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

Jean Bender lofideadbeat at gmail.com
Fri Jan 24 20:48:01 CET 2014

So much thanks for your answers...
Well, i get it working now, had some troubles with my breadboard..
well, breadboarding.

But i really appreciate your shared knowledge.. that's great !
Thanks Mr Snazelle, i'll be really happy to read these elektron papers...

Have a good day / night, depends where you are on earth !

2014/1/24 John Speth <jspeth at avnera.com>:
>> I'm turning around some schematics, and i found this one :
>> http://yusynth.net/archives/WirelessWorld/Chatterbox-1976.pdf
>> I'm not interested yet by the formant filters, but.. the touch
> sensitives
>> switches ( shown on P6, on the whole schema). I don't really figure how
> they
>> work. I've tried to breadboard one with my cd4049UBE, but it doesn't
> seem to
>> do anything.
>> I'm powering my breadboard with a 13,3v supply. With the bias built as
> shown
>> in the schematic, i can get around 6v at the CD4049 inverter input, when
> i'm
>> touching the points, it's going down, but i don't have nothing at the
> output.
>> Still at 0, touching points or not.
>> Could somebody explain me how it should work ? I mean, i think i get the
>> idea, but i can't see where i'm wrong. Maybe my component ?
> 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.
> The bottom line is the physical design is just as important as the
> electrical design.
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Jean Bender

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