Introduction

                A force sensing resistor a variable resistor that can be used to measure force. Force sensing resistors are the core component of load cells, pressure transducers, digital scales and many other things.

Theory

                A force sensing resistor is basically a shaped resistor, see the figure below:

                                                                                                                                                                                                 

                                   Figure 1: Force sensing resistor with and without force applied.

As force is applied to the resistor, its shape and thus its resistance both change, by measuring this change in resistance, the force can be calculated.

Technical Specifications

The force sensing resistor featured in this tutorial is shown below.  Its sensing area is

  
                                                               Figure 2: FSR 402 force sensing resistor.

12.7mm in diameter, and it is .45 mm thick. Its resistance ranges from about 1 MΩ with no applied force to 100 kΩ with an applied force of 100 N.

Connections

Connect the FSR in series with a resistor to the Arduino as shown below:

                                                                                                                                                                                                   

                                                                              Figure 3: Force sensing setup.

The red and black wires are 5v and ground respectively while the light blue wire should be connected to analog in. The resistor should be around 10 – 30 kΩ to give a good range of values when force is applied to the resistor.

Code

void setup()
{
  // put your setup code here, to run once:
  Serial.begin(9600);   // to display data
}

int value;

void loop()
{
  // put your main code here, to run repeatedly:
  value = analogRead(A0);       // read sensor output
  Serial.println(value);        // output value
  delay(50);                    // delay 50 ms to make easier to read
}

 A figure showing sample output is shown below, the spikes correspond to when the sensor is pressed, calibration would be required to convert the analog output to force values.


                                                                                                               

                                                                                      Figure 4: Sample FSR output.