# high resistance switch question

Discussion in 'Electronic Design' started by Hal Leemux, Aug 6, 2004.

1. ### Hal LeemuxGuest

HI guys,

I'm looking for a bit of newbie advice re. switching circuits.

Basically I'm trying to create a book which can tell which page it is open
on. One of the ways I'm trying is by using conductive paper for the pages -
using each page as a simple switch so when two pages aren't touching the
page knows its open. Crude I know...but it should be enough for what I
need. The problems is that without using black carbon paper I've only
managed to source anti-static paper with a very high resistance.

My question is, is there a way I can still use this low level of
conductivity to form a simple switch?

Any comments would be gratefully appreciated as I am a little out of my
depth!

Hal Leemux

Paper:

@ 50% RH is 5x106
@ 20% RH is 6x109

I don't quite understand how to read an area resistance but I know that it
doesn't even show on my multimeter! (I assume the above means at 50%
humidity the resistance is 10 to the power 6 ohms??? so 10m??)

2. ### John WoodgateGuest

For a uniform sheet of conducting material, the resistance from one side
to the other of a square of any dimensions is the same. For any
rectangle, R = Sl/w, where S is the area resistivity, l is the length of
the conducting path and w is the width of the path. If l = w, R = S, for
all l or w.

You forgot the '5x', so it's 50 Mohms; 10 m is 10 milliohms. You can
certainly detect 50 Mohms using a FET or a FET-input op-amp. You main
problem will be keeping mains frequency and a lot of other stuff out of
you circuit. A 100 nF capacitor directly across the input from your
pages would be a good start.

6 Gohms at 20% RH is quite a bit more difficult; you probably can't use
a printed board and surface leakage across components and wiring will be
significant.

Have you thought of printing conducting strips on the page margins close
to the spine? You can get much lower resistances, although the lower the
resistance the more costly the ink is. But you would then be using
ordinary paper.

Leemux,