Analog and Digital Sensors

Though every sensor has a different construction and working principle, in the end, the output of the sensor matters the most. Some sensors provide a discrete value as output while some other sensors just provide a binary value. Sensors are classified as analog and digital, based on output. The below article explains these sensors in detail.

Contents:

  1. Analog Sensors and Digital Sensors Overview
  2. Comparison of Analog and Digital Sensors
  3. Temperature Sensor and Thermometers
  4. Pressure Sensors
  5. Analog Tachometers
  6. Switches and Push Buttons
  7. Detection Sensors
  8. Digital Tachometers

Analog Sensors and Digital Sensors Overview

Analog sensors measure an environmental parameter and provide the amount of that particular parameter. It displays the discrete value in the form of an integer or a decimal number.

  • For instance, a humidity measurement sensor can be connected to an Arduino board and placed into the soil.
  • When placed in the soil, the amount of water content in the soil is displayed in the IDE of the Arduino board to which the board and the sensor are connected.
  • The value measured from the sensor will be between the following range – 0.69 V to 0.825V.
  • The humidity to voltage conversion is done by the sensor itself.

A digital sensor is a device that provides an output in binary form. It does not provide a distinct output but it provides an output if it detects a particular parameter and does not provide any output if there’s no input up to a certain level.

  • For example, some infrared sensors are digital.
  • If there’s a black object in front of an energized IR sensor, it does not provide any output.
  • If an object of any colour (except black) or no object is detected, the IR sensor provides a 5V.
  • This mechanism can be used in line follower robots, in which a small robotic module follows a line of a particular colour, mostly black.

Comparison of Analog and Digital Sensors

Analog and digital sensors have entirely different construction and working as their output is of different forms. The other such comparisons of analog and digital sensors are shown in the below table.

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Parameter Analog Sensor Digital Sensor
Input Environmental parameters like temperature, pressure, humidity, etc. An object with a particular colour or a certain material which is to be detected
Output Analog output – discrete value Digital output – Either 0V or 5V
Complexity in construction More complex as it needs to measure the exact value Less complex as it works only within a certain range
Efficiency More efficient Less efficient
Cost/Pricing Comparatively lesser Higher in most situations
Example Thermometer, thermocouple, etc. IR sensor, Pushbuttons, etc.

Temperature Sensor and Thermometers

A temperature sensor is the best example of an analog sensor. Both electronic temperature sensors like thermal sensors and thermistors, and manual temperature sensors like thermometers fall under this category.

  • Manual thermometers have a bulb of mercury in one end which expands as the temperature around the bulb increases.
  • The mercury is allowed to pass through a capillary tube to which the measurements are marked.
  • The bulb can be read through the amount of mercury expanded and an analog value.
  • A thermistor is a thermal sensor that is used in electrical circuits to measure the temperature and converts the temperature change into a precise resistance change from which the temperature can be monitored.

Pressure Sensors

Pressure sensors are also analog sensors that measure the amount of pressure applied or to be applied to fluids. Most of the time, pressure sensors are used for industrial purposes. The generalized working of a pressure sensor is given below.

  • A pressure sensor is placed in the tube in which the fluid flows.
  • When the fluid with some applied pressure passes through the sensor, the pressure creates a movement on the elastic material which is present inside the sensor.
  • The amount of movement of the material is directly proportional to the amount of pressure applied.
  • The movement is then converted into small electrical signals for easier manipulation.
  • By measuring the amount of current produced, the amount of pressure applied can be calculated.
  • The calculated pressure is then displayed in a form of a number.

Analog Tachometers

An analog tachometer is a device that is used to measure the number of rotations of a wheel or a gear in a machine. Using the rotations per minute (RPM), the speed of rotation can be calculated. The working and construction of a simple analog tachometer is explained below.

  • Generally, there are two types of tachometers – Contact type tachometers and non-contact type tachometers.
  • Contact type tachometers are directly mounted inside or on the wheel while non-contact type tachometers are mounted away from the wheels and use radiation for measurement purpose.
  • Tachometers works based on the relative motion created by the rotating object and the magnetic field that is generated by the tachometer itself.
  • The above mentioned magnetic field is created by the motor inside the tachometer.
  • The amount of magnetic field produced is directly proportional to the rotating speed of the wheel.

Switches and Push Buttons

Switches and push buttons are mandatory components in almost every device in electronics. They are very important in power plants, day-to-day life, industries of all kinds, etc. The switches and push buttons are a very good examples of digital sensors. Though they don’t explicitly sense anything, they can affect the flow of work across a circuit according to the input given to them.

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  • Switches are simple devices that can perform only two operations – allow or deny.
  • In allow state, a switch is said to be fully on or closed and allows the current to flow through it.
  • In the deny state, a switch is said to be off or open and it does not allow the current to flow through, interrupting the whole working itself. A simple circuit with a switch is shown below.
    A Circuit with a Switch
  • The above figure shows a simple circuit with a switch in open and closed state.
  • A push-button is similar to a switch except for the fact that it has only one state. A push button circuit is as follows.
    A Circuit with a Push Button
  • As shown in the figure above, when the button is pressed once, the circuit becomes closed and there is a flow.
  • If the button is pressed again, it acts like the denied state of an ordinary switch and does not allows the current to pass through.

Detection Sensors

Detection sensors or proximity sensors are also an example of digital sensors. If the object to be detected is within the range of the sensor, it provides an output, mostly 5V and no output at all when there is no object detected. The below circuit is a diagrammatic representation of a proximity sensor which is connected to a buzzer.

A Simple Proximity Sensor Circuit
  • In the above diagram, the proximity sensor acts as a switch.
  • As stated above, when there is an object detected, the proximity sensor acts like a closed switch and the buzzer rings.
  • If there is no object detected, the sensor acts like an open switch as shown in the figure and the buzzer will not be energized.

Digital Tachometers

Just like analog tachometers, digital tachometers are also used to measure the seed of a rotating object like a wheel. This doesn’t create a magnetic field or a radiation for measurement. Instead it uses digital encoders. The working of a digital tachometer is as follows.

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  • Digital tachometers are also classified as contact type and non-contact type tachometers.
  • The contact type digital tachometers are highly efficient when compared to other kinds of tachometers.
  • They can even measure very low speeds using an optical encoder.
  • The non-contact type digital tachometers use laser for measurement and can measure a higher speed but the measurement angle of this type of tachometers is relatively low.

Key Points to Remember

Here’s the list of key points we need to remember about “Analog and Digital Sensors”.

  • Analog sensors provide a discrete number as the output value.
  • Digital sensors provide either on or off as output affecting the flow in the circuit or process.
  • A temperature sensor is an analog sensor which is used to measure the exact temperature value.
  • Pressure sensor or a pressure transmitter is an analog sensor which is used to pressure applied on a fluid that flows.
  • Tachometers can be both analog or digital by the measurement method they use.
  • Switches and push buttons are digital sensors which are used to restrict or allow the flow in the process.
  • Detection sensors or proximity sensors are just like switches which reaches the closed state when an object is detected.

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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