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Using Electronic Modules with Tim Blythman Thin-Film Pressure Sensor Being able to sense force and pressure is handy as it allows properties like weight to be measured. While industrial-grade pressure sensors are available at higher prices, thin-film pressure sensors use a simpler technology and are much cheaper. F orce and pressure sensors are used in industrial applications. In addition to directly measuring pressure (such as in a gas reaction vessel), they can measure liquid volumes and weight. Pressure can be related to liquid volume since the height of a liquid column and its density dictate the amount of pressure it exerts. If you can apply the pressure over a known area, the applied force can also be known and thus the weight-derived force due to gravity can be determined. Just about any product you can buy by weight or volume has been precisely measured out using a sensor such as a strain gauge. These are among the more common types used for this purpose since they have the necessary accuracy. Of course, accuracy comes at a cost, and many projects don’t need the kind of accuracy these devices provide. That said, strain gauge sensors and their interface electronics are readily available to the hobbyist if that level of accuracy is needed. Thin-film sensors So-called thin-film pressure sensors are also known as force-sensitive resistors; simply put, they are devices that change their resistance when force is applied to them. This makes them quite easy to use since a simple resistive voltage divider is sufficient to get a reading using an ADC (analog-to-­ digital converter). The force-sensitive resistor consists of a polymer containing conductive particles. The polymer is applied as a thin film (hence the name) to an array of conductive electrodes. As pressure is applied, the conductive particles touch the electrodes and each other, reducing the resistance. Fig.1 shows the construction of a typical device. We’ve seen similar sensors created by sandwiching a layer of conductive foam (such as used for packaging DIP ICs) between two blank PCBs or similar conductive plates. As the foam is compressed, its resistance decreases. Thin-film pressure sensors have hysteresis and thus poor accuracy; error figures of around 10% or higher are typical. Not only does the reading vary quite a bit, but it will also depend on the sensor’s recent history. So they are not suitable for precise measurements. However, they are often used as touch sensors since touch sensing does not require a high degree of accuracy. As long as the touch force can be coupled to them, they can work behind a protective surface in harsh conditions. Some force-sensitive resistors are constructed as long, thin devices with three terminals, like a potentiometer. A touch moving along the length of the resistor is analogous to moving the pot’s wiper, so the touch position can be estimated. Sensor modules Fig.1: pressure applied to the sensor brings together conducting particles within the substrate and closes the gap between the active area and substrate, reducing the sensor’s resistance. The black region of the sensor is a high-resistance polymer that’s embedded with carbon. The silver areas are conductive electrodes that expand the sensor’s active area. The sensor electrodes are connected to terminals that are soldered to a module PCB featuring a resistor, mounting holes and a 3-way pin header. It is possible to purchase bare force-sensitive resistors, but they are also available attached to a module with a pin header, making them easy to interface to a microcontroller board such as an Arduino main board. We tried the Duinotech XC3738 Arduino Compatible Thin-Film Pressure Sensor from Jaycar Electronics. It consists of a sensor attached to a module PCB. The PCB has a three-way header and a single 510kW resistor, marked as R1. There is an unpopulated Australia's electronics magazine siliconchip.com.au 34 Silicon Chip Fig.2: The circuit on the Duinotech Thin-Film Pressure Sensor module is a simple voltage divider. As the sensor is in the upper half, the output voltage increases as pressure is applied. There is an empty footprint for a capacitor, which we recommend fitting. Fig.3: the module provides an analog voltage related to its supply voltage, so its connections are simple enough. The V (or +) pin should be fed from a voltage that matches the ADC reference used to measure the voltage from the S pin. Our sample code uses analog input pin A0. Screen 1: Test Sketch 930.00 933.00 930.00 929.00 798.00 901.00 907.00 917.00 920.00 916.00 923.00 918.00 924.00 926.00 925.00 927.00 925.00 928.00 926.00 51000.00 49196.14 51000.00 51603.88 143796.99 69056.60 65226.02 58953.11 57097.83 59574.24 55254.60 58333.33 54642.86 53423.33 54032.43 52815.53 54032.43 52209.05 53423.33 The output from the test sketch shows the raw 10-bit ADC reading and a calculated sensor resistance based on the module’s nominal 510kW resistor value. Even with a steady weight, there is some drift. footprint for a capacitor on the module; this is marked C1. Fig.2 shows its simple circuit. A 5V or 3.3V supply is applied between the V and G (alternatively labelled + and −) pins. Since the sensor’s resistance decreases as pressure is applied, the voltage at the S pin will increase with more pressure. Circuit and software Fig.3 shows the simple circuit we used to test the module with an Uno R4. Since the Uno R4 has socket headers and the module has plug headers, we made the connections using plugsocket jumper wires. We expect that almost any Arduino board with an analog input can be substituted. The “XC3738_test.ino” sketch uses the ADC to read the voltage at its A0 pin and displays the raw 10-bit ADC reading (from 0 to 1023) and the calculated force-sensitive resistor resistance (siliconchip.com.au/Shop/6/502). This was a simple way to get a feel for how the module responds to being squashed and squeezed. When no pressure was applied, we got a reading of 25, indicating a sensor resistance of around 20MW. We could get a reading over 1000 with firm pressure between our fingertips, indicating a resistance near 10kW. siliconchip.com.au As you can appreciate from Fig.1, the sensor is quite thin, and it’s not immediately clear how it could be used to weigh an object or vessel. We measured the sensor tip with callipers to be around 0.3mm thick. The Jaycar website offers a basic data sheet, and we found some