On Mon, 14 Dec 1998, Paul B. Webster VK2BZC wrote:

> Peter L. Peres wrote:
> 
> > The usual way is to connect the metal plate through a high value
> > resistor to a diode-protected CMOS input.  With a PIC input and 220V
> > mains, 2M2 will work.
> 
>   I'd make that two resistors in series.  I am not happy with the
> thought or one little resistor 'twixt me and the mains.

The R must be rated 400 V at least. This means, 0.5W or larger. I use 1W I
think. The resistor connects the touch plate to the board (i.e. it is the
'wire') and is dressed in shrink tubing. 

> > A capacitor is also used in series with the plate and resistor
> > sometimes (0.1 uF PMP 400Vdc will be enough).
> 
>   More than enough!  Do you mean on its own, as the limiting/ protective
> device?  0.1 µF will pass 7.5mA which would be quite a tingle.  I'd have
> picked 0.0047 µF or so.

0.1 uF is a value we stock up on... This is in series with the R of
course.

> > Since the PIC input was never meant to be used like this, it is good
> > to give it an initial DC polarization. I use a 47Meg resistor from the
> > PIC input to GND for this.
> 
>   The internal diodes will do a better job of controlling teh input
> range.  Isn't this just to bias the input and reduce spurious
> triggering?

It sets a defined input voltage on the input pin and prevents it from
drifting into the forbidden zone and causing extra power drain etc.

Peter