Overview

The FSR 402 is a robust single-zone Force Sensitive Resistor (FSR) manufactured by Interlink Electronics. Featuring a 0.5-inch ($12.7\text{ mm}$) active circular sensing diameter in a thin ($0.45\text{ mm}$) polymer package, it exhibits a non-linear inverse relationship between applied mechanical force and electrical resistance.

Unpressed, the FSR 402 acts as an open circuit with a baseline resistance exceeding $1\ \text{M}\Omega$. Applying a light touch ($\approx 20\text{ grams} / 0.2\text{ N}$) breaks the contact threshold, causing resistance to drop sharply to $\sim 100\ \text{k}\Omega$; pushing firmly with heavy force ($\approx 2\text{ kg} / 20\text{ N}$) drops resistance below $1\ \text{k}\Omega$. It is widely used in electronic drum pads, touch-sensitive musical instruments, robot gripper force feedback, and bed occupancy sensing.

Quick reference

Terminal Layout & Mechanical Geometry

       ┌───────────────────────┐
       │     (O) Active Area   │  [=== Pin 1
       │       Ø 12.7 mm       │  [=== Pin 2
       └───────────────────────┘

• Pin 1 & Pin 2: Non-polarized 2-pin female solder tab connector ($0.1\text{"} / 2.54\text{ mm}$ spacing).
• Actuator Tip: For maximum force transfer, apply force through a soft rubber pad or hemispherical rubber bumper (actuator tip) placed over the active circular center area. Avoid pressing with sharp metal edges.

Specifications

Voltage Divider Circuit Math

To read force on an analog microcontroller ADC, place the FSR 402 in series with a fixed $10\ \text{k}\Omega$ pull-down resistor ($R_M$):

$V_{OUT} = V_{CC} \times \left( \frac{R_M}{R_{FSR} + R_M} \right)$

• Unpressed State ($R_{FSR} > 1\ \text{M}\Omega$):

$V_{OUT} \approx 5.0\text{V} \times \left( \frac{10\text{k}}{1000\text{k} + 10\text{k}} \right) \approx 0.05\text{V} \quad (ADC \approx 0)$

• Light Press ($R_{FSR} \approx 30\ \text{k}\Omega$):

$V_{OUT} = 5.0\text{V} \times \left( \frac{10\text{k}}{30\text{k} + 10\text{k}} \right) = 1.25\text{V} \quad (ADC \approx 256)$

• Firm Press ($R_{FSR} \approx 1\ \text{k}\Omega$):

$V_{OUT} = 5.0\text{V} \times \left( \frac{10\text{k}}{1\text{k} + 10\text{k}} \right) = 4.54\text{V} \quad (ADC \approx 930)$

Wiring

Example

const int fsrPin = A0;
const float VCC = 5.0;       // ADC Reference Voltage
const float R_DIV = 10000.0;  // Fixed 10k resistor

void setup() {
  Serial.begin(9600);
}

void loop() {
  int fsrADC = analogRead(fsrPin);
  
  if (fsrADC > 10) { // If pressure detected
    float fsrV = fsrADC * (VCC / 1023.0);
    float fsrR = R_DIV * (VCC / fsrV - 1.0); // Calculate FSR Resistance

    Serial.print("Raw ADC: "); Serial.print(fsrADC);
    Serial.print(" | Voltage: "); Serial.print(fsrV); Serial.print(" V");
    Serial.print(" | Resistance: "); Serial.print(fsrR / 1000.0); Serial.println(" kOhm");
  } else {
    Serial.println("No force applied (Unpressed)");
  }

  delay(200);
}

Common mistakes

• Soldering directly to the silver flexible solder tabs: Direct soldering melts the flexible polyester substrate. Use male/female jumper header pins or crimp connectors.
• Using FSRs for precision weight scales: FSR 402 sensors are qualitative force/touch sensors ($\pm 15\%$ to $\pm 25\%$ accuracy) rather than precision weight measurement load cells. For precision scale builds, use load cells paired with an HX711 ADC.

Notes

• FSR Family: FSR 400 (5mm micro disc), FSR 402 (12.7mm round disc), FSR 406 (38mm square), FSR 408 (600mm long strip).