Hannah Perner-Wilson uses conductive yarn to make a stretch sensor, here’s how:
Use a circular knitting machine to knit a stretch sensor with regular and conductive yarns in five minutes! The values of the sensor range from roughly 2.5 Mega Ohm when relaxed, to 1 Kilo Ohm when fully stretched.
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.
She uses it in a bracelet that lights up when you stretch it, and her colleague Boni Kaufmann used one to make an exercise rep counter.