Felted Crochet Stretch Sensor

Sensors come in many forms, you can buy them already manufactured, or make them yourself. 

The latter is my favorite for many reasons. This sensor is meant to be integrated in to wearable technology projects as an alternative to buying manufactured sensors which are made of rubber, metals, plastics and ceramics. My experience with a manufactured stretch sensor hasn't been good. It is one made of rubber with crimped metal hooks at each end. It broke easily and did not hold any properties found in materials that a wearable tech piece would mainly be constructed of; fabrics, thread and so forth. You can buy this conductive rubber in any length, but still, rubber isn't always the material you want for a project.

I've been experimenting with different ways of making soft sensors, particularly ones that utilize felting techniques. >>huge felt dork<<

This sensor is crocheted in a tube, it can be crocheted in a strip and other shapes. I myself have not tried these yet, but this is why I am sharing the technique!

This technique is a starting point and meant to be ran with.
Use this for any project you see it being applicable towards and please post them when you can, I would love to see what people come up with! 

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Required Tools

Crochet tools
* multimeter
* alligator clips
* scissors or any sharp blade
* merino wool yarn - Frog Tree brand worsted weight 
* crochet hook - size H/8 5mm
* conductive thread - Lame Lifesaver
* elastic thread - Stretchrite brand

Felting tools

* sushi mat
* towel
* soap
* cup of warm water
* access to a sink or a large, deep container filled with cold water (not pictured, I used a sink)

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Optional

* measuring device
* needle nose pliers
* heat shrink tubing
* The pins from D-Sub connectors (male and female)
* Ribbon cable

Substitutes

* sushi mat
You can use many materials here that you can find at various kinds of stores. The idea is to get something that you can roll up you item in and that has a textured surface. Here are some suggestions:
   - tulle netting
   - carpet saver
   - placemats made of rubber or bamboo
   - bamboo blinds
   - bubble wrap
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All of the required tools are ones that will guarantee a successful outcome. If you choose to change or substitute any of them the outcome may differ. Some tools are more exchangeable than others such as the sushi mat, however, even if you were to use the tulle netting instead of the sushi mat, you would need to rub the sensor with your hands rather than roll it back it forth as you would in a sushi mat or blind. So some material substitutions may take some finesse to work with.
The ones that will definitely make a difference will be changing the crochet hook size and gauge of yarn, I have tried some larger ones of both without much success. You are more than welcome to experiment, but please keep this in mind!

IMPORTANT!
the yarn must be wool if you want it to felt! Merino is hands down the easiest and best for this purpose.

  

Combine all three elements: wool yarn, conductive thread and elastic thread

Slip Knot
* Hold all three strands like so in your hands making an loop with the pointer and thumb of your left hand and the ends between the pointer and thumb of your right hand.
* Let go of the ends with your right and grab the strand with your left hand.
* Pull the strand through the loop, creating another loop as the previous one tightens creating the slip knot.
* Put your hook through the loop and tighten the knot by pulling on the ends, don't pull too tight! You want your hook to have a little bit of movement through the loop.

Chain Stitch

* Throw yarn over back to front (this is called a yarn over or yo)
* Hook and pull through loop
* You have created one chain stitch!
* Repeat until you have made 4 chain stitches

When you have your 4 chain stitches done, skip the chain stitch closest to your hook and go to the second stitch. This is the stitch you are going to single crochet in.

Single Crochet Stitch

* Notice there are three strands of the chain stitch. You are going to push you hook through between the two top and one bottom while holding your hook in your right and the piece in your left. You will always be working from left to right once you do your base of chain stitches.
* Push your hook through the stitch.
* yo
* Hook and pull through stitch, you now should have 2 loops on your hook.
* yo
* Hook and pull through 2 loops, this is a single crochet!

Now you know 2 basic crochet stitches (and one knot) that you need to construct this sensor, it is time to move forward and finish the starting end of our tube while learning one more basic stitch.

* Push your hook through the same chain stitch as before, we are going to put 2 sc stitches in each of the 3 chain stitches (this is called increasing).
* make single crochet
* Continue to make the remaining 4 sc in the next 2 chain stitches, making a total of 6.

Slip Stitch

* Now you want to create a circle by going to the 6th sc from your hook and pushing your hook through. You want to push it through under the top two strands that make the V.
* yo
* Pull through sc stitch and loop on hook. This is a simple slip stitch!

After closing the circle with the slip stitch, you should have a circle made of 6 sc. From here you want to go around and around making sc stitches in the 6 sc building up the length.

* Make sure to push your hook through the sc stitch keeping the two strands of the V on top. 
* Proceed to make your sc, yo, pull through, yo and pull through again.

