Mechanism for preventing the simultaneous locking for use in a push button device
BACKGROUND OF THE INVENTION 
     The present invention relates to a push button device equipped with a plurality of push buttons, and more specifically, to a mechanism for preventing simultaneous locking of more than one pusn button in a device having a plurality of push buttons. 
     Electric circuits often operate erroneously when two or more push buttons of a push button device having a plurality of push buttons are inadvertently actuated at one time, particularly when the push buttons are of the type locked in their actuated condition. Consequently, such a locked-type push button device should be so constructed that all of the buttons actuated return to their initial state and do not lock whenever two or more push buttons are simultaneously actuated. 
     Lock-type push button devices have been proposed for use as the channel selector employed as a tuner for modern television sets since conventional channel selectors typically involved clumsy operation for selecting the channels, particularly when the channels are to be selected in the UHF band. 
     FIG. 1 is a diagram illustrating the appearance of a channel tuner employing push buttons for selecting various channels. After a switch 1 for selecting the appropriate band has been set to a desired band, one of the push buttons 2 for selecting the channels is actuated to select a desired channel easily. With channel tuners of this type, however, the many number of channels in the VHF, UHF, and perhaps, CATV bands require increased number of push buttons arrayed with only a small distance between them. Therefore, unless they are actuated carefully, neighboring push buttons will often be depressed simultaneously and become locked, and the desired channel would thus not often be selected properly. 
     With the locked-type push button device for use in channel tuners, therefore, the push buttons should not be locked but should return to the non-depressed position when two or more push buttons are simultaneously depressed to prevent erroneous operation or erroneous selection of a channel. 
     Conventional channel tuners employing push buttons have not typically been provided with structures preventing simultaneous locking of two push buttons. Devices for preventing the simultaneous locking of two adjacent switches used in other fields are complex in construction and their increased manufacturing costs make them prohibitive for use in channel tuners. 
     SUMMARY OF THE INVENTION 
     The object of the present invention therefore is to provide a mechanism for preventing the simultaneous locking which is based primarily upon the push button arrangement, which is, therefore simple in construction, and has an advantageous manufacturing cost, so as to be employed easily for the channel tuners. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating the outer features of a channel tuner embodying the present invention; 
     FIG. 2 is a perspective view illustrating, in a disassembled manner, a push button device employing a mechanism for preventing the simultaneous locking according to this invention; 
     FIG. 3 is a cross-sectional view of the device of FIG. 2; and 
     FIG. 4 is a diagram illustrating the forces which act upon a cam plate of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention will now be explained below in conjunction with the accompanying drawings. 
     Reference numeral 11 denotes a push button made of a plastic material, which consists of an operation portion 11a, a generally L-shaped member 11c extending in an inverted manner from the rear of the operation portion and having a shaft portion 11b which serves as a pivot for rotating the operation portion 11a, a contact depressing projection 11d extending from the bottom of the operation portion to depress a contact, as will be described below, and a cam-engaging projection 11e extending from the lower front portion of the operation portion and which can depress a cam plate as described below to move it out of engagement with any previously depressed push button when a push button is depressed. Reference numeral 12 denotes a leaf spring made of metal and having spring elements 12a formed in a V-shape, and holes 12b through which the contact depressing projections 11d of the push buttons 11 can penetrate. The spring elements 12a come into contact with the under side of the L-shaped member 11c of the push button 11 to urge the corresponding push button 11 in a direction opposite to the depressing direction. Reference numeral 13 denotes a cam plate which is commonly provided for the push buttons. Holes 13b for mounting the frame are formed in the lower portion of both ends, and at the center of the base plate 13a. On the upper portion of the base plate 13a are formed cam surfaces 13c each corresponding to a respective push button 11. The cam surfaces, however, may be formed in a continuous form. 
     When the push button 11 is depressed, the cam surface 13c is urged outwardly by the cam engaging projection 11e of the push button and the cam plate 13 starts to rotate. As the push button 11 is further depressed, the cam engaging projection 11e engages with the cam surface 13c. At this moment, the cam plate 13 is urged upwardly by the resilient force of coil spring 14 attached to the frame, and the push button 11 is locked or latched in the depressed state. 
     The coil spring 14 works to urge the cam plate 13 in direction opposite to the direction depressed by the push button 11, to maintain the engagement between the cam-engaging projection 11e and the cam surface 13c, and to hold the depressed button 11 in the depressed state. Reference numeral 15 denotes a contact having V-shaped contact portions 15a each corresponding to a respective push button 11. When the push button 11 is depressed, the contact depressing projection 11d depresses the corresponding contact portion 15a via the hole 12b in the leaf spring 12 to perform the switching operation. Reference numeral 16 denotes a frame having shaft supports 16a for rotatably supporting the shaft portions 11b of the push buttons 11, a leaf spring mounting portion consisting of plate-like projections 16b which engage with the notches 12c of the leaf spring 12 and leaf spring support portions 16c. A cam plate mounting portion which consists of projections 16d formed on the side wall of the frame and projections 16e that will be inserted in the holes 13b of the cam plate 13 is also provided and rotatably supports the cam plate 13 by these projections. A contact mounting portion 16f is provided for fastening the contact point 15. 
     The mechanism for preventing the simultaneous locking, i.e. latching of a push button, will now be explained below with reference to FIG. 4. 
     FIG. 4 is a diagram illustrating the forces which act upon the cam plate 13. Here, the force of the coil spring 14 which urges the cam plate inwardy is denoted by F 1 , and the force produced by the leaf spring 12 urging the push button 11 upwardly is denoted by F 2  &#39;. The component of the resulting forces in the direction opposite the force F 1 , i.e., the force of the cam engaging projection 11e as it urges the cam plate 13 outwardly, is denoted by F 2 , the frictional force between the cam surface 13c of the cam plate and the cam engaging projection 11e is denoted by F 3  &#39;, and the component in the direction of the force F 1  is noted by F 3 . According to this invention, the force F 3  &#39; is determined so as to satisfy the following relations (1) and (2): 
     
         F.sub.2 &lt;F.sub.1 +F.sub.3                                  (1) 
    
     
         2F.sub.2 &gt;F.sub.1 +F.sub.3                                 (2) 
    
     If the forces F 1  and F 2  remain constant, the shape of the cam surface 13c of the cam plate 13 is so determined that the force F 3  resulting from the friction will satisfy the above relations (1) and (2). Therefore, when only one push button 11 is depressed, the leaf spring 12 is not capable of expelling the cam plate 13 (relation (1)); the push button 11 is locked in the depressed state. When two or more push buttons 11 are depressed, the resilient forces of the two leaf springs expel the cam plate 13 (relation (2)); the push buttons are not held in the depressed state but are all allowed to return to the non-depressed state. 
     As mentioned above, this invention provides a mechanism for preventing the push buttons from being simultaneously locked, which is very simply constructed requiring reduced manufacturing cost.