Well, excuse me if I submit again this question, maybe I haven't well =
explained this problem... :-((

I'm experimenting with a common LM358 OP-AMP arranged as a squarewave =
oscillator (as example in the datasheet, see below). My idea is to use a =
fixed value for C, substitute R1 with two electrodes and to use this =
circuit as an inexpensive sensor  to check water conductivity.
The problem is that I don't obtain a  linear variation of the frequency =
as expected, and this non-linearity becomes remarkable when I change =
from 2.2K to 1K Ohm (or less). =20

Is it correct that the frequency values aren't linear?
I imagined a frequency formula "similar" to this:    F =3D 1/(1.4*R*C)

These are the values recorded (supply =3D ~ 5.2V):

R1 (Ohm)  Freq.(Hz)
15000         745
10000       1092
4700         2220
2700         3500
1000         7360

And this is the circuit:

              R1 (probe)____
                   +---|_____|---------+=20
           C 47nF  |                   |
               ||  |    |\ -           |            ____     ____
         +-----||-------| \  1/2 LM358 |           |    |   |    |  =20
         |     ||       |  \_____ _____|__  Vo   __|    |___|    |__
    GND  |         +----|  /           |
        ---        |    | /            |
                   |    |/ +           |
          _____    |    _____          |
  V+  ---|_____|---+---|_____|---------+
                   |
       R2 100K    --.   R4 100K
                 |  |=20
                 |  |
                 |  |  R3 100K
                  --
                   |
             GND   |
                 --+--              =20

If anyone has a suggestion...
thanks

Marco



--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu