Understanding and Using Force-Sensitive Resistors (FSRs)

A Force-Sensitive Resistor (FSR) is a type of resistor whose resistance changes in response to the applied force or pressure. Think of it as a variable resistor that you control with your touch!

FSRs are also known as force sensors, pressure sensors, or touch sensors. They’re typically made from a conductive polymer material that decreases in resistance as you apply force to its surface. The amount the resistance changes is proportional to the applied force, which makes FSRs really handy for measuring force or pressure.

FSR Construction, Structure, and Working

The core of an FSR is a layer of conductive polymer material. Here’s how it works:

• At Rest: When no force is applied, the conductive particles within the polymer are relatively far apart, resulting in a high resistance.
• Under Pressure: When you apply force, these conductive particles get pushed closer together. This creates more pathways for electricity to flow, thereby reducing the resistance.

This conductive polymer layer is usually mounted on a flexible substrate. This allows the sensor to bend and conform to the shape of whatever it’s pressed against. Electrodes are attached to this layer to measure the resistance changes.

Arduino Interfacing with a Force-Sensitive Resistor

Here’s how you can connect an FSR to an Arduino and start reading force data:

Wiring it Up

1. Connect one end of the FSR to the 5V output on the Arduino.
2. Connect the other end of the FSR to an analog input pin (e.g., A0) on the Arduino.
3. Connect a resistor (e.g., 10kΩ) from the analog input pin to the ground. This resistor forms a voltage divider with the FSR, allowing you to measure the voltage change caused by the FSR’s resistance change.

Visual Aid

Image showing how to connect an FSR to an Arduino.

Arduino Code

Here’s what you need to do in your Arduino code:

1. Read the Analog Input: Use the analogRead() function to read the voltage from the analog input pin. This value will correspond to the resistance of the FSR, which is related to the applied force.
2. Conversion Formula: Use a conversion formula to translate the analog input values into meaningful force or pressure readings. The exact formula will depend on your specific FSR and its characteristics. Often, you’ll need to experiment to find the best formula.
3. Calibration: Calibration is often required to establish an accurate relationship between the FSR’s resistance and the actual force or pressure being applied. You’ll want to apply known forces and record the corresponding analog readings. This data can then be used to create a calibration curve or formula.

Example Code

Here’s a simple Arduino code example to read values from an FSR:

const int fsrPin = A0;  // Analog pin connected to the FSR
const int analogMax = 1023; // Maximum value for analogRead

void setup() {
  Serial.begin(9600);  // Initialize serial communication
}

void loop() {
  int fsrReading = analogRead(fsrPin);  // Read analog input
  float force = map(fsrReading, 0, analogMax, 0, 100);  // Map the reading to a force range

  Serial.print("FSR Reading: ");
  Serial.print(fsrReading);
  Serial.print("\tForce: ");
  Serial.println(force);

  delay(500);  // Add a delay to avoid rapid serial printing
}

This code reads the analog input from the FSR, maps it to a force range (in this case, 0 to 100), and prints the FSR reading and corresponding force value to the serial monitor. Remember that the “force” value here is arbitrary and depends on your FSR’s characteristics and your desired units.

Applications of FSR

Force-sensitive resistors are used in a wide variety of applications, including:

• Touch Sensing: Touch-sensitive interfaces for electronic devices. Detect touch and pressure changes on surfaces.
• Medical Devices: Pressure-sensitive pads in medical applications for monitoring pressure variations (e.g., in beds to prevent bedsores).
• Robotics: Force feedback and touch-sensitive robotics. Allow robots to “feel” their environment and apply appropriate forces.
• Gaming Controllers: Detect varying levels of pressure in buttons and triggers.
• Biomechanics: Measure pressure distribution on surfaces (e.g., in shoes to analyze gait).
• Human-Machine Interfaces: Create interfaces that respond to touch and pressure, providing more intuitive control.

Advantages of FSR

Here are some of the key benefits of using FSRs:

• High Sensitivity: FSRs are very sensitive to applied force or pressure.
• Thin and Flexible: This makes them suitable for applications where a conformable and compact force-sensing solution is needed.
• Easy Integration: They can be easily connected to analog input channels on microcontrollers or interface directly with other electronic circuits.
• Versatile Force Ranges: FSRs are available in different force ranges, allowing you to choose a sensor that suits your specific application.
• Dynamic Response: They provide a dynamic response to force changes, making them suitable for applications requiring real-time force feedback or continuous monitoring.

Conclusion

Integrating an FSR with an Arduino makes it easy to add force-sensing capabilities to your projects. FSRs are also often used alongside collision impact sensors to measure the impact of collisions. The benefits outlined above make force-sensitive resistors valuable in various fields, including human-machine interfaces, robotics, medical devices, and industrial applications where precise force sensing is essential.