Introduction
Sensors are devices that detect events or changes in physical conditions such as temperature, humidity, pressure, light, or motion. Based on these changes, sensors generate appropriate electrical signals as output. Sensors can be classified into different categories such as sound, automotive, electrical, chemical, and more. Some of the most common types include pressure, force, proximity, light, heat, temperature, and position sensors.
Sensor Technology
The use of sensors in real-time electrical and electronic circuits is rapidly increasing for the development of innovative projects.
For example, proximity-based automatic door systems are widely used in shopping mal
ls, offices, banks, and other public places. Similarly, sensor technology is expanding in fields such as embedded systems, robotics, and automation. One important sensor used in such applications is the force sensing resistor.
What is a Force Sensing Resistor?
A resistor is a circuit element used to control current flow and reduce voltage levels in electronic circuits. Based on construction and application, resistors are classified into different types such as fixed resistors, variable resistors, wire-wound resistors, metal-film resistors, and special-purpose resistors.
A force sensing resistor (FSR) is a special type of resistor whose resistance changes when force or pressure is applied to its surface. The FSR technology was invented and patented by Franklin Event off in 1977.
FSR sensors are made of conductive polymer material that changes its resistance according to the applied force. Because of this property, they are considered a combination of resistor and sensor technology.
Construction and Working
An FSR is usually manufactured as a thin polymer sheet or conductive ink layer used for screen printing. The sensing film contains both conductive and non-conductive particles. These particles are typically very small and are designed to reduce temperature dependency while improving durability.
When force is applied to the sensing surface, the conductive particles make contact with the electrodes, which decreases the resistance of the film. As the applied pressure increases, the resistance decreases further.
FSR sensors are known for their thin structure (less than 0.5 mm), low cost, and good shock resistance. However, their main limitation is lower measurement precision, with possible variations of 10% or more.
Force sensing resistors are also called Polymer Thick Film (PTF) devices. Their resistance decreases as the applied pressure increases.
Steps to Design a Force Sensing Resistor
The following four simple steps can be used to design an FSR sensor:
1. Gathering Materials
Collect the required components such as PCB plates, conductive foam, wires, solder, hot glue, and tools like a soldering iron, glue gun, and cutter.
2. Sizing
Cut the PCB into two identical plates. Solder the red and black wires to the plates. Then cut the conductive foam into the same size and shape as the plates.
3. Connecting
Place the conductive foam between the two plates and secure them using glue. Ensure proper electrical contact between the plates and the foam.
4. Testing
Use a multimeter to test the sensor. When no force is applied, the resistance will be very high. As pressure is applied, the resistance decreases accordingly.
Applications of Force Sensing Resistors
- Foot pronation and pressure measurement systems
- Automotive sensors
- Resistive touchpads
- Musical instruments
- Keypads and control panels
- Portable electronics
Touch-Controlled Load Switch Application
A touch-controlled load switch is a simple project that uses a touch-sensitive input to control a load. The circuit typically includes a 555 timer, relay, diodes, capacitors, resistors, and a load such as a lamp.
The 555 timer operates in monostable mode and is triggered by the signal from the touch plate. When the plate is touched, the timer output activates the relay for a preset time. The duration can be adjusted using the RC time constant, after which the load turns off automatically. The touch plate generates a small voltage from the human body, which is used as the triggering signal.
