[sdiy] Need some advices... cd4049 touch switches

Ian Fritz ijfritz at comcast.net
Fri Jan 24 21:13:02 CET 2014


At 12:23 PM 1/24/2014, John Speth wrote:
>To my recollection, old style low cost touch switch circuits usually used
>4000 series CMOS inputs because of their very high input impedances.  Hi-z
>inputs are well known for their susceptibility to open circuit capacitance
>in switching the logic states.  Designers exploited that behavior by
>trying to bring the effect under control somewhat as what appears to be
>the case in the Chatterbox circuit with the huge 10M resistor and tiny 10
>pF cap.
>
>Your hand waving in front of it is additional capacitance (or field
>disturbance) that will flip the logic state IF all of the physical design
>is "just right".  The physical design must take into account everything in
>the physical area that will be part of the electric field (including
>humidity, etc!).  I think touch plates can be designed to bring that under
>control.  Touch plates have been easily implemented in modern circuit
>board designs.
>
>Remember that the CMOS chips from the 1970s were probably a lot more
>cooperative in terms of static capacitance for touch interfaces than they
>would be on CMOS chips from later years.  It appears you knew that because
>you tried the UBE variant.


If you read the text you will see that the switch is intended to be 
resistive, not capacitive.  Dry skin often prevents resistive switches from 
operating.  Try rubbing some kind of lotion into your skin and see if that 
helps.

Ian 




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