7 and the contact sheet 9 have not yet come into contact with each other. In the phase according to FIG. 5c, the push button has been pressed so that the envelope surface of the enlarged part 11 slides against the bent part 7d and also the two legs 7a, 7b of the contact spring 7 spring outwards. Thus, by making both the tongue 7c and the legs 7a, 7b to spring outwards, a double spring action of the contact spring 7 is obtained. The contact profiles now contact each other, but just at the beginning of the contacting the contact profiles 7f and 9a will, during a small time interval, slide against each other, and for that reason eventual oxidation on these parts can be removed. In FIGS. 5a-5c, for the sake of simplicity, the different phases are shown only for a pair of contact springs-contact sheets, but it is understood that the same process will occur at the opposite situated pair on the base plate 1. 
     The contact springs are manufactured from endless strips from which a desired number of combined contact springs are stamped and, being connected together, are mounted on the base plate 1 included in the push button set. FIG. 6 shows a set of such connected contact springs, for the FIG. 4 pieces intended to be included in a push button set which consists of 3×4=12 buttons. In FIG. 7, four contact sheets 9 are shown stamped from a strip, the sheets of which in connection with the mounting on the base plate may be cut to obtain individual sheets as shown in FIG. 1. The contact profiles 7f, 9a consist of, for example, cut gold wire, the wire being mounted on the contact spring along the two taps 7e on smaller tongues 7e of the tongue 7c and horizontally on the upper part of the contact sheet 9. The contact springs and the contact sheets may be individual or may be connected along the base plate. The contact sheets as well as the contact springs are, during the mounting, combined in strips, whereby it is achieved that the same automatic mounting principle can be used. 
     The proposed design of the contact springs and contact sheets gives the advantage that the spring as regards spring action is twice as long as allowed by the space, which implies lower stress on the spring compared with a single spring. Furthermore, greater contact sliding is obtained owing to the design. The complete mounted base plate can, by means of the contact sheets formed as soldering tags, be soldered directly on an underlying circuit card.