Passive Sensors

Passive sensors are sensors that detect anything just by analyzing it without sending any prior radiation or light. Most of the time these sensors measure a physical parameter from its environment and displays the amount in a certain format. The below article clearly explains the concept of passive sensors.


  1. Introduction of Passive Sensors
  2. Types of Passive Sensors
  3. Resistive Sensors
  4. Capacitive Sensors
  5. Inductive Sensors
  6. Measurement of Electromagnetic Flux using Radiometers
  7. Light Analysis by Spectrometers
  8. Difference between passive components and passive sensors

Definition of Passive Sensors

A passive sensor is a device that can detect and measure a physical parameter. The type of parameter that a passive sensor can measure can be any type like radiation, vibration, heat, light etc.

In other words, the moon can be compared to a passive sensor. It doesn’t have any illumination on its own but it shines by reflecting the light from the sun. It depends upon the type of the sensor and mostly used in the measurement of environmental variables.

  • They can also be embedded in a circuit or a microcontroller board like Arduino.
  • Most commonly a passive sensor requires a minimal external power supply for its operation.
  • Some complex passive sensors are even used in satellites for communicational purposes.
  • Passive sensors can only be used when no radiation or light has to be transmitted from the sensor.
  • Unlike active sensors, passive sensors do not emit any kind of radiation or light and wait for the reflected radiation for its work.

Types of Passive Sensors

Passive sensors are most commonly classified into three types based on the type of output they provide. They are resistive sensors, capacitive sensors, and inductive sensors. The pictorial representation of types of sensors is given below.

Different types of sensors

Different types of sensors mentioned in the above diagram are explained below.

  • A resistive sensor provides the output in the form of resistance.
  • A capacitive sensor provides the output in the form of capacitance.
  • An inductive sensor provides the output in the form of inductance.
  • Other than the above-mentioned classification, passive sensors can also be classified based on
    their inputs too.
  • There are photographic passive sensors that are used to detect electromagnetic radiations and converts them into an image like output.
  • Thermal passive sensors detect thermal wavelengths for remote sensing systems.
  • There are also various passive sensors like chemical passive sensors, seismic passive sensors, vibration sensors, etc.

Resistive Sensors

Resistive sensors are electromechanical devices that measure a physical parameter ad converts it into resistance and provides it as the output. Resistive sensors are commonly used to measure parameters such as displacement, pressure, etc.

  • A resistive sensor works on the principle that some external factors can affect the resistance.
  • If there’s a rod or a block of conducting material, the resistance of the block is directly proportional to the length of the conductors, and the resistivity of the conductor.
  • The resistance is also inversely proportional to the area of the conductor.
  • When any one of the above-mentioned parameters is changed, the resistance will also change.
  • By analyzing the resistance change, the change in a particular parameter can be found.
  • Some of the examples of resistive sensors are photoresistors, thermistors, potentiometers etc.

Capacitive Sensors

Capacitive sensors are much similar to resistive sensors. It consists of a capacitor, which is used to detect the change of intensity in an electric field. It is used to measure parameters such as displacement, proximity, etc. Though capacitive sensors are most commonly used in proximity sensors. The generalized working of capacitive proximity sensors is explained below.

  • Commonly, capacitive proximity sensors are non-contact devices that are used to detect motion across a specific range.
  • They are capable of sensing all the presence irrespective of the type material of the object i.e., a capacitive proximity sensor can sense both human presence and presence of other objects.
  • There will be a metal plate that is connected to an oscillator circuit which generates an electromagnetic field.
  • When there is no object detected, the electric field remains undisturbed and thus there will be no change in the initial capacitance.
  • When any motion or an object is detected, the electric field that is produced by the oscillator circuit is affected, and the capacitance also varies accordingly.
  • The change is directly proportional to the distance between the proximity sensor and the object.

