force sensitive resistor

[Trumpkin]

Looky what I found! http://www.instructables.com/id/DIY-Force-Sensitive-Resistor-FSR/ I certainly never thought that could be done with such simple materials. does anybody have Ideas of what you could use it for?

[krich]

Now that's pretty cool.  It would be very easy to use something like that inside a gripper.

I seem to remember seeing something similar made with window screen.  Can't remember what the middle conductor material was.  They put it on a punching bag or something like that.  I'm going to have to look that up to see if I can find it.

Edit:  I found it, and it's virtually identical to what you posted.  Wonder where you can get large sheets of that foam...hmm...

Another Edit:  Wow, 10+ pages of conductive foam in the Google shopping section.  This inter-web thing never ceases to amaze me.

[Trumpkin]

http://www.altex.com/product_info.php?products_id=6242 pretty cheap stuff, I wonder what thickness is the best.

[krich]

Well, I gather the thickness would simply define the resistance at rest vs. resistance while compressed.  (similar to a photocell)

So, (all theoretical numbers here), a FSR with 1" thick foam might have an "at rest" resistance of 100M Ohm and a "compressed" resistance of 1MOhm, while a FSR with 1/8" thick foam might have an "at rest" resistance of 100k ohm and a compressed resistance of 1000 ohms.

This is truly groundbreaking for me.  I bet I could build myself a nice little pressure pad for my Halloween display.  I just need to find myself a rather large and thin sheet of that foam.

Any ideas on a conductive adhesive?  They say in the instructable you find that the adhesive can interfere with the conduction between the foam and the copper board.

[Trumpkin]

it couldn't hurt to try conductive adhesive, i googled it and one of the first results was this http://scientificsonline.com/product.asp?pn=3035852&sid=2008FS&eid=2008FS&mr:trackingCode=C8DE8CFA-D14D-DD11-AFF6-000423C27502&mr:referralID=NA&bhcd2=1215724616

[krich]

I wonder how varying thicknesses of the conductive adhesive would affect the accuracy of the sensor.  I found some conductive double sided adhesive sheets, but they're super expensive.  Will keep looking.

I'm definitely putting this sensor on my build list.

[Admin]

This is actually an old idea - I first saw this like 10 years ago

IC's and other electronics often come with that conductive foam as part of the packaging. I always keep some around just in case.

To maximize the signal, follow the schematic and equation as if it was a photoresistor:
http://www.societyofrobots.com/schematics_photoresistor.shtml

(but instead of light intensity, its foam compression %)

You may have a problem with sensor hysteresis though . . . google that for more info

[krich]

Oh, hysteresis, like when I burned my hand real good with the soldering iron, or when my daughter's kitten jumped on my face in the middle of the night?   

[Admin]

no, thats hysteria

(and perhaps a hysterectomy for the cat)

[krich]

The cat got a smack-terectomy.   

Seriously though...

I can see how the foam would not perform consistently.  Each compression would change the characteristics of the foam and thus the sensor itself.  I'm good with that.  If I need something more reliable (like for grippers), I'll just shell out the bucks for a commercial FSR.