How do Resistive Sensors work?

Sensors are devices that detect physical or chemical changes in its environment. The parameters of change could be temperature, pressure, force, light or an electrical entity. They sense changes in parameters and send signals which can be used to control or monitor a circuit or a parameter.

Resistive sensors use components whose resistance changes in response to external parameter. The sensor element changes its resistance in known relation to external stimulus, and the change can be measured using potential divider circuit. Usually a resistance is connected in series with the element, and a fixed voltage is applied across the combination. Voltage across this second known resistance can give indication of element resistance value. A change in resistance value can then be calibrated as the entity (external stimulus) measurement.

The resistance of conductor is given by the well-known relation

Resistance of transducer varies predictably with external environmental factors, and change in physical properties of environment can be predicted by measuring the change in resistance of conductor. Simplest of sensor can be imagined as a conductor whose resistance varies with length and its area of cross section. Humidity can change the resistivity of a material, while change in resistance with temperature is commonly recognized. Most commonly used transducers are variable resistance transducers, or resistive sensors. They measure temperature, force, vibrations, pressure, displacement etc.

Resistive sensors offer the following benefits:

• Small size
• Ease in integration
• Simple in reading / monitoring
• Low cost

Applications of resistive sensors are found in a wide range of applications like consumer electronics, pressure and temperature measurement, power tools, medical devices, human machine interface- to mention a few.

General circuits of a sensor are given below.

This voltage divider circuit is the most basic set up for sensor.  Common current passes through the series combination of sensor and known reference resistance. Voltage across reference resistance gives an idea of sensor voltage, and thus sensor resistance. From the knowledge of sensor resistance, parameter being checked can be estimated for this measured value.

Above circuit (an I-V converter) is used more often, where an amplifier converts the ratio of current and sensor resistance (V_in / R_sens) into output voltage.  This provides more accurate output than voltage divider circuit.

There are four types of sensors in available:

1. Light Dependent Resistance (LDR)

LDR has its resistance variable with light Intensity. It is used to detect light and send signal or operate a device at certain light level. This gives an indication of light intensity of incident light. LDRs, or photoresistors, can be used for light sensing (day / night) switches, and circuits for automatic sensing and control of light on streets, room / space. They can be used for motion sensors, burglar alarms and many other uses.

Sensitivity of LDR to light changes is quite high, and it is easy to use. LDR is quite inexpensive, and variation from darkness-light resistance is very high, which makes detection very easy. For example, dark resistance of LDR could be 1 MΩ, at 10 Lux it could go down to a low of 9 KΩ, and at 1000 Lux, it can go down drastically to 400 Ω.

2. Thermistor

These are resistors whose resistance varies with temperature. These are cost effective devices used to sense, measure and control temperature. Thermistors are available in different shapes, sizes and mounting designs.

Thermistors are the most widely used temperature sensors. Their advantage lies in that they are the cheapest, small in size, as also long lasting sensors. They also most convenient and simple to use. Though their sensitivity is high However, their range of temperature is rather limited. Most common uses include digital thermometers, oil and coolant thermometers in vehicles, ovens, refrigerators and several other applications.  Thermistors are available with both positive and negative temperature coefficients.

3. Flex Sensors

Resistors are made by depositing conductive ink on a flexible substrate or film strip (creating a resistance), on which a segmented conductor is placed, to create a flexible potentiometer. The resistance of this strip varies as it bends. The strip (usually 50 mm to 125 mm in length) can bend only in one direction. Resistance is proportional to angle of bend (e.g. it may change from 25 K in straight position, to 100 K at 90° bend.

Flex sensors are used to measure degree of bend or deflection in an object. It does so by means of change in its resistance, which increases with bending. Sensors are available in many shapes and sizes, and made of different materials like metal sheets, plastic, carbon or optical fiber. They are used widely in robotics, automobiles, medical sciences, bionics and several other applications.

4. Force Sensing Resistors (FSR)

The resistor is usually circular or rectangular in shape. Its resistance decreases when a pressure, or mechanical stress, is applied on this area. Force sensitivity could be from few grams to kilograms. The sensor can be sensitive to human touch also.

Force sensing resistors are widely used in several applications like strain gauges, artificial limbs, car occupancy sensors, musical instruments, medical equipment, toys, sports etc. They work by measuring strain or deformation caused by force or torque, and this information is used to make adjustments and corrections in a system. Force sensors in robots make possible tasks such as grinding, polishing, deburring etc. They can be retrofitted silos and tanks to estimate the weights or liquid levels. Force sensors play crucial role in various applications, improving the performance and safety of systems.