Inductive Sensors

Inductive sensors are devices that use the principle of eddy currents and electromagnetic induction for detecting a presence or measure the distance between the sensor and the object. Though they are equally effective as capacitive sensors, inductive sensors are not used as much as capacitive sensors. The working of an inductive sensor is as follows:

  • An inductive sensor consists of a coil that induces an electromagnetic field when energized.
  • When a metal object comes within the range of the electromagnetic field, it creates an eddy current in the object. Thus, there will also be a magnetic field which is caused due to the eddy current produced.
  • As there are two magnetic fields, the self-inductance of the coil is converted into mutual inductance.
  • If there’s any change in the self-inductance, then it means that there is some metal object present within a certain range.
  • Inductive sensors cannot be used in the detection of non-metallic objects as they use the mutual inductance caused by both the coil and the object to be detected.
  • Inductive sensors are widely used in metal detectors.

The below table characterizes the parameters of three types of passive sensors easily.

Sensor Input type Output Working Principle Application
Resistive Sensors Displacement, pressure Resistance change Resistance changes when certain parameters are changed Displacement measurement, Pressure measurement etc.
Capacitive Sensors An object whose presence is to be detected. A change in capacitance Capacitance changes when there is some other object in between Used in proximity sensors
Inductive Sensors An object Inductance change Self-induction is converted into mutual inductance when another metal is brought near the coil Used to detect whether the object is metallic or non-metallic.


Measurement of Electromagnetic Flux using Radiometers

A radiometer is a passive sensor that is used to measure the amount of flux produced by electromagnetic radiation. They use photocell sensors or infrared sensors or even ultraviolet rays sensors to measure the intensity. The working of a radiometer is explained below.

  • A radiometer is situated at a vacuum where there are two or four double-colored vanes connected to the brass tube.
  • When light or radiation is passed to the vanes, the vanes start rotating as there is no air around the meter.
  • The effect caused by the light varies according to the colour of the vane. It depends upon the darkness of the colour.
  • The blackened side absorbs a lot of photons from the light and the lighter side reflects them.
  • By the amount of radiation reflected and the number of radiations absorbed, the intensity of the radiation can be measured.

Light Analysis by Spectrometers

A spectrometer is a passive sensor that is used to analyze various properties of light such as frequency, energy, wavelength, etc. It works on the principle that an element can absorb or reflect light at a certain wavelength. Spectrometers use various kinds of lamps for illumination and the working of the spectrometer is explained below.

  • The light from the lamp is allowed to pass through a narrow slit.
  • A solution or a glass prism is mounted to the table on which the narrowed beam of light is allowed to fall.
  • Based on the wavelength of the light, angle of the prism or the concentration of the solution, the wavelength or the frequency of the output wave is also changed.
  • Just like the above-mentioned sensors, by analyzing the output beam, the frequency, wavelength, and energy of the input light can be measured.

Difference between Passive Components and Passive Sensors

One of the biggest misconceptions is that passive sensors are the same as passive components that are used in electronic circuits. Passive components are devices that serve a certain purpose but passive sensors are used to measure certain parameters. Comparison of some passive components to their corresponding sensor is discussed below.

  • A resistor is used to limit the current flow across a circuit while a resistive sensor uses the concept of resistance to measure displacement.
  • A capacitor is used to store charges in the form of the electric field while a capacitive sensor detects motion or presence using capacitance.
  • An inductor is used to store charge in the form of an electromagnetic field, while an inductive sensor is used to detect metallic substances.

Key Points to Remember

Here is the list of key points we need to remember about “Passive Sensors”.

  • Passive sensors are devices that are used to measure a certain environmental variable.
  • It does not emit any kind of radiation or light for measurement purposes.
  • Passive sensors are broadly classified into three types namely resistive sensors, capacitive sensors and Inductive sensors.
  • Resistive sensors use the resistance change in the block of metal to measure the displacement, pressure, etc.
  • Capacitive sensors work on the principle that the capacitance of a capacitor changes when there is a foreign object in between the plates and is widely used in proximity sensors.
  • Inductive sensors work on the principle of mutual inductance and it is used in metal detectors.
  • Passive sensors are different from passive components.

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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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