How does a Force-Sensitive Resistor work?
Does it work by sensing the number of tracks the user covers meaning that the more force the user pushes on the sensor the less resistance it causes?
Here is a picture of one:
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\$\begingroup\$ One possible method for a home built FSR can be found here: instructables.com/id/Pressure-Sensor-Matrix This makes use of 2 strips semi-conductive foam. The harder you press, the more contact area is made between the two pads, lowering the resistance. I didn't post this as an answer because I don't know if that's a commonly used method, or just one for DIY FSRs. \$\endgroup\$– PonkadoodleJan 20, 2011 at 22:43
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3 Answers
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A piece of flexible conductive material with a high resistance is under the tracks, and the resistance between the tracks reduces when the sensor is pressed.
I once made a crude one from a piece of conductive foam, which worked in a similar fashion.
Piezo film is another technique that is sometimes used. I've also experimented with that, and it works very well.
answered Jan 20, 2011 at 21:53
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\$\begingroup\$ The one in the picture looks like it's sealed and you push the tracks against the dark material (probably something resistive), rather than using human skin, which has terrible manufacturing tolerances and temperature coefficients. \$\endgroup\$– Nick TJan 20, 2011 at 22:14
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\$\begingroup\$ I think you are right. It's more like my second suggestion, then. I've amended my answer. \$\endgroup\$ Jan 20, 2011 at 22:16
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The one you picture here is a polymer film that has conducting particles held within it (think of something like novolac resist or SU8 with metal micro-particles evenly distributed within it). The film has a non-zero conductivity.
When you press on it some of the micro-particles touch, increasing the conductivity. Pretty simple.
It's not the same as the piezo-resistive effect, which depends on the shape of a crystal lattice in the atomic structure. when you bend it, the crystal lattice either expands or contracts, causing the resistance to increase or decrease because of a slight change in the energy bands of the crystal.
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\$\begingroup\$ Answering with a question is normally not productive, as noone can answer you. \$\endgroup\$– KortukJan 20, 2011 at 22:18
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\$\begingroup\$ @Kortuk... you'll rue the day you met me Trebek! \$\endgroup\$– vicatcuJan 21, 2011 at 1:31
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\$\begingroup\$ I can answer him. The answer is no, it's not a piezo-resistive effect. It's completely different. Piezo-resistance is a quantum effect related to the change in the inter-atomic distances and the related band-gap. The effect that is questioned here is a mechanical effect, causing particles to touch by squeezing a film with a higher-than-the-particles resistance. \$\endgroup\$– bevFeb 16, 2011 at 5:28