We travel back in time about two weeks. I had some discussions with friends and I decided to have a position sensor based on a string instead of based on pipes as described in the last blog post. Also the string will between the middle of the tree stem and the ground, not between two trees. The sensor itself is basically a string which wraps around a roller which is connected to a rotary sensor. When the string is pulled it rotates the roller and therefore the rotary sensor, well..., it rotates. It is similar to the designs positional sensor manufacturers are using:

My design there are two spools, so the string does not wind up, and my design uses a weight as the pullback force, not a spring:The next step is to print out the holding structure, the spools and assemble everything.