Overview

The Flex Sensor 2.2" (manufactured by Spectra Symbol and distributed by SparkFun and Adafruit) is a flexible carbon-based variable resistor strip. As the sensor is physically bent, conductive carbon particles printed on its flexible polymer substrate separate, causing electrical resistance to increase proportionally to the bend angle.

Unbent (flat), the 2.2-inch sensor has a nominal baseline resistance of $10\ \text{k}\Omega$. Bending the sensor $90^\circ$ away from the ink-printed surface increases its resistance to approximately $30\ \text{k}\Omega$ to $40\ \text{k}\Omega$. It is widely used in VR data gloves (finger flexion tracking), animatronic puppet controls, robotic joint position sensing, and wearable motion capture.

Quick reference

Terminal Layout & Physical Structure

       ┌──────────────────────────────────────────────┐  [=== Pin 1
       │  Spectra Symbol Flex Sensor 2.2" (Ink Facing) │  [=== Pin 2
       └──────────────────────────────────────────────┘
       <--------------- 55.88 mm Active --------------->

• Pin 1 & Pin 2: Non-polarized 2-pin solder tab header ($0.1\text{"} / 2.54\text{ mm}$ spacing).
• Direction of Bend: Bend away from the printed text/ink side (so the ink layer is stretched on the outer curve of the bend) for maximum resistance increase and longest mechanical life.

Specifications

Voltage Divider Circuit Math

To convert variable resistance into a voltage readable by a microcontroller ADC, place the flex sensor in series with a fixed $10\ \text{k}\Omega$ pull-down resistor ($R_F$):

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

• Flat State ($R_{flex} = 10\ \text{k}\Omega$):

$V_{OUT} = 5.0\text{V} \times \left( \frac{10\text{k}}{10\text{k} + 10\text{k}} \right) = 2.50\text{V} \quad (ADC \approx 512)$

• $90^\circ$ Bent State ($R_{flex} = 30\ \text{k}\Omega$):

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

Wiring

Example

const int flexPin = 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 flexADC = analogRead(flexPin);
  float flexV = flexADC * (VCC / 1023.0);
  
  // Calculate Flex Sensor Resistance
  float R_flex = R_DIV * (VCC / flexV - 1.0);

  // Map estimated angle (10k = 0 deg, 30k = 90 deg)
  float angle = map(R_flex, 10000, 30000, 0, 90);
  angle = constrain(angle, 0, 90);

  Serial.print("ADC: "); Serial.print(flexADC);
  Serial.print(" | Resistance: "); Serial.print(R_flex / 1000.0); Serial.print(" kOhm");
  Serial.print(" | Angle: "); Serial.print(angle); Serial.println(" deg");

  delay(200);
}

Common mistakes

• Bending near the base connector: Concentrating mechanical stress at the solder tabs snaps internal metal contacts. Keep bends within the middle 2-inch flexible zone.
• Forgetting per-sensor calibration: Resistance values vary $\pm 30\%$ between manufacturing lots ($R_{flat}$ can range from $7\ \text{k}\Omega$ to $13\ \text{k}\Omega$). Always calibrate flat and bent ADC thresholds in code using map().

Notes

• Flex Sensor Lengths: Available in 2.2-inch (short) and 4.5-inch (long) formats.