How to Make Bi-Directional Flex Sensors

How to Make Bi-Directional Flex Sensors
Flex sensors are passive resistive devices that can be used to detect bending or flexing. The flex sensor shown in this article is a bi-directional flex sensor that decreases its resistance in proportion to the amount it is bent in either direction. The sensor we are building is about 3/8" wide by 5" long. You can easily make a sensor wider and longer depending upon your application:

Applications

Flex sensors may be used in robotics to determine joint movement or placement. They may also be used like whiskers for wall detection. The sensors we are making are also pressure sensitive so they can also be used as bumper switches for wall detection or pressure switches on robotic grippers. For bio-metrics, the sensor can be placed on a moving joint of athletic equipment to provide an electrical indication of movement or placement. A few of the sensors can be incorporated onto a glove to make virtual reality glove.
 
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Step 1Components:

Components:
Components:

The materials needed for the construction of the bi-direction flex sensor is shown in figure 1 and listed below. The size of the materials listed is only a guideline to the sensor we are constructing in this article. These types of sensors can be manufactured to larger widths and lengths.

1. Copper foil laminate 1/4" x 4.5" (see text)
2. Acetate 1/4" x 4.5" x .010 thick
3. Heat shrink tubing 3/8" dia x 5"
4. Resistive material 5/16" x 5" (see text)

Copper foil laminate is used in the electronics industry to make flexible circuits. It is thin copper cladding on a plastic material substrate like acetate. The material we are using is single sided copper. Copper on one side and the substrate (plastic) on the other, The copper cladding material is cut into two pieces 1/4" wide x 4.5" long strips. The material is easily cut with a scissors. Solder about 6" of wire to one end of each strip. You may fine it easier to solder the wire to the strip if you tin the bottom 3/8" of each strip. Solder each wire to one corner side of the strip. It doesnt matter which side you choose, just make sure you solder both strips on the same side, see figure 2.

Resistive Materials

There are a variety of resistive materials available; cloth, plastic and paper. The common element of all the appropriate materials is that the material is somewhat conductive or resistive. The degree to which the material is resistive will determine the scale at which your flex sensor operates. For the example here I am constructing here, I using conductive black plastic poly bags conductive bags used in the electrical industry. These bags are used to store components that are static sensitive. The bags are made from single layer of carbon-loaded polyethylene and its conductivity does not depend on humidity. I cut the bags into the 3/8 " wide by 5" long strips.
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49 comments
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Jan 21, 2012. 5:10 AMdpotthast says:
just did a five minute proof of concept with tinfoil and a static bag from some old computer part--- I was skeptical because the resistance on the bag was too high for my multimeter to read it, but the sensor works like a charm--- thanks! Now to get some copper foil --- ( p.s. stained glass supply outlets have a variety of copper laminates)
Mar 16, 2011. 2:25 AMT-Hawke says:
Cool!! Very Cool!! Going to give this a try. Great Job!!!!
Nov 29, 2010. 1:39 AMCapt.Phil says:
I am looking to make five of these for a robotic hand project that I'm doing. Do you know how easy it would be to make then single directional, and if so would I still need all the original materials?
Thanks in advance, The Captain
Dec 1, 2010. 8:15 AMCapt.Phil says:
Also I noticed in a previous comment that someone stated that heating the heat shrink would destroy the sensor. Why is this exactly?
Aug 27, 2010. 7:36 AMsaintkyumi says:
Looks very cool! By the way, when the resistive material is sandwiched between the copper clad laminates, adhesive is necessary? If so, could you recommend what type of adhesive is proper?
Apr 20, 2010. 9:31 PMbowmaster says:
This is cool. Now I won't need as many sensors on my exoskeleton, which means less input, which means simpler code!!
Mar 28, 2010. 8:53 PMlane29 says:
where can u get that stuff
Mar 4, 2010. 8:16 PMWesley666 says:
I was planning on using tinfoil glued to strips cut from those paper slips for binders, the ones made of or of similar material to acetate.  As well I have anti-static bags, but I use them to store components.  I made a little list of materials that may work.  I would really appreciate if you looked it over and gave a little feedback on which you think would be the best to try:

- Camera Film
- Synthetic fabric such as Rayon
- That specialty photo printer paper
- Velostat? (Not sure what this is, heard it on a fabric sensor Instructable)
- Conductive fabric

If none of these work, I will try with one of my anti-static bags.
Mar 6, 2010. 4:00 PMWesley666 says:
Thanks for the speedy response and the help!

Also if I haven't said already, excellent Instructable!
Feb 14, 2010. 6:38 AMsolarrobor says:
The only copper foil I can find in the UK are ones which have oneside that is adhesive. Where the ones that you used also adhesive on one side?
Jan 31, 2010. 8:48 AMdavelarave says:
Hey, brilliant instructable, for a substitute resistive material, would 'mercury caribonum carbon film' work? ive tried it but not sure im doing it right. Thanks
Feb 1, 2010. 7:24 AMdavelarave says:
Turns out it didnt work as it was not conductive/resistive at all, I also had problems with the copper strip, the one I used was very thin and actualy a coating on solder (strange). Trying again with the proper materials now, found that computer stores are quite happy to give you thier old anti-static bags! we all love a freebie. Thanks for the quick reply.
Will post my instructable of when Im done (making a glove/ replicating hand).
Nov 18, 2009. 1:14 PMDELETED_madscientist167 says:
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Oct 27, 2009. 3:20 PMDELETED_madscientist167 says:
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Oct 27, 2009. 6:46 PMDELETED_madscientist167 says:
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Jul 25, 2009. 2:29 PMmicroman171 says:
Wow! You're John Iovine!? When I was growing up, I used to LOVE your books! Great instructable by a great author.
Oct 27, 2009. 3:24 PMDELETED_madscientist167 says:
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Oct 27, 2009. 6:43 PMDELETED_madscientist167 says:
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Aug 20, 2009. 3:23 PMrclark says:
I was wanting to make a light, portable scale, maybe like a rubber mat. It would be something like a bathroom scale that can be slipped in a suitcase. Would it be possible to make a flex sensor that could used for such a scale?
Aug 5, 2009. 7:49 PMLuminousObject says:
Wait, so do you actually heat the heat-shrink tubing, or is it just like a protective cover? Cool instructable, by the way. I'm planning on making one just to say I have a bi-directional flex sensor.
Aug 6, 2009. 6:25 AMLuminousObject says:
Allright, thanks.
Jul 25, 2009. 8:24 PMMarathon says:
This looks really cool, but I'm confused as to how it works. How does bending change the resistance? I understand how the copper creates a capacitor (I thinks) but does the bending move the copper apart?
Aug 3, 2009. 1:33 PMrachel says:
One kind of resistive-conductive material is Velostat, which is also available from LessEMF.com: http://www.lessemf.com/plastic.html (search the page for Velostat). You can make hundreds of bend or pressure sensors from just a foot or two of yardage.
Jul 26, 2009. 2:43 AMvenkboy says:
Wow! Do your know of any other resistive material? Because anything I can find has either too much resistance (more than 2000K) or have zero resistance(almost). I still don't understand how this works, I think that it works by changing the distance (on the resistive strip) between the two copper strips.
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