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:


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.

Step 1: 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.

Step 2: Making the Bi-Directional Flex Sensor: (Sandwich the Materials)

Making the Bi-Directional Flex Sensor:

The resistive material is sandwiched between the two copper clad laminates. The copper sides of the laminates are both facing toward the resistive material; see figures 3, 4 and 5. If you soldered the wires on the same side of the laminates, when you assemble the sensor, the wires will be positioned on opposite corners allowing the base of the sensor to lay flatter.

To this sandwich we add the acetate strip, see figure 6. The purpose of the acetate strip is to make the sensor more resilient and to spring back after it has been flexed.

Step 3: Packing the Sandwich

This entire sandwich is inserted into the heat shrink tubing, see figure 7. The base of the sandwiched materials is shown in figure 8, just before it is inserted completely into the heat shrink tubing.

Step 4: Sealing the Sensor

You are almost finished. At each end of the heat shrink tubing place a small amount of clear silicon sealer, see figure 9. Allow the sealer to dry according to its directions, usually around twenty-four hours.

Step 5: Testing the Bi-Directional Flex Sensor:

Testing the Bi-Directional Flex Sensor:

Set a VOM meter to read ohms. The sensor we built had a nominal resistance of approximately 20K ohms, see figure 10. As the sensor is bent in one direction Figure 12the resistance decrease in proportion to the bend to approximately 2k, see figure 11. Bending the sensor in the opposite direction also decreases the resistance in proportion to the bend to 2K ohm, see figure 12.

The sensor is also pressure and force sensitive. So pressing down on the sensor will also decrease it resistance. The basic sensor can be modified in size and shape to fit custom applications. The resistive material may also be modified to obtain different resistances and characteristics. Most materials are easily obtained from electronic sources. Images will supply the following materials to get you started.