* Repeat your sc stitches until the length is what you desire, this will be in relation to the finished resistance of your sensor.

The length of the tube that you are crocheting is in relation to the finished resistance of the sensor. What you are reading is the conductive thread, the longer piece of the thread, the higher the resistance. 

So, you want to test the resistance before you finish the sensor at some point. 

I stopped this one at 6", slipped out the hook and and attached the alligator clips to each end of the tube.

When you stretch the sensor, the resistance will go down. Once let go, the elastic helps it go back to the original length, but still takes awhile. The last step of felting will ensure a quicker snap back. The slow recovery of the sensor can be used as is, if this is what you desire, I think it could make for an interesting sensor, but usually you want the snap back. 

6" hovered around 40 Ohms and went down to about 10 Ohms.

I finished this sensor around 9", it meant to be longer, but I was at the end of the elastic thread. No big deal though... :)

Once the length is good, you are ready to decrease and end it off. 

* Go through the next sc as you normally would
* yo
* Pull through, you now have 2 loops on your hook
* Push hook through the next sc stitch
* yo
* Pull through, now you have 3 loops on your hook
* yo
* Pull through all 3 loops decreasing 2 sc to one stitch.

Repeat 2 more decrease stitches in the next 4 sc stitches. Decreasing the 6 sc to 3 stitches. End with one more chain stitch.

Cut strand, pull out end and pull tight to end off.

Once done, read the resistance once more.

8.5" gave me 45-50 Ohms
stretched - around 18 Ohms
scrunched up - around 10 Ohms

You can see how stretching it once, really stretches it out, which also means that the resistance will slowly go back to the starting range as it creeps back to the original length. Again, once we felt it, this will be solved. Or use it as is from here!

Lay down the towel on the work surface, place the sushi mat on the towel.

Add a couple drops of soap to some warm water. The soap helps the fibers stick together while they lock up together. 

* Dip the sensor into the warm water
* Gently squeeze out water, you don't want it sopping wet, but you still want the water to penetrate all the fibers.
* Lay the sensor horizontal towards the front of the mat
* Roll mat completely up

* Roll back and forth applying downward pressure at least 200 times
* Throw the sensor into cold water either in a stopped up sink or a large, deep container so it's submerged. The cold water shocks the fibers and helps them shrink faster.
* Keeping it scrunched up, take the sensor and squeeze it in your hand. Pass it back and forth between hands, scrunching and squeezing. Passing it back and forth is the friction and movement it needs to felt up in the cold water. Do this at least 75 times. 
* Squeeze out excess water and go back to the workstation, dunk the sensor in the warm water a couple times and repeat.

Go back and forth between warm and cold water so it totals to 7 times
so, it will be play out like this:
- roll 200 times with warm water at least 7 times including the first time.
- rinse and pass between hands 75 times in cold water at least 7 times.


Just make sure that you end with a cold rinse, this will tighten up the fibers nicely for the last time.

It helps to measure every now and then to monitor shrinkage, you want it to shrink as much as possible. You should see a dramatic change between the 3rd and 6th repeat cycle.

Measuring it, the sensor shrunk up .75". I know this is enough from experience but also because it stopped shrinking between the 6th and 7th cycle.

Other yarns might take more or less.

The texture will be tighter and fuzzier. The elastic and conductive thread will be looser and there should be loops visible, since they are not shrinking with the wool.

Leave the sensor out to air dry.

When the sensor is dry, it is ready to use.

Depending on what you intend to do with it, you may or may not want to finish it in the way that I do in this step. I just find this a really handy way to finishing off any end of conductive thread and connecting it to wire and/or your breadboard.

* Separate the conductive thread and elastic from the yarn, it may be a bit tangled and take some finagling. 
* Take the male half of the Sub-D connector and floss the elastic and thread through the prongs of the crimp end.
* Take needle nose pliers and squeeze the prongs shut really tight.
* You can also insulate and protect the connections with heat shrink tubing.
* Take some ribbon wire (it's thin and flexible!) and do the crimp the female half on to the end of that.
* You can now connect and disconnect the wire to the sensor!
* The male pins also fit into your breadboard quite nicely, so you can either plug the sensor straight in without the wire extension, or crimp some more male halves on to the end of your wire.


This is one way to make a connector, you of course can use thread, crimp beads, eyelets and other methods to attach this sensor to your project.

This is really up to you, but I have included a couple vidz of the sensor made here as a frequency mod in a tone generator circuit using a 555 timer.


If you would like to see other ways I have used this sensor and how the design came about you can check the page from my personal blog and a video of them being used as musical controllers here and here.

I used three of the sensors made here in one large green cable, so look for that... that's the stretch sensor.


>^..^<