Keyboard Switch Assembly Having Flexible Rung Ladder Contacts

The invention relates to a keyboard for introducing data of the digital type or logical instructions into an instrument for processing or utilising such data such as office calculators or pocket calculators, keyboard arrangements for composing telephone numbers or any other pushbutton arrangement providing an instantaneous return and intended to establish electrical contacts which transmit an order or information.

Known keyboards of this type even if they provide satisfactory performance are generally of a relatively complex construction which places them in a high price level, causes them to occupy more volume than is desired and does not favor their utilization for arrangements where miniaturization is desired.

Swiss Pat. No. 545 991 has, however, suggested that in order to avoid such inconvenient features, a network might be employed comprising a single conductive plate in which elastic tongues have been cut out, these tongues forming moving contact organs having a snap action. The contact of a tongue with a conductive base corresponding thereto is established by an associated pushbutton. The proper functioning of this arrangement requires that such tongues be arched by a permanent deformation after they have been cut out and tempered, thereby causing their manufacture to be relatively complex and expensive. The arrangement of this network as a single piece has the disadvantage that in case of the failure of a single contact the entire network is rendered useless and must be replaced.

The invention comprises a keyboard in which the keys of pushbuttons are arranged in a matrix so as to actuate movable conductive elements in order to establish an electric contact with fixed conductive elements corresponding thereto, each movable conductive element comprising a rung of a conductive elastic foil member in the form of a ladder, cut from a flat foil and arranged according to a row (or column) of the matrix. The rungs are arranged to respond in a snap fashion to pressure externally applied on the pushbutton corresponding thereto so as to be brought into abrupt contact with their corresponding fixed conductors such contact being broken as soon as the pressure on the pushbutton is relaxed and the pushbutton restored to its initial position by the movable conductor. The ladders are introduced longitudinally into slots provided in an insulating support and arranged so as to give lateral support. They are slightly prestressed by transversal arching thereof at the moment of introducing them into the slots giving them by simple elasticity an initial bias enabling the snap action contact.

The design and manufacture are extremely simple due to the fact that when relaxed the ladders are perfectly planar and no permanent arching is initially necessary. Should there be failure of a contact, there is no necessity to replace the entire network but only the defective ladder serving a single row (or column) of the matrix.

For a better understanding of the description following, reference will be made to the drawings in which:

FIG. 1 shows schematically a transversal section of a keyboard according to the invention along the line of a row or column of the matrix,

FIG. 2 is a top plan view of the keyboard with the cover removed showing the arrangement of the ladders,

FIG. 3 is a plan view of a ladder in the relaxed position before assembly.

FIG. 4 shows a detail cross section of one of the contact arrangements.

From the cross section as shown in FIG. 1 and the plan view of FIG. 2 it will be seen that the keyboard is formed of a rectangular insulating support member 1 which may to advantage be made from an elastic material in which are arranged a plurality of identical parallel slots 2 having straight rectilinear sides and crossing from one edge support member to the other. At the bottom of each of these slots 2 fixed conductive elements are arranged at regular intervals in a manner so as to form an ordered matrix. These are in the form of studs or rivets 3 which traverse the bottom of the insulating support 1 and of which only the head is presented within the slot 2, the other extremity being coupled in a known manner to a data processing circuit (not shown) printed under the insulating support 1 for example by silk-screen methods.

The side walls of slots 2 are each provided at the same height from the bottom with a longitudinal groove 4 of triangular cross section. Each pair of grooves 4 belonging to a slot 2 thus forms a slideway which is intended to receive and to give lateral support to elastic metallic foil 5. Foils 5 may be punched from a planar plate having a length slightly less than that of the slots 2 and a width slightly larger than the distance separating the edges at the bottom of grooves 4 of each slideway. They are put into place in the corresponding slideway by simply pushing them after having elastically flexed them upwardly. This permits their introduction and lateral maintenance without play in the corresponding groove 4 and gives them a bias which permits a snap action. Before the insertion into the slideways 4 rectangular windows 6 are made at regular intervals by punching out foils 5 in a manner so as to form a ladder as shown in FIG. 3 having rungs 7 of small width. The rungs 7 are spaced along each ladder by a distance equal to that which separates the centers of conductive elements 3 at the bottom of slots 2. Rungs 7 comprise the movable conductive elements of the keyboard and, as taught in the aforementioned Swiss Pat. No. 545,991, may be connected to one side of a potential source (not shown).

