A circuit for a touch keyboard

1,134,704. Typewriters &c. keyboards. INTERNATIONAL BUSINESS MACHINES CORP. 12 Feb., 1966, No. 6315/66. Heading B6F. [Also in Division H3] A circuit for use in a " touch keyboard " comprises a capacitor 6 arranged to be charged via one path 8 and discharged via another path and provided with means for blocking the discharge path so that a charge builds up on the capacitor 6. As described, capacitor 6 tends to charge via diode 8 during positive half-cycles of a positively-biased square wave at terminal 5 and tends to discharge via diode 10 during negative half-cycles at 5. The introduction of a capacitance 4 of an operator touching a terminal 3 or a dielectric applied thereto, will result in capacitance 4 charging via diode 11 during positive half-cycles at 5 so as to block diode 10 and thus prevent discharge. The voltage on capacitor 6 thus builds up incrementally until the cold cathode tube 2 becomes triggered, illuminating the key associated with terminal 3. In Fig. 3 (not shown) anti-phased or other square waves are applied to separate terminals associated with resistors 7, 9.