Desk-Top Calculator Keyboard Switch

1. Field of the Invention

This invention relates to an electronic desk-top calculator and more particularly relates to an improvement in the structure of such type of electronic desk-top calculator which is small and thin in size as well as simple in structure.

2. Description of the Prior Art

The calculators of this type according to the prior art are so structured that pairs of contacts and wiring conductors associated therewith are provided on a rigid printed circuit board particularly prepared for this purpose and the pairs of contacts are, upon actuation, brought in contact with movable contacts provided over the pairs. Therefore, in the prior art calculators, key signals have to be led out from the printed circuit board, which necessitates connectors at connecting portions between the circuit board and the display device of the calculators. Moreover, electronic parts have to be mounted on the circuit board per se. Since this structure requires soldering, electrically conductive bonding agents and the like, the manufacturing process of the calculators inevitably becomes complicated.

Accordingly, it is a general object of the present invention to eliminate the above described disadvantages involved in the conventional electronic desk-top calculators.

A more specific object of the present invention is to provide an electronic desk-top calculator of the above mentioned type in which a plurality of point-like contacts are provided on a bottom casing of the calculator keeping the contacts insulated from each other and a flexible plate is disposed above the contacts, with flexible plate pairs of contacts and wiring conductors associated therewith.

A further object of the present invention is to provide an electronic calculator of the above mentioned type which has a keyboard in which electrically conductive paints are advantageously used.

Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings illustrating the preferred embodiment of the present invention.

FIG. 1 is an exploded perspective view of an electronic desk-top calculator in accordance with the present invention; and

FIG. 2 is a sectional view of a portion of the calculator shown in FIG. 1.

Referring now to FIG. 1, reference numeral 1 designates a bottom casing of an electronic desk-top calculator according to the invention. The bottom casing 1 is made of a suitable organic material such as plastics and has therein a battery receptacle 1a integrally formed therewith by molding. On the inner surface of this bottom casing 1, there are disposed a number of point-like fixed contacts 2 which are insulated from each other and arranged in rows and lines of a matrix. These point contacts may be formed by screen printing or off-set printing with electrically conductive paint. Also, a stamp printing method using a rubber plate, or a spray method employing a stencil or roller method may be suitably used to form the contacts 2. Alternatively, the contacts 2 may be formed using any of the above mentioned methods on an insulating, flexible plate overlying the inner surface of the bottom casing 1.

Reference numeral 3 designates a spacer having therein a number of openings 4 in registry with the point contacts 2 on the inner surface of the bottom casing. The spacer 3 may be made of any suitable insulating material such as rubber or plastics.

Reference numeral 5 designates a flexible plate made of polyester, nylon, ethylene propylene rubber or chloroprene rubber. The under surface of the flexible plate is coated with electrically conductive paint by using any of the above described thick film printing methods so as to provide on its surface a number of pairs of contacts and 6A.sup.1, wiring terminals 6B and wiring conductors 6C. Each of the terminals 6B is electrically connected with the associated one of the contacts 10A on a circuit board 10 by using a suitable connecting method such as pressure welding or conductive bonding. On the base plate 10, there are further provided a display device D, operational circuit elements L1 and L2 and other necessary elements.

The function of the fixed contacts 2 is only to make electrical connections between the associated pairs of the contacts 6A and 6A.sup.1 when actuated. Therefore, in order to form these fixed contacts 2 on the inner surface of the bottom casing 1, there may be advantageously used such conductive paint which is prepared by dispersing inexpensive carbon particles with relatively high electric resistance. On the contrary, in order to form the electrical members 6A, 6A.sup.1, 6B and 6C, it is desirable to use conductive paint having a small resistance value prepared, for example, by dispersing conductive particles of metal, such as silver or copper. Otherwise, some deterioration of signals may be caused by the increase in resistance of the circuits because of relatively long electric paths required for the members 6A, 6A.sup.1, 6B and 6C.

Moreover, with the exception of gold, metal is generally apt to be subjected to oxidation and sulfation which will in turn give rise to an increase of electric resistance thereof. For the reason, if the members 6A 6A.sup.1, 6B and the like formed by a coating of conductive paint containing metal particles remain exposed, their contact resistance will be gradually increased. Therefore, to obviate such a problem, it is desirable to overcoat the members 6A 6A.sup.1 and 6B with conductive paint containing carbon particles so as to form a protecting layer 61' therefor. While the same overcoating with the carbon-containing conductive paint may be applicable also to the wiring conductors 6C, it is rather preferable to apply a print-coating of moisture-resisting and insulative paint 5A onto the wiring pattern of conductors 6C or to all over the area of the surface except the contacts 6A and 6A.sup.1 terminals 6B. By doing so, the direct exposure of 6C to the atmospheric environment can be prevented and, therefore, any possible oxidation and sulfation of the conductive particles can be prevented. This is also effective for preventing silver migration which would often occur particularly in the case of silver paint.

Reference numeral 7 designates a rubber plate having a number of domes 8. As best seen from FIG. 2, the rubber plate 7 functions as a returning spring for key tops 9.

Reference numeral 11 designates a top casing for the electronic desk-top calculator.