SparkFun: 0.5" Force Sensitive Resistor

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The force-sensitive resistor.

Overview

The SparkFun 0.5" Force Sensitive Resistor varies its resistance based on the amount of force applied to it. The larger the force, the lower the resistance of the device. When no force is being applied, the resistance is larger than 1MΩ. Like a regular resistor, it only has two leads. The FSR Integration Guide can be accessed here.

Bone Usage

Physical

The force-sensitive resistor works well when used with one of the BeagleBone's analog inputs. The images below outline how to hook it up. This schematic on the left is modified and originally from the SparkFun tutorial on the force-sensitive resistor. The example on the right outlines one way to hook-up the resistor to read values from the Bone.

Schematic Bone example

A 27kΩ resistor is used to connect the 3.3V source (pin 3) from the Bone to one pin of the force-sensitive resistor. Then, the other pin of the force-sensitive resistor is connected to ground (pin 1). An analog pin (AIN) on the Bone connects to the node containing both resistors, and values can be read off of that AIN. I'm using AIN5 (pin 36) to read values on the Bone.

Reading Values

Reading values off of the analog input is the same as explained in Exercise 10. You can "cat" ain6, and the value will range from around 0 to 4096. When no force is applied to the force-sensitive resistor, the value is close to 4096. When maximum force is applied, the value is close to 0.

Programmatically, this means that you can read that ain6 value, which is based on how much force is applied, and do anything you want with it. The following code reads the analog value and saves it as an int named analogValue:

 FILE* file = fopen("/sys/devices/platform/omap/tsc/ain6", "r");
 int analogValue = 0;
 fscanf(file, "%d", &analogValue);
 fclose(file);

The example program below uses this idea to light an LED based on how much force is applied to the force-sensitive resistor.

Example Program

Bone config for example program

I've written a program which demonstrates usage of the force-sensitive resistor, available here. The input is the force-sensitive resistor on pin 36 (AIN5), and the output is an LED on pin 12 (GPIO1_28). The physical configuration is shown in the image on the right.

The result of this program is that the harder one squeezes on the force-sensitive resistor, the brighter the LED illuminates. It is configured to require quite a bit of force for maximum brightness.