<p>Once the sensor is working, Is there a method to validate the sensor?<strong> How do I compare the performance of this sensor with commercially available ones?</strong> Most papers put reliability as a parameter to show the performance of DIY sensors, but actually, do not compare with commercially existing sensors.</p>
Can the size of resistive material smaller than copper foil? My resistive material is anti-static bag
<p>It can, but it becomes easier for the two copper foils to touch and short the sensor out.</p>
<p>Is there any replacement for Copper-foil? </p>
Technically you can use any conductive material such as aluminum foil. The difficulty is that it is difficult to solder to aluminum and without the conductive foil having a plastic laminate base the foil will deteriorate pretty fast when used.
<p>Hi</p><p>Is rubber a good resistive material?</p>
Usually , yes.<br>John<br>
<p>Thank you!</p>
<p>what is the use of the acetate? just to protet the upper copper layer??</p><p>plz answer me ...</p>
The acetate adds the stiffness and flexibility to the sensor. It allows the sensor to bend and then return to its original shape when the pressure or load is removed.<br>
<p>Check the resistive material you are using with an ohm meter. If the resistive material is so high in ohms that it doesn't register using a ohm meter it will not work in the sensor.</p><p>The copper foils of the sensor go on opposite surfaces of the resistive material. You should be able to get a resistive reading from the copper-foil, resistive material, copper-foil sandwich before assembling the bi-flex sensor.</p>
Check the resistive material you are using with an ohm meter. If the resistive material is so high in ohms that it doesn't register using a ohm meter it will not work in the sensor.<br>
No! <br><br>If the two copper foils touch the sensor is short circuited and will not function.<br><br>John<br>
Awesome DIY
what can be used as resistive material ?
<p>you could try a piece of the heat shrink</p>
<p>hi,</p><p>nice project ,can you tell me what is 'acetate', is some kind of plastic?</p>
<p>Yes, it's a plastic. Typically sold as transparent sheets . Amazon.com sells or you can buy them locally in a craft store.</p><p>http://www.amazon.com/Clear-Acetate-9x12-003-25/dp/B004QJX1JI/ref=sr_1_3?s=arts-crafts&amp;ie=UTF8&amp;qid=1396480689&amp;sr=1-3&amp;keywords=acetate+sheets</p>
<p>hi, you mentioned other resistive materials. would they work instead of the conductive bag? like if you put the acetate itself in the middle of the copper laminates.</p>
<p>Plastic acetate is not resistive at all. It is an insulator and will not work. Using acetate, you would obtain very high resistance that would not change when sensor is bent. Other potential materials that may work are conductive cloth, thread, inks, etc..</p>
ahh okay i guess i misunderstood the note about resistive materials. thanks
Say i don't need it to spring back, do i still need to put the acetate strip?
You can experiment with any flexible material that has a resistive value, cloth, paper and plastics. <br>
nice job...its very informative. <br>thanks!!!!
Awesome project! Instead of shrink tubing could I use packing tape? <br>Thanks
Never tried that, I don't think so because the sticky tape may prevent things from sliding around on the inside. If you do try it let me know if it works for you.<br>
does it give good values a broad range
yes. however the value range depends upon many variables in the physical construction of the sensor so that's hard to predict. <br>
If i used this with a glove is it easy to bend or not is it the same as the ink flex sensor thing the expensive ones.
No it is not. Different technology. The ink flex sensors are more accurate and repeatable. <br>
but could i put it in a glove. <br>And does it bend easily.
Yes it does.
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)
Cool!! Very Cool!! Going to give this a try. Great Job!!!!
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?<br>Thanks in advance, The Captain
I have not come up with a way to make the sensor one directional. However I never worked on this application, because I consider the bi-directional nature of the sensor a plus.<br><br>
Also I noticed in a previous comment that someone stated that heating the heat shrink would destroy the sensor. Why is this exactly?
As it shrinks it forces the inside components together, making contact, and effective turning the sensor into a somewhat fixed resistor.
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?
No adhesive is used in the: copper laminate / resistive material /copper laminate sandwich. Once sandwich is inserted into the heat shrink tubing, its the tubing that keeps the sandwich from coming apart.
This is cool. Now I won't need as many sensors on my exoskeleton, which means less input, which means simpler code!!<br />
where can u get that stuff<br />
&nbsp;Components are available here:<br /> <br /> http://www.imagesco.com/sensors/flex-sensor.html<br /> <br /> John<br /> <br /> <br /> <br /> <br />
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.&nbsp; As well I have anti-static bags, but I use them to store components.&nbsp; I made a little list of materials that may work.&nbsp; I would really appreciate if you looked it over and gave a little feedback on which you think would be the best to try:<br /> <br /> - Camera Film<br /> - Synthetic fabric such as Rayon<br /> - That specialty photo printer paper<br /> - Velostat? (Not sure what this is, heard it on a fabric sensor Instructable)<br /> - Conductive fabric<br /> <br /> If none of these work, I will try with one of my anti-static bags.<br />
They don't make tinfoil anymore do they? You mean aluminum foil right? Aluminum foil glued to strips may work, but for how long? I don't know. I never tried. Soldering to aluminum foil, haven't tried that either. If you talking about a base material, the camera film I think is the best to secure your alum. foil too.<br /> Velostat, conductive fabric and some anti-static bags will work as the resistive material.<br /> <br /> Good luck.<br /> <br /> <br />
Thanks for the speedy response and the help!<br /> <br /> Also if I haven't said already, excellent Instructable! <br />
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?<br />
&nbsp;No, the copper used has a plastic or kapton backing. Its used in the flexible pcb market.<br /> <br /> You can buy it here:<br /> <br /> http://www.imagesco.com/sensors/flex-sensor.html<br /> <br /> If you can't find it anywhere else.<br /> <br /> <br />
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
&nbsp;I am not familiar with the material you asked about. But the sensor is pretty&nbsp;resilient.&nbsp;&nbsp;What is the resistance of the material before you placed it in the sensor, and what was the resistance of the bi-flex sensor using the material?

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