Maker David Huang has shared a trick for creating a surprisingly capable tension sensor at practically zero cost — by rubbing some string over a pencil.
"Our goal is use the cheapest and the easiest way to make your own sensor for your project," Huang explains by way of introduction. "The sensor we are making today is a tension sensor. A tension sensor will react to a pulling motion. The most common material we use for the tension sensor is conductive rubber, or weaved conductive rubber wire - but the problem is the conductive rubbers are very expensive, and they are also hard to get."
The solution is surprisingly low-tech: Replacing the conductive rubber with simple string, treated by rubbing it repeatedly against a pencil lead to cover it in a layer of graphite — making it conductive. "If you think you are almost done, test it with the electric [resistance] meter," Huang notes. "If the electric resistance is smaller than 1MΩ the sensor will be more stable."
Connecting the treated string to wires and then to an Arduino Leonardo board, Huang demonstrates how what has now become a sensor reacts to being pulled about. "The principle behind this tension sensor is because the fiber of the cotton string is flexible, when you pull the cotton string and the fiber will be tightened it will made the graphite powder of the pencil lead that we colored tighten as well. Once it tightens, the electric resistance will go down."
Despite its simplicity and low cost, the sensor proves useful: Combined with elastic, it proves capable of reading a variety of movement — including, towards the end of Huang's video, tracking the arm and leg movements of a full-sized adult with surprising accuracy. "I made four tension sensors for it," Huang adds. "You can pin the tension sensor on you with safety pins. I pinned them on my arms and legs and it will sense my movement. I use Processing to track my movement."
The full video is now available on Huang's YouTube channel.