The stretch sensing is actually due to the structure of the conductive yarn which is made up of lots of short steel fibers mixed with polyester. Even without knitting the yarn into a structure you can use it as a stretch sensor by simply pulling it taught or relaxing it. But the yarn is not very strong and easy to tear. The knit structure allows you to accumulate more yarn and thus more resistance in less length and also by combining the conductive yarn with regular yarn you can gauge the sensitivity of the sensor by choosing a thicker or thinner yarn - thicker yarn gets more in the way of the conductive yarn making extra contact through the knitted loop structure. Plus knitting creates the stretchy structure giving you some natural tangible feedback.
Circular knitting machines
Circular knitting machines range from 20-250$ in price and differ in diameter, number of needles and spacing of needles. Spool or wire knitters have only four needles and resemble manual knitting dolls, while the knitting machines resemble manual knitting wheels or looms. See these posts on HOW TO GET WHAT YOU WANT for a more detailed summaries of circular knitting machines:
and circular knitting looms:
Conductive yarn is hard to come by, but hopefully soon online shops like www.plugandwear.com will offer such yarns in various thicknesses, conductivity and at a good price. The conductive yarn used in this tutorial is called Nm 50/2 and is made up of 80% Polyurethane and 20% Inox steel fiber and is manufactured by the Austrian company Schoeller (www.schoeller-wool.com) but they only produce and distribute it in industrial quantities of 300KG. See the following post for more information:
Circular knit stretch sensor post:
Buy a circular knit stretch sensor on Etsy:
narrow >> http://www.etsy.com/view_listing.php?listing_id=37778885
wide >> http://www.etsy.com/view_listing.php?listing_id=37805729
The following Instructable will cover the knitting of stretch sensors using conductive and regular yarn with the following models of circular and spool knitting machines:
- Play Go Knit Knit (17 Euro from Amazon Germany)
- Singer Spool Knitter (19$ from Amazon USA)
Video of the full process of knitting and testing a circular knit stretch sensor using the Play Go Knit Knit circular knitting machine (6 minutes)
Video of the full process of knitting and testing a circular knit stretch sensor using the Singer Spool Knitter (5 minutes)
Video demonstration of both wide (circular knitting machine) and narrow (spool knitter) stretch sensors
Step 1: Materials and Tools
* conductive yarn
* regular yarn
* circular knitting machine or spool knitter
* 3 crocodile clips
(* or multimeter)
For inflation sensor:
* regular party balloon
Step 2: Threading Machine
To start, thread the yarn through the tension rig and through the hole into the center of the knitting machine. During the entire knitting process, if you machine makes strange noises, check that the thread is able to run through the tension rig with no extra friction on it, even having to unwind itself from the ball of yarn can sometimes be too much weight for the machine and it will help if you unwind the yarn ball so that the yarn can feed into the machine with absolute ease.
Step 3: Knitting the Beginning
Step 4: Knitting Sensitive Part
Step 5: Knitting the End Piece
Step 6: Finishing Off
here are two ways to finish off so that your knitting won't unravel:
* You can either turn the handle several times until all the loops are free from the hooks any the tube come off the machine. This way you have to be careful not to stress the knitting in any way that would unravel it. Hold it gently in your hands, thread the end of the yarn through a needle and pick up the loose loops. then pull the thread tight, closing the tube. You can also finish the knitting any crocheting the loops, this will maintain the stretchiness.
* Instead of releasing the whole tube from the machine in one go you can also turn it around slowly, picking up each loose loop as it slips off the needle.
Step 7: Seeing That What You Made Works!
power+ ---(connected to)--- LED+
power- ---(connected to)--- one end of sensor
other end of sensor ---(connected to)--- LED-
Make sure that you make contact between the resistive yarn and the crocodile clips on each end of he sensor. See video and pictures for reference.
Now stretch the sensor in all directions, squish it up into a ball or press it tightly, all of these things should lead to a change in resistance big enough to fully light the LED when pressured/stretched and fully dim the LED when relaxed.
Step 8: What You Can Do With a Knit Stretch Sensor?
Video of inflation sensor: