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The eyes of an IoT system

The sensors are responsible for making measurements of the variables of interest, be they temperature, humidity, position, etc. The obtained values are processed and sent to the IoT platform, where they are stored, analyzed and displayed in the user interfaces.

Choosing the right sensor is crucial in every IoT project, as it determines the reliability of the information obtained. We will now see what factors must be taken into account when choosing the sensors.


iot raspberry pi

Choice of sensor

what are we going to measure

Naturally, the first thing is to determine the variable to be measured. For example: temperature, voltage, speed, gas concentration, etc. 

 

Measuring range

The second stage is to select the range over which the measurement will be made. 
Here are two things to keep in mind:

  • That the sensor can measure over the entire range of interest.
  • In some cases it may be useful to select a sensor that does not exceed this range too much, to improve the definition of the measurement.

To clarify this last point, let's look at an example: 

Suppose we have to measure the concentration of O2 in the air. There are sensors that measure the concentration of O2 on the scale of 0 to 30% and others that measure it on the scale of 0 to 100%. Since the concentration of O2 in the air round the 21% and this amount will not change significantly up (it will down if O2 is consumed), it is better to select the sensor with scale up to 30%. In this way, the resolution of the sensor will be better used and more precise measurements will be available.

 

Precision

This parameter is given as a percentage of the full scale value or with relative values (+/-) of the measured value. 

It is important to check what precision is needed in the project.

 

repeatability

This factor is important and is often not considered. This is the ability of the sensor to provide the same measured values for the same sensed values over time. 

That is, the correspondence between the measured value and the real value is maintained.

This factor can be affected for two reasons: 

  • Agents external to the sensor, such as temperature, humidity, electromagnetic interference, etc;
  • Internal factors of the sensor, such as sensor degradation.

Returning to the O2 sensor example, manufacturers provide sensors with different life cycles, including the maximum deviation of the measurement for that time. For example, the manufacturer citytech provides sensors with useful lives of 2 and 7 years.

 

Installation conditions

It is important to consider the environment in which the sensor will operate. Some of these factors can be:

  • Extreme temperatures.
  • Presence of corrosive or toxic agents.
  • explosive environments.
  • Impermeability.
  • Presence of strong electromagnetic fields.

In short, any condition that can affect both the measurement and the useful life of the sensor.

 

Interface

Another aspect to take into account is to evaluate with which interface the sensor will be connected to the acquisition system (microcontroller, PLC, etc.). 

If it is an analog interface, the voltage range it offers, the resolution of the A/D converter, and other factors must be considered.

If it is an ON/OFF type of sensor, it must be verified that the logic voltages between the sensor and the microcontroller are compatible.

Finally, if the interface is digital, take into account that the microcontroller has that interface available (I2C, SPI, 1-Wire, etc).

 

Electrical power supply

All sensors require some power to function. Sometimes the sensor power supply must be done independently from the microcontroller, while other times the same power supply can be used. 

Sometimes it may be necessary to add capacitors in parallel to filter noise and keep the sensor supply voltage stable. 

You also have to consider the length of the conductors, their insulation, shielding if necessary, etc.

Finally, in the nodes that are powered by batteries, it is of vital importance that the sensors consume as little as possible, to prolong the battery replacement period.

Conclusions

As we have seen, the choice of the correct sensor depends on several factors and will be something to solve in each project. Fixed recipes cannot be given, but I hope this guide will help you.

I invite you to leave your questions or comments below and if you found this article useful, please share it with your contacts ;). 


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