Durkee Gilbert H, Wareberg Per G, Yoder Alan C: Calculator keyboard switch with disc spring contact and printed circuit board. Bomar Instrument Corporation, March 12, 1974: US3796843 (47 worldwide citation)

A keyboard switch assembly including a printed circuit board having four switch terminals on one side of the board, three of the terminals being arranged in a triangle and the fourth disposed within the triangle. Conductors on the one side of the board are respectively joined to the interior termina ...