GB1399251A - Electrical signal generators - Google Patents

Abstract

1399251 Push-button switching systems POST OFFICE 19 May 1972 [19 May 1971] 15775/71 Heading G1N [Also in Division H1] A variable capacitor functioning as a signal generator comprises a pair of plates the separation between which is varied by actuating a pushbutton or the like, a metallized electret being mounted with its metallized surface in contact with one of the plates as shown, a hermetically Sealed housing is constituted by insulating base plate 11, rigid plastics wall 10 and resilient plastics plate 3 having a rim 5. Actuation of pushbutton 1 through stem 4 on the convex portion of metal member 6 constituting one of the capacitor plates deforms it to a concave arrangement with an audible indication. The resilience of member 6 causes the return movement on releasing the pushbutton. As shown electret 13, consisting of h.t.f.e., f.e.p., or polypropylene has its metallized surface in contact with pick-up electrode 12. The electret may alternatively be on the underside of the moving plate, or the pick-up electrode may be movable with the electret on the stationary plate. In a further alternative embodiment the electret material is applied to both surfaces of a thin metal sheet located centrally between parallel pick-up conductors, either the electret or the pick-up conductors being movable. As shown plate 6 is connected to earth via one output terminal 17 and the other output terminal 16 is connected to pickup electrode 12. Output terminals 16, 17 are connected across the gate and source of MOS NOR gate 18 acting as an impedance transforming buffer interface between the signal generator and a monostable multi-vibrator 19 to which the drain is connected. Diode 20 ensures that the multivibrator conducts on either the leading or trailing edge of the wave form of the drain pulse of interface 18. A plurality of signal generators may be c onstructed as a keyboard as described in Specification 1399252, or the actuation means may be coupled to a cam to generate coded signals. Alternatively a plurality of generators may be used in traffic controllers or burglar alarms.