Stretching with Arduino

The Stretch Sensor

On Kate’s suggestion, I explored using a stretch sensor to communicate with Arduino and Processing. Stretch sensors are very simple, and quite similar in design to flex sensors. A conductive length of rubber is connected to a voltage divider circuit, and the resistance or the rubber changes as it stretches or relaxes to its original state. Here is a photo of the circuit I used:

I followed the tutorial I found here to make sure that I understood what the circuit was supposed to be (and learned that voltage divider circuits are much simpler than they sound). The tutorial mentioned that a resistor should be used which is approximately the same resistance as the stretch sensor itself. I didn’t know what that was, so I experimented with a couple of different resistors. I found that even a single 10K resistor gave a relatively small change in voltage across the circuit, but with two 10K resistors in series I was able to get a significant change in the readings the Arduino received.

I set up a Processing sketch where the radius of a circle could be altered by stretching the sensor:

Kate had mentioned that stretch sensors had been used to measure breathing, and so I looked up some of the ways this had been implemented. Here is one example of this idea, implemented as a chest band breathing monitor with a wrist band which provides feedback on breathing patterns. I found that the sensor I was using wasn’t quite sensitive to pick up changes in breathing, although I tried several positions around the chest where I thought displacement would be maximized. I was eventually able to get consistent changes with breath patterns, but the changes that occurred through shifting position or changing the tension on the sensor were much more dramatic that the slight stretch from breathing itself, which made it hard to track consistently. Looking around at other similar projects, it seems that many people are using knitted versions of stretch sensors with conductive yarn rather than the traditional rubber cable. These have the advantage of making it much easier to integrate stretch sensors into clothing in a comfortable manner. Here are some examples of knit stretch sensors.

I found a very interesting project which aimed to used stretch sensors embedded in the garments of performers to allow them to create additional MIDI controlled sounds while playing traditional instruments. The researchers collaborated with a professional cellist to create a shirt with stretch sensors around the chest and on the elbows which could be used to control software instruments. Although the project didn’t quite end up working as intended, it seems like a very interesting idea which could be explored further.

Personally I would be interesting in seeing a simple biofeedback experience which allowed you to focus on a virtual or physical object which was coordinated with your breathing. A system like this could help you go more deeply into meditative practices by giving you a more substantial way to visualize your breathing. I’m not sure if stretch sensors would be the ideal way to realize this, but it certainly seems like one sensor that could be used.

I enjoyed learning to use this sensor, there is definitely something magical about creating digital input with a physical object in this way!