Touch switch, was: Possibly goofy idea...

[Ingo Debus]

Scott Gravenhorst wrote:
> 
> Touch switches!  Didn't I read here that a CMOS gate input can
> work as a touch switch?  If so, are there biasing requirements?

Here's my circuit:

       Vdd
        |
        -
       | | 10MOhm
       | |
        -     1/6 40106              1/6 40106
        |        |\                     |\
o-------+--------| o--->|----+----+-----| o---o Output
Touch            |/  1N4148  |    |     |/
plate                        -    |
                            | |  ---
                    470kOhm | |  --- 0.1uF
                             -    |
                             |    |
                            GND  GND

The idea was, when the touch plate is touched, there's a high impedance
DC path to ground, so the first Schmitt-Trigger/inverter sees logic 0 at
its input. This requires that the person who touches the plate needs to
touch something at ground level too (this is how the AKAI Wind
Controllers work I think). However, since the input imedance is so high,
there so much mains hum that there's a nice 50 Hz rectangle at the
output of the first inverter as soon as someone touches the input in
most cases. This is what the second stage is for: as soon as the
rectangle at the first inverter's output appears the output of the
second inverter goes low. This even works without galvanic contact to
the touch plate: its sufficient to grab the insulated wire to the touch
plate with the hand (just touching it with the tip of a finger doesn't
work, though). So yes, coating the touch plate with insulating laquer
could work, if there's enough mains hum around... might not work when
the device is battery powered and operated outside ;-). It's not
necessary to touch something at ground level the same time. Perhaps its
necessary that the circuit's GND is connected to mains earth - didn't
try without that connection, since I used a scope to monitor the output.

I breadboarded the circuit first, this worked well. Small parasitic
capacitances don't seem to matter. I tried a 4049 instead of a 40106, this
worked too, but the 4049 has a strange pinout.

> ESD protection things?  Output conditioning?  Could I make the
> pads by etching a board and coating it with clear lacquer?

Well, I just forgot to do anything for ESD protection. Perhaps it's a
good idea to insert an 1 MOhm resistor in series with the input of the
first inverter.

Ingo