Practical guidance for designing with Force Sensing Resistors — from basic circuits to production-ready integration.
An FSR is a variable resistor — its resistance changes with applied force. Reading that resistance requires a circuit that converts resistance change into a voltage your microcontroller or data acquisition system can measure.
The simplest approach connects the FSR in series with a fixed reference resistor (Rref) between the supply voltage and ground. The voltage at the junction is read by an ADC input. When no force is applied, FSR resistance is very high and the output voltage is near zero (or near Vsupply, depending on orientation). As force increases and FSR resistance drops, the output voltage changes proportionally.
The value of Rref determines the sensitivity range. A good starting point is to set Rref roughly equal to the FSR resistance at the midpoint of your desired force range. For light-touch applications (under 100g), try 100kΩ to 1MΩ. For heavier force ranges (1-10kg), try 1kΩ to 10kΩ.
For a more linear and consistent output, use an operational amplifier in an inverting configuration with the FSR as the input element. Apply a fixed voltage to one end of the FSR, and connect the other end to the inverting input of the op-amp. A feedback resistor (Rf) from the output to the inverting input sets the gain. The output voltage is then proportional to the current through the FSR, which varies with force.
This configuration eliminates the loading effects of a simple voltage divider and provides better linearity across the force range. The feedback resistor value controls the output scale — larger Rf gives higher sensitivity to light forces.
If you only need to detect whether force exceeds a threshold (press/no-press), a comparator circuit is the most straightforward approach. Use a voltage divider to set the FSR output, feed it to one input of a comparator, and set the threshold voltage on the other input with a trim potentiometer or resistor divider. The comparator output switches cleanly between high and low states.
Raw FSR signals often benefit from conditioning before they reach your application software. The right conditioning depends on your application's speed, noise environment, and resolution requirements.
For human-interface applications (buttons, sliders, grip sensing), a simple low-pass filter removes high-frequency noise while preserving the relatively slow force signals. A single-pole RC filter with a cutoff frequency of 10-50 Hz works well for most touch applications. For musical instruments or fast impact detection, you may need higher cutoff frequencies (100-500 Hz) to preserve the attack transient.
Digital filtering in software (moving average, exponential smoothing, or median filter) is often easier to tune than analog filters and can be adjusted without hardware changes.
The FSR's resistance-vs-force curve is inherently nonlinear. If your application needs a linear force output, you have two options. The hardware approach uses the op-amp current-to-voltage converter described in AN-1, which produces a more linear voltage-vs-force relationship. The software approach applies a lookup table or polynomial correction to the raw ADC reading, mapping the nonlinear input to a calibrated force value.
For applications requiring quantitative force values (not just relative changes), calibrate each sensor assembly against known weights at several points across the force range. Store the calibration data in firmware as a lookup table. Calibrate the complete assembly — including the overlay, mounting, and actuation mechanism — since these all affect the force transfer to the sensing element.
Mechanical design is often the most critical factor in FSR system performance. The same sensor can give excellent or poor results depending on how it's mounted and actuated.
The sensor should be mounted on a firm, flat, smooth surface. Any irregularity on the mounting surface — bumps, ridges, screw heads, rough textures — will create localized pressure points that affect readings. If the mounting surface isn't perfectly flat, use a thin compliance layer (silicone sheet, firm foam) between the surface and the sensor back to distribute contact pressure evenly.
How force reaches the sensing area matters enormously. Direct finger pressure through a thin overlay works well for touch interfaces. For mechanical actuation (a lever, cam, or spring), ensure the force is applied through a flat, smooth surface that covers the active sensing area uniformly. Avoid point loads on the sensor edge — they produce unreliable readings and can damage the sensor over time.
The actuation area relative to the sensing area also affects sensitivity. A puck or button that concentrates force onto a small portion of the sensor will produce a stronger signal per unit of applied force than distributed pressure across the entire sensing area.
The overlay — the material between the user's finger (or actuator) and the sensor surface — affects force transmission and feel. A rigid overlay (polycarbonate, acrylic) transmits force efficiently but provides no tactile give. A compliant overlay (silicone rubber, TPU) improves feel and distributes force more evenly, which can improve sensor-to-sensor consistency. The overlay thickness and stiffness interact with the spacer design to determine the activation force and feel.
