This project explains how to create a cheap and nice sensor good to measure tension changes in a slackline when someone is walking or jumping on it. We will be using conductive yarn in form of a crochet lace, and in particular its property of changing its conductivity when stretched.

There is a bit of very simple electronics/Arduino involvement as well to show how this sensor can be read and what could be done next with these readings.

In order to make a musical slackline using this instructable you need:

Materials for the sensor:

2 alligator clips
2 meters (6 feet) of stranded wire
approx. 10 meters (32 feet) of conductive yarn thread (one at Adafruit)
1.25 meters (4 feet) of 2-way stretchable fabric in the form of a ribbon. The ribbon should be approx. 5cm (2in) wide
3 meters of stiff cotton fabric in the form of a ribbon. Approx. 5cm (2in) wide;
approx. 1 meter (3 feet) of cotton thread

Tools to build the sensor:

1 needle
a pair of scissors
1 knitting mill (optional, but highly recommended to save time. One from Prym)
1 sewing and embroidery machine (optional, but recommended)

Materials for the circuitry:

one 10K resistor
little piece of solid core wire
breadboard or prototyping shield for the microcontroller (optional)

Tools for the circuitry:

soldering iron
a microcontroller that you know how to output readings to your computer or other device that will play the actual sound. In our case - Arduino Uno.

Step 1: Making the Cotton/stretch Ribbon Base for the Sensor

There are many ways this sensor could be built, and we are showing here only one of those that we tried so far. In this case the part that hosts the sensor, which we will call the "base" further on, is made out of fabric itself. We can say it right away - please share your ideas as well.

So, off we go. Cut the cotton ribbon into two equal parts, 1.5m each. Then sew each end of the stretchable fabric ribbon to a separate cotton ribbon from the two you just made. You can do it by hand or use a sewing machine. In any case, the stitching should be strong and the resulting three-part ribbon should be durable to a good point.

You might ask - why such a long base? Good question, and here is our answer - we need to register as much of a change in slackline's length as we can while it actually doesn't change much when one walks on the slackline. So we need a base long enough to contract itself considerably (i.e. more than just a couple of centimeters) along with the contraction of the slackline.

Step 2: Making the Conductive Lace

Now it is time to make the sensor itself. We use conductive yarn and make a crochet lace out of it as it brings good conductivity for a 5 Volt circuit we will use eventually, and such lace also stretched very well.

The desired length of the lace should be around one meter (3 feet). For other measuring purposes you can use a shorter lace, yet for the sake of measuring slackline tension we need longer one simply to make the base long enough at the end to be fixed to a tree.

We suggest using a knitting mill machine to do the work in 5 mins. Doing this by hand is a totally different story. Here is some jazzy video on how to use Prym mill machine.

Step 3: Attaching the Conductive Lace to the Base

With cotton thread and a needle, make 3 to 5 hooks on the stretchable fabric and pull the lace through them.

Next is to sew each end of the lace to the stretchable fabric. The overall goal is to sew it in such places that when the base is stretched to its full capability, the lace attached to it will be close to, but not entirely at its own full stretching capability. This will require you to do some stretching tests to identify the appropriate spots.

Failing to do the sewing in an appropriate place would either leave the lace loose when the base fully stretched or, vice versa, will stretch the lace too tense when the base is still loose.

Step 4: Circuitry and Wiring: Arduino Example

Here comes the part where Arduino is involved.

Next we need to make a pair of wires that would connect both ends of the conductive lace with the Arduino board. The length of these wires should be considerable as you probably want to host your Arduino somewhere behind a tree or a pole to which one side of the slackline is attached. And given our long base that means - long wires.

Once you cut the stranded wire in two equal parts, solder an alligator clip to one end of each of them. This clip you will be attaching to the lace.

Using a small piece of solid core wire and the 10K resistor, make the voltage divider circuit on a breadboard or prototyping shield. Connect the wire to an Analog pin.

Hook up the non-alligator ends of the wires to the voltage divider circuit at the places highlighted by the red circles.

Step 5: Making Any Sound at All: Ableton + Connection Kit Example

We are already making an assumption that you will be using Arduino. Time for one more assumption which you, of course, could break. We will show how this setup can work with Ableton Live using the Connection Kit plugin that Ableton announced in early 2016.

Retrieve the code from the Firmata protocol on Github. Firmata is a standard protocol made to "marry" together Arduino and software that runs on computers. It should be said that this software must itself support Firmata protocol.

Load the Firmata code to the Arduino. Keep Arduino connected via USB.

In Ableton Live, upload the free Connection Kit plugin for Ableton.

Create a new Ableton set, add the Connection Kit plug-in to it. In the plug-in, choose the USB port where you connected the Arduino. In the interface of the plugin pick the pin that you chose in the previous step. Map it to the desired knob or other controller in Ableton Live sketch by first clicking "map", and then clicking on the knob. Now, the reading from the sensor controls that knob.

Step 6: Finally, Putting This All Together

Fix the middle of the base to the slackline (we usually do it close to the ratchet or directly to it) and tie both ends of the cotton ribbon behind the tree/pole to which the slackline is attached. The slackline, the tree and the sensor form a right-angled triangle.

If you tie the base to the tree above the slackline level, then the lace should be a bit loose (walking on the slackline would stretch it) and vice versa - if you attach it to the tree below the slackline level, then the lace should be tense (walking on the slackline would contract it).

The alligator clips of the wire are to be connected to the two ends of the conductive lace. You can use the calibration mechanism in the Connection Kit plugin to let the kit know what are the lows and highs your setup produces by walking/jumping on the slackline while the calibration is on.

Enjoy the sound of your movement in the air!