How Does a Force Sensing Resistor (FSR) Work?

Force‑sensing resistors (FSRs), sometimes called resistive force sensors or force‑sensitive resistors, are thin, flexible components that detect applied force by changing their electrical resistance. In this guide, we’ll explain how an FSR force‑sensing resistor works, how to calibrate one, and common applications in modern device design.

How FSRs Measure Force

At the heart of every FSR is a piezoresistive element: a conductive ink layer sandwiched between two substrates. When no force is applied, the sensor’s resistance is very high, often in the megaohms (MΩ) range. As you press on the sensing area, the conductive particles within the ink move closer together, lowering the resistance into the kiloohms (kΩ). The inverse of resistance, conductance, responds in an approximately linear fashion over the sensor’s specified force range. By measuring voltage changes across the FSR in a simple voltage‐divider circuit, you can infer the magnitude of applied force.

FSR Construction: Shunt Mode vs. Thru Mode

Manufacturers typically produce FSRs using one of two architectures:

• Shunt Mode FSRs consist of two polymer‐coated membranes separated by an air gap. One membrane carries interdigitated conductive traces, while the other holds the textured resistive ink. When force presses the membranes together, the ink “shunts” across the traces, reducing resistance.
• Thru Mode FSRs use a polyester film with front and back electrodes sandwiching a conductive polymer layer. Under load, the polymer compresses, bringing the electrodes closer and dropping resistance. These are often favored for their thin profile and ease of integration into tight spaces.

Calibrating a Force‑Sensing Resistor

Raw FSR outputs don’t equate directly to engineering units like newtons or pounds‐force. To convert resistance changes to force values, you must perform a two‑ or three‑point calibration using known weights or loads.

1. Record the FSR reading with no load (zero force).
2. Apply a known force (for example, 10 N) and record the output.
3. (Optional) Use a second known load (e.g., 20 N) for improved accuracy.

By fitting these data points to a linear or piecewise‐linear model, you create a calibration curve that translates raw resistance into force measurements. Adjustable circuit components, like potentiometers, can fine‑tune sensor sensitivity for specific force ranges

Signal Conditioning and Readout

Most FSRs are passive elements requiring external electronics to interpret their resistance changes. A typical setup includes:

• A voltage divider or current source to generate a measurable voltage proportional to conductance.
• An ADC (analog‑to‑digital converter) on a microcontroller or data acquisition system converts the voltage into digital values.
• Filtering may be applied to reduce noise, especially in dynamic applications where force changes rapidly.

With proper conditioning, FSRs can resolve small changes in force and provide fast response times suitable for tactile sensing, robotics, and wearable devices.

Applications of Force‑Sensitive Resistors

FSRs are prized for their low cost, thin form factor, and flexibility. Common uses include:

• Human–machine interfaces (e.g., pressure‑sensitive buttons or touch pads).
• Robotics for grip force feedback and object manipulation.
• Medical devices such as bedsore prevention mats and prosthetic socket pressure mapping.
• Consumer electronics including smart textiles and gaming controllers.

Their versatility and ease of integration make force‑sensing resistors a popular choice in R&D and product development.

Tips for Maximizing FSR Performance

• Choose the right size and force range: FSRs come in various active‐area diameters and force ratings; match these to your application’s expected load.
• Consider temperature effects: Extreme temperatures can shift baseline resistance, account for this in your calibration.
• Protect with overlays: Thin silicone or fabric covers can shield the sensor from wear, moisture, and contaminants without significantly affecting sensitivity.

Final Thoughts

A force‑sensing resistor is a simple yet powerful resistive force sensor technology. By understanding how an FSR force‑sensing resistor works, its piezoresistive principle, construction modes, calibration steps, and typical applications, you can select and integrate the right sensor for your project. Whether you’re building a prototype or scaling a production line, FSRs provide an accessible path to accurate force detection in a thin, flexible package.