more detailed data sheets for similar devices from Interlink Electronics (www. interlinkelectronics.com). That firm appears to be one of the pioneers of this technology. The sensor on the XC3738 looks quite like Interlink’s FSR 400 sensor. We also found an Integration Guide on the SparkFun Electronics website with numerous tips for this type of sensor (siliconchip.au/link/abx5). This guide doesn’t exactly correspond to the Duinotech sensor, but we found it very helpful. They state that the sensors should not be exposed to sharp surfaces. They are not waterproof and have an air vent that runs parallel to the external leads, allowing their internal pressure to equalise. The guide seems to focus on measuring weights and notes that a pressure measurement would require the vent to be in contact with air at atmospheric pressure. So we will concentrate on applications that measure weight rather than pressure. Testing The guide notes that the sensors are tested by applying force via a silicone rubber ball. We recommend adding small rubber feet (see Fig.1) to help spread the load on the sensor and protect it from impacts. We also added a 100nF capacitor to the vacant C1 footprint on the module. August 2025  35 Rubber is recommended in designs where some degree of movement is expected. It also protects the sensor from sharp edges and impacts while spreading the force uniformly across the active area. With that in mind, we found some self-adhesive rubber feet about 5mm in diameter, similar in size to the sensor’s active area. We attached one to each side of the sensor’s tip. Screen 1 shows the output of the XC3738_test sketch with a half-full (half-empty?) glass resting on the modified sensor. The ADC reading is moving around a bit; the sensor measures around 50kW. We then rigged up a container to balance on the sensor to see if it could be used to measure weight. The blue trace in Fig.4 shows the results of our first experiment. The curve indicates quite a narrow working range, with a notable offset from zero grams before a meaningful reading is registered. The values near the centre of the graph tended to drift around a bit, even with a steady weight, sometimes by up to 100 ADC steps. To test the hysteresis, we noted the values as we filled and then emptied the container, but due to the large amount of drift, we couldn’t draw any firm conclusions about hysteresis. Many microcontroller ADC peripherals recommend a source impedance of no more than 10kW. The data sheet for the RA4M1 microcontroller on the Uno R4 suggests 6.7kW at most. The divider on the Thin-Film Pressure Sensor module is typically dominated by the 510kW resistor, so it would usually have a much higher impedance than the recommended value. That could lead to ADC readings being affected by noise and even the ADC sampling process. The typical solution is to fit a capacitor here to provide a low-impedance voltage source; we generally use a 100nF part for this role. Such a value results in a time constant of around 50ms, which we figure should not affect any weight-­ measuring applications. It might be a bit high if you are using the module as a touch sensor to detect brief touches, though. So we fitted a 100nF M3216 (1206 imperial) SMD capacitor to the C1 footprint on the module, visible in our photo. We then repeated the weight experiment and recorded the red curve in Fig.4. We still noted quite a bit of drift around the middle of the graph. Overall, the response is similar, although the values span a wider range; the capacitor clearly makes a positive difference. The useful working range in either case is approximately 150-300g. There is some response to changing weights above this range, but it is not as distinct. We wonder if replacing the resistor with a lower value might provide better resolution at higher weights at the cost of losing resolution at lower weights. In use The narrow working range sounds quite limiting, but it could be expanded with the appropriate arrangement of levers and pivot points. With the sensors being relatively cheap, a second Fig.4: the blue curve shows the raw 10-bit ADC readings from the sensor with different weights applied. The red curve shows the effect of fitting a 100nF capacitor to the module on the readings. As you can see, the module has a useful response between about 150g and 300g when fitted with rubber feet. 36 Silicon Chip Australia's electronics magazine or third sensor could be added to share the load and thus the measured weight. Many electronic scales use an array of four strain gauges to ensure the weights are measured consistently, even if they are unevenly distributed. The thin film pressure sensors do not produce a change in reading near zero, which is not ideal. Adding an extra weight could help offset the reading, allowing it to measure lower weights. That said, the accuracy is not great, and we suspect that the sensors will be more useful in indicating a full or empty state (with perhaps a handful of steps in between) than a precise weight. The integration guide noted earlier also suggests that calibration is necessary if precision is needed. This section of the guide also states that temperature compensation may also be included in the calibration, with an expected resistance change of up to 10% with temperature. The guide mentions that humid conditions (95% RH) can change the sensor’s resistance, so this should also be considered if the sensor is used in a moist or humid environment. Conclusion Thin-film pressure sensor modules such as the Duinotech XC3738 are handy for detecting changes in weight or pressure, but they are not wellsuited to precision applications. They are more realistically useful when you want to detect the presence or absence of weight. We recommend adding a capacitor and rubber feet to the sensor to help in weight-measuring applications. Without the rubber feet, we’re not sure how it would be possible to apply a meaningful force to the sensor. The capacitor helps ensure it has the correct source impedance to suit a typical ADC. The module’s response is expected to vary under different conditions and between different units. Individual calibration is probably the best way to counteract any of those sorts of variations. So, these devices are better suited to one-off projects than production devices. The XC3738 Arduino Compatible Thin-Film Pressure Sensor is available from Jaycar Electronics; see: www.jaycar.com.au/p/XC3738 SC siliconchip.com.au