Here’s a useful tutorial by i.saralynn over on Instructables on how to measure breathing by knitting a sensor with conductive yarn. I can do all the bits apart from the knitting! Check it out here.
And here is another one by Erin Gee, which doesn’t require knitting over on Instructables here
I had some issues with getting good readings of the resistance in the actual rubber cord, and ended up using a longer length, which gave a wider range of resistance, eventually settling on a 35cm long bit. I also had some issues with the Arduino code. I figured that the ideal would be if the sensor did a bit of calibration to begin with to get max and min values and then scaled incoming sensor values to be inside the range 0-255 – which is whats needed for the LED to be completely on or off and also capped so values cant be above or below 0-255 – because I wont just be controlling an LED.