# Flex sensor to determine a force

Hi guys,

I want to start a project, and i want to be able to determine a force using a flex sensor like in the attachment.

I know that flex sensor return a resistance value. How can i use that resistance value to determine the necessary force?

Thank you very much.

mariusdej:
I know that flex sensor return a resistance value. How can i use that resistance value to determine the necessary force?

Use a second resistor to make a voltage divider (that's a good term to google if you don't know it) and then you can measure the voltage at the center of the divider with an analogRead. I find it hard to believe that there aren't any good tutorials on the net for working with a flex sensor.

Once you are getting analog readings from your sensor, apply various known forces and note the analog reading for each. Graph them to see if they form a straight line. If so, use map() to convert from analog reading to force. If not, use multimap() to interpolate between samples.

In this circuit, we will use a flex sensor to measure, well, flex! A flex sensor uses carbon on a strip of plastic to act like a variable resistor, but instead of changing the resistance by turning a knob, you change it by flexing (bending) the component. We use a “voltage divider” again to detect this change in resistance.
The sensor bends in one direction and the more it bends, the higher the resistance gets; it has a range from about 10K ohm to 35K ohm. In this circuit we will use the amount of bend of the flex sensor to control the position of a servo.

"About" 10k to 35k, means that you will need to calibrate it, but I would be more worried about stability and repetition of readings. I would understand a force sensor like this is not meant for the preciset readings that you would require.

The diagram you have supplied is indicating you need a "Strain Gauge" which is bonded to the material under test which you then calibrate to your force F.

The response in strain gauge or flex will not be linear to the force applied.

Tom...