Developer Zeke Medley has shown off a low-cost flex sensor build, created using an LED at one end and a photoresistor at the other end of a simple fabric tube.
"The flex sensor works by putting an LED on one end of a fabric tube and a photoresistor on the other," Medley explains of his homebrew sensor device. "When the tube bends the amount of light reaching the photoresistor decreases which an Arduino can read as a change in voltage. Using this approach saves about $19 relative to the cost of purchasing a real flex sensor."
The sensor was created by gluing strips of plastic — cable ties with the raised end cut off — onto a piece of fabric which was then rolled into a tube and taped into place. An LED and a current-limiting resistor sit at one end, with a simple and low-cost photoresistor — also known as a light-dependent resistor, or LDR - at the other.
As the tube flexes, less and less light gets through — causing the resistance to change. Eventually, the tube flexes to the point where none of the light from the LED reaches the photoresistor — but releasing the tube allows the plastic ribs to spring it back into place again.
It's by no means a new approach: The original prototype of the infamous Power Glove accessory for the Nintendo Entertainment System used optical flex sensors to track the bend of each finger — though these were replaced with carbon-based sensors in the consumer version to bring down the cost.
A full series of build images are available on Medley's website to help you build your own flex sensor.