## Force-Sensitive Resistor (FSR) Overview A **Force-Sensitive Resistor (FSR)** is a passive sensor component that changes its electrical resistance based on the amount of physical pressure (force) applied to its surface. --- ### 1. Working Principle The physics of an FSR is based on **Piezoresistivity**. * **No Pressure:** When no force is applied, the sensor acts like an open circuit (infinite resistance). * **Increased Pressure:** As force increases, a conductive layer is pressed harder against a resistive layer, creating more contact points. This reduces the overall electrical resistance. * **The Curve:** The relationship is non-linear. Resistance drops sharply at the beginning (low force) and levels off at higher force levels. ### 2. Physical Construction An FSR typically consists of four main layers: | Layer | Description | | :--- | :--- | | **Protective Film** | Top plastic layer (Mylar/Polyimide) to protect internal traces. | | **Conductive Electrodes** | Interdigitated silver or carbon traces (finger-like patterns). | | **Spacer Layer** | An air gap or thin adhesive that keeps the layers apart when idle. | | **Resistive Material** | A specialized polymer thick film (PTF) containing conductive particles. | --- ### 3. Electronic Specifications When selecting an FSR for a circuit, pay attention to these parameters: * **Actuation Force:** The minimum force required to trigger a change (typically ~20g to 100g). * **Sensitivity Range:** The maximum force it can detect (e.g., up to 10kg or 20kg). * **Resolution:** Depends on the ADC (Analog-to-Digital Converter) of the microcontroller used. * **Response Time:** Usually very fast (< 3 microseconds). --- ### 4. Basic Circuitry: The Voltage Divider Because an FSR is a variable resistor, it cannot be read directly by a microcontroller. It must be placed in a **Voltage Divider** circuit to convert resistance into a measurable voltage. ```text VCC (5V/3.3V) | [ FSR ] | +-----> Output Voltage (To Analog Pin) | [ R_fixed ] (e.g., 10k Ohm) | GND ``` **Formula for Output Voltage ($V_{out}$):** $$V_{out} = V_{cc} \times \frac{R_{fixed}}{R_{FSR} + R_{fixed}}$$ --- ### 5. Applications * **Human-Machine Interface:** Electronic drum pads, MIDI controllers, and pressure-sensitive buttons. * **Robotics:** Tactile sensors for grippers to detect if an object is being held. * **Medical:** Smart insoles to analyze gait or bed-occupancy sensors. * **Industrial:** Measuring seal pressure or weight distribution on surfaces.