Electricity: circuit makers and breakers – Electric switch details – Contact
Patent
1995-08-02
1997-02-25
Walczak, David J.
Electricity: circuit makers and breakers
Electric switch details
Contact
200243, 200239, H01H 106
Patent
active
056052235
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an arrangement of contact surfaces for switches, pushbutton switches and keyboards.
Switches, pushbutton switches or keypads, in particular entry keyboards for data processing systems, are often constructed on carrier boards. In this case, it has proved to be advantageous to use a printed circuit board as the carrier board and to implement fixed key contacts as specially formed conductor tracks, so-called contact surfaces. When a key is pressed down, a contact bridge, which is often made of conductive rubber, is pressed against the contact surfaces by the movable part of the key.
An arrangement of two adjacent semicircles as in German reference DE A 37 14 382 is already used for the contact surfaces. In this case, the contact surfaces composed of copper are coated on the surface, because otherwise any possible corrosion would prevent reliable contact-making. A gold layer which is applied by electroplating is used as the surface coating. In order to avoid the high production costs of gold plating, use is also made of a conductive lacquer which is applied, for example, by screen printing. However, the contact-making of this arrangement is not reliable enough for keyboards which have stringent requirements placed on the contact reliability. Therefore, use is also made of a form as in U.S. Pat. No. 4,818,828, in the case of which the contact surfaces engage into one another in the form of fingers. However, in this form it is possible to carry out the surface coating by means of conductive lacquers using screen printing only when there are few fingers or large key spacings, since the meandering interspace between the fingers would have to be so narrow that the tolerances of the printing method for the conductive lacquer would not permit reliable production.
SUMMARY OF THE INVENTION
The object of the invention is to implement contact surfaces in such a manner that conductive lacquers can be used as the surface coating and better contact-making is achieved in comparison with the form of semicircles.
The preferred embodiment of the invention uses a contact zone of four contact surfaces at the corners of an imaginary square, in each case two diagonally opposite surfaces being electrically connected to one another. In this way, respectively adjacent contact surfaces are connected to different poles of the switch. Therefore, contact is always made when the contact bridge touches two adjacent contact surfaces. Therefore, as illustrated in FIG. 4, a connection is already established even in the event of incomplete key actuation, when only that part of the contact bridge which is drawn in bold is effective, whereas this is not the case, under the same conditions, in the case of the known semicircle arrangement according to FIG. 3. As a result, the contact-making reliability is considerably improved because the number of possible contact-making surfaces is doubled. In the case of the diagonal form according to the invention, the contact surfaces can be dimensioned in such a way that they can be produced, as in the case of the known semicircle arrangement, from an electrically conductive lacquer, preferably a carbon lacquer, because the spacings are large enough to tolerate inaccuracies of a simple printing method. They are applied over, and so as to overlap, copper pads, from which the connection is carried out by means of a suitable printed circuit board track configuration.
An advantageous development is to connect diagonally to one another a pair of contact surfaces within the contact zone. This connection contributes to the contact-making; in addition, only one electrical connection routed around the contact zone is now necessary.
The contact surfaces preferably have a square configuration, but they can also be implemented as segments of a circle. Likewise, more than four, therefore six, for example, contact surfaces are possible.
Since the contact surfaces can be applied using devices which are present in any case for every printed circuit board production pro
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patent: 4987275 (1991-01-01), Miller et al.
patent: 5471021 (1995-11-01), Malesko
"Gummischaltmatten CRS Produkt Informationen", ITT Shadow, pp. 5, 6, 7, 10 and 11. No date.
"Konstruktions-hinweise, Kontaktmatten, Schaltelemente aus Silikonelastomer fur die Elektronik" Carl Freudenberg, pp. 12 and 13. No date.
Gutmann Klaus
Luckehe Hans-Werner
Siemens Nixdorf Informationssysteme AG
Walczak David J.
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