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

John Speth jspeth at avnera.com
Fri Jan 24 20:23:48 CET 2014

> 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
> switches ( shown on P6, on the whole schema). I don't really figure how
> 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
> in the schematic, i can get around 6v at the CD4049 inverter input, when
> touching the points, it's going down, but i don't have nothing at the
> 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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