Damp cloth sensor

Hi,
A little project I’m considering need to detect when a piece of fabric gets damp or wet. My brainstorming fizzled, so I thought I’d ask for ideas. There has to be a simple and cheap way to do this.

What I’m looking for is a way to tell when a piece of cloth at least 18" square gets water on it in one or more places. I can’t use a single spot sensor because of the large area. The sensor needs to be flexible, somewhat durable and safe to wear. The available humidity and rain sensors don’t seem to fit, and I can’t use the houseplant soil monitoring trick with the nails.

So, anybody have any suggestions on how to do this? It doesn’t have to be very accurate, or pretty.

Thanks,

tie one end of the cloth to ground & one end to an analog input pin, with its internal pullups turned on. You will get a nice measurement that will correspond to the resistance between the 2 sides of the cloth: 1023 w/ no water & something less than that when there is water between the 2 points. Use conductive fabric or thread to make the connections to the cloth if you want a softer sensor that extends into the fabric.

I am not sure that last method would work. As I see it the problem would be the conductivity when the cloth was dry would not change much when it was wet in just one spot and not the whole length.

What I would try is to get some embodying metallic thread and make sure it conducts, that is it hasn’t got a plastic coating. Then use it to stitch lines from top to bottom closely spaced. Connect up alternate lines so you in effect have inter meshing fingers only a few mm apart. Then if any part of the cloth got wet there would be a measurable resistance change.

conductive thread here: http://www.sparkfun.com/commerce/product_info.php?products_id=8549

all of the “metallic” thread that you get in fabric/craft stores is not conductive

a p.s edit: mike is right. your sensor won’t trigger unless there is a solid area of water touching both of your sensing electrodes.

Hi,
Thanks for the idea. I tested it on a small area and it works pretty well.

Mike is right about the distance between the wires being a factor. However, I think I can get good coverage with creative use of tape and some thin wires. Or maybe even aluminum foil ;). I’ll make an interlacing 'E’s pattern so that any wet spot with a diameter larger than an inch or so will bridge between ground and analog in. I think it will do just what I need.

Thanks!

Might try something like this http://www.hobby-boards.com/catalog/product_info.php?cPath=21&products_id=1544

Hey I had one of these sensors in an electronics projects kit I got for Christmas in 1963! :slight_smile:
I didn’t know they were still around.
After a time the traces become detached from base board, it helps if you don’t rub it to dry it.

An Update:
The damp cloth sensor worked great on the workbench. I taped aluminum foil strips to the piece of cloth and wired that to the ground and analog pin. It could detect any wet spot more than about 1.5" across, which was good. To make it less sensitive to noise, I set a threshold that had it had to cross for at least 30 readings in a row. The tests I ran on it (after debugging) all went really well.
Unfortunately, it did not fare so well in “field” conditions. For some reason the analog readings would drop over time until the threshold was tripped, even if the cloth was dry. So over a period of a couple of hours, I almost always got a false positive, which means it wouldn’t run unattended.

Anyway, it was just an idea. If anyone else wants to run with it be my guest.