CONTACTLESS_KEYBOARD

Published:2009/7/13 4:29:00 Author:May | From:SeekIC

Touching one of 16 metal pads at inputs of 74150 multiplexor produces corresponding 4-bit BCD output from 74194 shift register. During scanning of multiplexer inputs by counter, output is produced only when finger of operator is on corresponding fingertip-size touch pad. Requires 10-kHz pulse from external source to strobe multiplexer and serve as clock for counter. Duration of clock pulse must be more than 20 ns so untouched pads charge up to threshold voltage but not long enough to let touched pad charge.-D. Cockerell, TTL IC Serves as Touch Keyboard, Electronics, Feb. 20, 1975, p 108-109.



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