Flex sensor

A flex sensor, the 2.2 inch Spectra Symbol strip: thin plastic with carbon ink along one side, two legs at one end. Aliases: bend sensor, flex strip.

Bend it with the ink on the outside of the curve and the ink cracks open into more, smaller islands, and the resistance goes up. It is another resistor whose value means something, and it goes in a divider like the others. The glove that reads your fingers is five of these.

Pins

Pin	What it does
1	One leg.
2	The other leg.

Properties

flat is the resistance lying flat, 25 000 ohms by default. bent is the resistance folded right over, 180 degrees, 100 000 ohms by default.

While it runs

Drag the slider to bend the strip, 0 (flat) to 180 degrees (folded over). The top of the strip curls as it bends.

What the model gets right

The two numbers the datasheet gives: 25 k flat, and a bend resistance of 45 k to 125 k depending on the bend. The model is a straight line between flat and bent, which puts a right angle at 62.5 k, inside the sheet's range. It bends one way only, as the real strip is meant to.

What it does not model

The datasheet's 30% tolerance (every strip here is the typical one), how the reading depends on where along the strip the bend is and how tight it is rather than just the angle, the slow creep back after a bend is released, and wear: a real strip reads a little higher every month. Bent backwards, a real one barely changes; the slider here does not go that way.

Common mistakes

Choosing the divider resistor too small. The strip lives between 25 k and 100 k, so a fixed resistor of about 47 k puts the reading in the middle of the ADC.

See it in action

Squeeze and bend puts a flex sensor on a multimeter beside an FSR lighting a lamp. Open it at /templates.