Re: modifying a circuit design for a touch switch

On a sunny day (Sat, 10 May 2008 10:26:27 -0700 (PDT)) it happened
markus.kayser@xxxxxxxxxxxxxxx wrote in
<d187fb6e-9e2d-411b-8077-59c1162ab0f7@xxxxxxxxxxxxxxxxxxxxxxxxxxx>:

There also is a space restriction to theswitchso it should be made
from as little parts as possible.

2 parts:

        C1           C2        
           touchhere
       ------------------- copper
=========================
====================== thin PCB
       -------    -------- copper
         |            |
         |            |
         |osc         |
        --------      |      
out    |       |comp  |
  <----| PIC   |----- |
       |       |      |
        -------       R
          |           |
         ///         ///

This is a capacitivetouchswitch.
The advantage is that there is no physical contact with the circuit if you=
touchit.
The PIC runs on the internal osc.
It can be a small 8 pin PIC, one with a comparator or ADC.
The internal osc is output at 'osc', and via the capacitance C1 and C2,
between top and bottom PCB, interrupts the PIC constantly.
The interrupt routine resets a counter that is incremented in main.
When one touches the PCB top side, then the RF clock is shorted,
and the interrupts stop, the counter reaches a preset point,
and the PIC will flip 'out'.

I have not tried this cicuit, it is just a minimal parts idea.
There are of coursetouchswitchchips, with more then one switches in
a package.



hi
thanks for these suggestions!

You are welcome.


i am sorry but i'm not very experienced in electronics and have
problems understanding the diagram and the description.

Well, the 'PIC" is a Microchip micro controller.
As you perhaps have no experience with those, it would
be better to look for an other solution.


if you could clarify what you mean (for stupid) that would be great!

Not you fault, the terms refer to programming techniques for those
micro controllers.
So you'd have to program one....


the object which this touch switch needs to go into has got a metal
plug-socket anyway so i would not mind using this as the touch plate
but if it would work without it ...that would be fine as well.
one more important thing is that it works with three AAA rechargeable
batteries, so 3.6v.

Oh, PIC will work on 3 V upwards.

Here is a more generic description, avoiding micro controllers:

optionally
your finger
|
oscillator ---- capacitor ----- capacitor ---- detector ---- D flipflop ---- load
A

The oscillator generates a high frequency (MHz), that is normally
passed via the 2 capacitors in series to the detector, causing a voltage
at the detector.
As the capacitors are very small in value, but sufficiently big
to pass the RF, touching point 'A' short the RF via the much bigger capacitance
of your body to ground.
The detector voltage will drop, and the D flip flop will trigger, switching
the load on or off.

Up to you now to design the circuit :-)
The oscillator could be a Schmit trigger NAND gate, the D flipflop a 74HC74,
detector a diode + cap + resistor (time constant).
The capacitors can of course be small capacitors of a few pF with some kV
voltage spec, does not need to be PCB oriented.

Hope this helps.






.

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