Smart O’clock: Description of the FSR (Force sensing resistors)

Hi everyone,

Here is the description of the sensor that we are going to use:

Force Sensing Resistors (FSR) are a polymer thick film which is a resistor that changes its value depending on the force detected.

Characteristics:

As we can see on the datasheet of the FSR 400 series, the minimal force that can be detected by the sensor is 0.2 N so 0.02 kg and the highest force which can be reached is 20 N so 2 kg. The sensitive surface of the sensor is: 0.0396*0.0396=1.56816*10-3 m².

The surface that we choose to use is a PVC support of 0.25 m².

So, the characteristics for this surface are:

  • Minimal value: 0.2*=1.96*10-3 N so 1.96*10-4 kg
  • Maximal value: 20*=3188.45 N so 318.845 kg

As the sensor detect at a really low value, we decide to choose our own value that must be detected to turn off our system. We will determinate this value when we own the sensor and when we already test it.

Theoretical value

If we choose to have a minimum of 10 kg on the support. We need to have a force of 0.063 kg on the sensor. It means a force of 63 g on the sensor, with the graph mentioned below, we can determinate the output voltage that we have to put on our program.

Theoretically, the output voltage is: Vout= . As we want to limit the current and the sensitive range, we decide to choose an Rm=10kΩ.

For 63 g and Rm=10kΩ, the system will turn off when it will detect a voltage of 2 V.

Datasheet of the FSR: http://www.adafruit.com/datasheets/FSR400Series_PD.pdf

Good luck everyone,

Smart O’clock group

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