The Flex Sensor from Electrotap and distributed by Cycling 74 is a sensor that responds to a physical range between a straight "line" and a 90° bend.
This sensor uses the same technology as many of the glove controllers used for video consoles or computer games. It outputs a conditioned signal in the industry standard voltage range of 0.25 to 4.75V.
Note! Flex sensors can be prone to produce signals that have some noise in them. It is highly recommended that you low-pass filter the sensor signal before using it to eliminate the noise or spikes from the signal.