Pressure-sensitive adhesive (PSA) applied to the sensor back is the standard attachment method. Use a thin, uniform adhesive layer — thick or uneven adhesive can affect sensor response. For high-temperature applications or environments with vibration, consider mechanical clamping or gasket retention rather than adhesive alone. The sensor tail (where the conductors exit) should be strain-relieved to prevent flex fatigue at the junction.
MatrixArray sensors consist of rows and columns of sensing elements arranged in a grid. Reading the array requires scanning electronics that address each intersection point sequentially.
The standard technique drives one row (or column) at a time while reading all columns (or rows) simultaneously. By cycling through all rows in sequence, you build a complete force map of the array. The scan rate must be fast enough to capture the forces of interest — for static loads, a few scans per second suffices; for musical instruments or impact detection, you may need hundreds or thousands of scans per second.
In resistive matrix arrays, current can flow through unintended paths via adjacent sensor elements, creating "ghost" readings. This crosstalk is inherent to passive resistive matrices. Mitigation strategies include driving one row at a time with all other rows at high impedance, using current-sensing (rather than voltage-sensing) readout circuits, adding isolation diodes at each intersection (which adds manufacturing complexity), and applying software correction based on the known crosstalk pattern.
For applications requiring high spatial accuracy, Sensitronics can design arrays with optimized electrode geometry and provide companion scanning electronics that minimize crosstalk.
Matrix array data is most useful when displayed as a 2D pressure map — a color-coded grid where each cell's color represents the force magnitude at that position. Real-time visualization helps with system debugging, calibration, and end-user applications. Our custom DAQ electronics service includes companion visualization software when needed.
FSRs operate reliably across a range of environmental conditions, but understanding how temperature, humidity, and sustained loading affect performance helps you design robust systems.
FSR resistance values shift with temperature — resistance generally decreases as temperature increases. For room-temperature applications (15-35°C), this shift is small and often negligible. For wider temperature ranges, you can compensate in software using a temperature sensor co-located with the FSR, or specify temperature-stable ink formulations during the design phase.
FSR sensors are constructed from sealed polymer layers, but extreme humidity combined with high temperatures can affect long-term performance. For outdoor, automotive, or tropical environments, conformal coating the sensor edges and tail area provides additional protection. We can recommend specific environmental sealing approaches for your application.
When an FSR is held under constant force for extended periods (minutes to hours), its resistance can drift slightly — typically decreasing over time. This is a fundamental characteristic of polymer thick-film resistive sensors. The magnitude of drift depends on the ink formulation, force level, and temperature.
For applications involving sustained loads — such as seat occupancy detection, weight measurement, or clamping force monitoring — our LowDrift technology uses specially formulated inks and optimized sensor construction to minimize this effect. If your application involves continuous loading, specify this requirement early in the design process.
Proper testing ensures your FSR system performs reliably in the field. Here are the key tests to consider during development.
Apply known forces to the sensor in your actual mechanical assembly (not a bare sensor on a bench) and record the electrical output. Use calibrated weights or a force gauge. Test at a minimum of 5-10 force levels spanning your operating range. Plot force vs. output voltage to understand your system's transfer function. Repeat on multiple samples to characterize part-to-part variation.
Apply the same force repeatedly (50-100 times) and measure the variation in output. Good FSR systems achieve ±2-5% repeatability. If variation is higher, check your mechanical integration — inconsistent force distribution is the most common cause of poor repeatability.
For applications involving repeated actuation (buttons, foot pedals, drum pads), perform accelerated life testing at your expected force level and actuation rate. Monitor sensor output over thousands or millions of cycles to establish the useful lifetime. Most FSR applications achieve millions of cycles without significant degradation.
If your product operates outside controlled indoor environments, test the sensor assembly across the expected temperature and humidity range. A basic temperature chamber test — cycling between your specified extremes while monitoring sensor output — will reveal any environmental sensitivity issues early in development.
Our engineering team can help you design, integrate, and validate FSR systems for your specific requirements.
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