high resistance switch question

H

Hal Leemux

Guest
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!

Thanks in advance,

Hal Leemux



Paper:

http://www.alxtechnical.com/products/wbesdpad.htm

@ 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??)
 
I read in sci.electronics.design that Hal Leemux <hal_leemuxNOSPAMPLEASE
@yahoo.co.uk> wrote (in <HHJQc.88462$28.49986@fe1.news.blueyonder.co.uk>
) about 'high resistance switch question', on Fri, 6 Aug 2004:
Paper:

http://www.alxtechnical.com/products/wbesdpad.htm

@ 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??)
Click to expand...
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.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Thanks John that makes a lot of sense now.. That is why you see area
resistance written per square but with no value..... seems obvious once its
explained!!

So from what you're saying I could rig up a simple Op-amp circuit that would
allow me to switch with the paper? - that sounds great. I'll look at some
circuits online and start playing around with it this evening.

Yes I have tried using conductive ink but its a bit of a messy solution. I
am also trying to see if I could use some kind of conductive tape in the
same way you suggest - possibly a transparent one with an indium tin oxide
coating - but that is all if I can't get the paper to work really...

I've also got some alternative paper which says its resistance is 10/7 so I
guess that must mean 100 milliohms which would probably be a bit better so
I'll try that.

Thanks alot for your help,

Hal



"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:AE3VHeDax2EBFwf5@jmwa.demon.co.uk...
I read in sci.electronics.design that Hal Leemux <hal_leemuxNOSPAMPLEASE
@yahoo.co.uk> wrote (in <HHJQc.88462$28.49986@fe1.news.blueyonder.co.uk
) about 'high resistance switch question', on Fri, 6 Aug 2004:
Paper:

http://www.alxtechnical.com/products/wbesdpad.htm

@ 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??)

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.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Click to expand...
 
Hal Leemux <hal_leemuxNOSPAMPLEASE@yahoo.co.uk> says...
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!
Click to expand...
Go down to a toystore and look at the solutions that Mattel and
LeapFrog are using to tell what page is open. It would be
worthwhile to buy an electronic book type of toy and tear it apart.
 
Hal Leemux wrote:
HI guys,

I'm looking for a bit of newbie advice re. switching circuits.
Click to expand...
Right- SURE YOU ARE!!!! It looks like you've been fantasizing about this
fairy tale project for several years now........consider taking up tole
painting or something.
 
Hal Leemux wrote:
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!
Click to expand...
Leemux,
I had an idea about your task. If you print on each paper a small circle
with conducting paint and then have on each side below the row of circles a
coil that you exite at a certain frequency, the induced losses by the
eddy-current of those printed conductors would be bigger with a lot of
pages, so the difference between the 2 coils would be a possibility to
determine the opened page. with a T-shaped core you might even shield the
bottom from metallic desks. I do not know if it can be done.
--
ciao Ban
Bordighera, Italy
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top