Resistive Sensors

If a sensor is used to measure a parameter like pressure, it is tough to directly measure it. Hence, it is converted into some other easily measurable parameter and then the pressure is measured. Sensors that use resistance for the measurement of other parameters are known as resistive sensors. The following article explains the concept of resistive sensors in detail.

Contents:

  1. What is a Resistive Sensor?
  2. Widely used Resistive Sensors and their Applications
  3. Sliding Contact Devices
  4. Rheostat or Potentiometer
  5. Strain Gauges
  6. Thermistors and Thermocouples
  7. Light Dependent Resistors (LDRs)
  8. Resistive Humidity Sensor

What is a Resistive Sensor?

Resistive sensors are devices that are most commonly used to measure physical parameters like displacement, pressure, temperature etc. As stated above resistive sensors converts the parameter to be measured to resistance and then the resistance is measured. The working principle of a generalized resistive sensor is explained below.

  • Resistance is a sensitive parameter that can be affected by many parameters like the area of the conductor, length of the conductor etc.
  • When some of the above factor changes, it also affects the resistance. This is the main working principle of resistive sensors.
  • The initial resistance of a wire or a metal is initially measured.
  • Once done, the parameter to be measured is used to affect one of the above parameters, resulting in the change in resistance.
  • The amount of that parameter is measured by calculating the resistance change between initial resistance and the final resistance.

Widely used Resistive Sensors and their Applications.

Though resistance can be used to measure a lot of parameters, some physical parameters can directly influence the resistance. Such parameters can be effectively measured using resistive sensors. Those parameters and how resistive sensor can be used to measure them is tabulated below.

Sensor Input External parameter used Output
Potentiometer A turning effect or moving effect Displacement Resistance change
Strain Gauge Weight or a mass Stress Resistance change
Thermistor Temperature Temperature Resistance change
LDR Light rays Change in intensity of light Resistance change
Resistive humidity sensor Soil or material with moisture Semi-conductivity Resistance change

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Sliding Contact Devices

Sliding contact devices are the base devices of various resistive sensors. Though they are not directly used in circuits, sensors that use sliding contact techniques are used in many electronic devices and circuits. The main working of a sliding contact device is as follows.

  • Sliding contact devices consist of a wire-like structure whose resistance is known.
  • A small metal bar or a pointed tip is allowed to slide on the wire throughout its length.
  • If the slider is placed at the starting point of the wire, the resistance is high as the length of the conducting material is high.
  • When the slider is moved further towards the end, the resistance gradually decreases and reaches zero or a negligible value at the ending point.
  • As mentioned above, this device is not complete instrument but is one of the easiest yet powerful techniques to convert a mechanical parameter such as displacement (displacement of a slider) to resistance.

Rheostat or Potentiometer

A potentiometer is an upgraded complete version of a sliding contact device. In other words, it can also be described as the main application of a sliding contact device. The construction and the working is discussed below.

  • Potentiometers are broadly classified into two types namely, Linear potentiometers and rotary potentiometers.
  • Both types use the same sliding contact device technique to measure the resistance change.
  • In linear potentiometers, there will be a long wire which acts as resistance in a circuit, but this wire has a variable resistance.
  • The wire is connected to a voltage source and a voltmeter is also connected to it. If the resistance of the wire is changed by the slider, it also affects the voltage in the circuit.
  • The rotary potentiometer consists of a semicircle shaped resistive track in which a slider moves.
  • As in linear potentiometers, the voltage or the current flow across the potentiometer varies according to the position of the slider on the resistance track.
  • Potentiometers are highly useful in applications that require variable resistance as it eliminates the process of changing the resistor every time.

Strain Gauges

A strain gauge is a resistive sensor that is used to measure the strain or force exerting on it. It is an application of the fact that the resistance is varied when the area of the metal is varied. The construction and working of a strain gauge are as follows.

  • A strain gauge consists of a thin wire which is glued across a resistive pad or foil in the shape of a rectangle in a zig-zag manner.
  • Usually, a strain gauge is connected to a circuit in the form of a Wheatstone bridge, making it easy to measure the stress.
  • When stress or force is applied on a strain gauge, the zig-zag pattern of the wire is disturbed.
  • As a result, there will be a change in resistance. This change in resistance will be directly proportional to the force applied.
  • By analysing the resistance change, the amount of force or stress can be calculated.

Thermistors and Thermocouples

A thermistor is a device that is used to measure temperature using resistance.

  • Consider a circuit with a resistor connected. The circuit is initially placed at room temperature.
  • The resistance of the resistor remains constant as long as there is no big change in the temperature.
  • If the temperature of the environment exceeds a certain value or is lowered than a certain value, the resistance of the resistor will be changed.
  • The resistance and the temperature are inversely proportional. Thus, by measuring the resistance across a thermistor, its temperature can be determined.

As the name suggests, a thermocouple is a temperature measurement device that uses a couple of elements, mostly metals.

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  • The ends of two dissimilar metals are connected and are connected to a circuit. When energized, the temperature of both the metals starts to increase. But not at the same rate.
  • One metal gets hot faster than the other one. In such cases, the heat starts to flow from the hot end to the cold end. This temperature change results in resistance change, which can be measured.
  • The temperature across the circuit can be calculated using the measured resistance value.

Light Dependent Resistors (LDRs)

A light dependent resistor is also known as a photoresistor or a photodiode which is used to measure the intensity of a light beam using resistance. The construction and the working of an LDR are explained below.

  • The LDR is made up of a semiconductor material whose resistance is relatively higher than a conducting material.
  • As the light beam consists of photons that can energize the free electrons in the semiconductor. Due to the new energy, increasing the current flow and decreasing the resistance.
  • Here the resistance is inversely proportional to the intensity of the light.
  • Again, the intensity of the light can be calculated using the resistance change.

Resistive Humidity Sensor

A resistive humidity sensor is again a resistive sensor that is used to measure the humidity of the environment, mostly soil. The main working principle of a resistive humidity sensor is explained below.

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  • The electrical impedance of atoms is measured by resistive humidity sensors, which use ions in salts.
  • The resistance of the electrodes on either side of the salt medium changes as humidity changes.
  • The humidity of the surrounding air causes two heat sensors to conduct electricity.
  • The amount of electricity conducted is calculated by measuring the resistance across the electrodes.

Key Points to Remember

Here are the key points we need to remember about “Resistive Sensors”

  • A resistive sensor is a device that measures a mechanical parameter by converting it into resistance.
  • Sliding contact devices are the base of various resistive sensors.
  • A potentiometer is used as a variable resistance in many applications.
  • A strain gauge converts the force applied on it to resistance for easier measurement.
  • The thermistor is a device that is used to measure the precise temperature in large scale applications.
  • Thermocouples consist of two metals, by which the temperature of the circuit can be calculated.
  • The LDR uses energy from the photons in a light beam to reduce the resistance of the semiconducting material.

If you find any mistake above, kindly email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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