> >> I have an application where I need to sense four touch pads with a PIC.
> ...
> There was an article, in Elektor I believe, which covered just this
> application. The author used CMOS discrete logic ...

I remember two different solutions published by Elektor, both using
discrete CMOS logic:


The first approach used a circuit like this:

+---------------+        C1a  C1b +-----------------+
|1MHz square    |---*----||-*-||--| Signal detector |--- out1
|wave oscillator|   |       |     +-----------------+
+---------------+   |       O
                    |  Touch pad (key1)
                    |
                    |    C2a  C2b +-----------------+
                    *----||-*-||--| Signal detector |--- out2
                    |       |     +-----------------+
           to other v       O
              keys     Touch pad (key2)

In normal conditions, the 1MHz square wave goes to the detector
through CXa-CXb. When someone touches a pad, the capacitance of
the human body short circuits the signal, and so the revelant
detector sees no signal. This circuit is simple, but requires
one detector (two inverters and some discrete components) for
each key.


In the other circuit the keypad is organized as a matrix, with
column directly connected to a digital logic, and rows to a
pulse detector. A short pulse is sent to each column (one at
time). If a key is touched, the amplitude of the pulse from
the column to the row is reduced at the point that the pulse
detector do not trigger. The keys are etched on a double sided
PCB: for each key, the top side is a square area of copper (may
be covered by an adhesive plastic sheet to avoid oxidation), and
the bottom is divided in two rectangular areas, connected to the
Row and Column lines (a key is composed pratically of two capacitors
in series):

SIDE VIEW                     TOP VIEW       BOTTOM VIEW
                               +----+          +----+
top     _______                |    |          |    | row
========___=___==== pcb        |    |          ======
bottom   T   T                 |    |          |    | col
        row col                +----+          +----+


The circuits appeared in the italian edition of Elektor, respectively
on the number of July/August 1979 (AC sensor) and November 1982
(capacitive keyboard), so I think you can find them in the international
edition some month before.

As Hugo Ahrens wrote, Elektor have the patent rights on the circuits
published, so you *can't* use them for commercial products.