The flex sensor pictured below changes resistance when bent. It will
only change resistance in one direction (out of the screen in relation
to the picture below). An unflexed sensor has a resistance of about
10,000 ohms. As the flex sensor is bent, the resistance increases to
30-40 kohms at 90 degrees. The sensor measures 1/4 inch wide, 4-1/2
inches long and 0.19 inches thick.
If R2 = 33 kohm, and the input voltage is 5 volts,
then the output voltage will be in the range from 2.8 to 4 volts.
The positive power to the op-amp can be 5 volts and the negative power
to the op-amp can be 0 volts.