On insulating support 1 is arranged a cover 8 which is likewise insulating and which covers the cavities formed by slots 2 from above. On cover 8 are mounted in a known manner pushbuttons 9 arranged according to a matrix identical to that of the fixed conductive elements 3 carried by support 1. As shown in FIG. 4 these pushbuttons 9 comprise preferably a cylindrical stem 10 transversing with slight play the cover 8 by a passage provided with a conventional O-ring seal 11 and a convex head 12 having a greater diameter than the stem 10 cooperating with the lower wall of cover 8 in order to limit in one direction the axial displacement of pushbuttons 9. These pushbuttons may be depressed in the other axial sense by a pressure exerted from the exterior on the end of their stem or key 10.

The arrangement and the respective dimensions of the fixed conductors 3, the rungs 7 and the pushbuttons 9 are such that at rest rungs 7 maintain through their elasticity the pushbuttons 9 in a non-depressed position. Under the effect of pressure exerted on a pushbutton 9, the corresponding rung 7 springs abruptly into contact with the conductive stud 3 associated therewith by the snap action shown in FIG. 4 and of which the operation is as follows:

With no force applied from the exterior to the key 10 of pushbutton 9 the pressure exerted on such pushbutton by rung 7 maintains it in stable equilibrium. This state is maintained when a pressure is applied on the key up until this pressure exceeds a certain threshold. The rung 7 begins then to be deformed in an asymmetric fashion in a vertical plane and takes a form resembling a reclining "S" greatly flattened (FIG. 4) the pushbutton then being deplaced towards the bottom. If the pressure continues to increase it ends by overcoming the opposing force exerted by the rung 7 and forces the latter to be suddenly deformed in order to arrive at a second position of stable equilibrium imposed by the prestressing and situated on the other side of the plane passing through the base of groove 4. Prior to reaching this second position of stable equilibrium rung 7 is stopped by conductive element 3 with which it comes into abrupt contact, this element having the tendency then to return it towards the initial position of equilibrium. The rung 7 remains however in contact as long as the pressure on pushbutton 9 remains sufficient. If such pressure is relaxed the contact immediately ceases, the rung 7 returns to occupy its initial stable position and brings the pushbutton 9 equally into its rest position.

It will be readily be seen that the ladder construction enables the actuation of each rung 7 or moving conductive element independently of the neighboring rung 7 carried on the same ladder 5. There thus exists no risk of establishing more than the contact which may be sought.

In a known manner each assembly comprising a key 10 or pushbutton 9, rung 7 and conductive stud 3 has associated therewith an item of numeric or alphanumeric data or a logical instruction such as an arithmetic operation.

This data or instruction will be transmitted by the arrangement and transformed into an electrical pulse train which the processing circuit will code and treat in an appropriate manner, this falling outside the scope of the present invention.

To assure a sufficient water resistance of the arrangement a seal may be arranged in a known manner between support 1 and cover 8. It is equally possible to avoid the employment of O-rings 11 equuipping each pushbutton 9 by utilizing a sealing membrane of elastic material covering the entire cover 8 and on which the position of keys 10 could be indicated by any appropriate sign.

The keyboard according to the invention is of extremely simple construction. The ladder form of the movable conductive elements enables a very rapid assembly of the entire arrangement and a failure of a single contact may be corrected by removing and replacing the single ladder corresponding without the necessity of disassembling the entire keyboard.