PIR and ultrasonic sensors: What's the difference and how do they work? | Engineers' Insight | Avnet Abacus

Display portlet menu

PIR and ultrasonic sensors: What's the difference and how do they work? | Engineers' Insight | Avnet Abacus

Display portlet menu

PIR and ultrasonic sensors: What's the difference and how do they work?

Alessandro Mastellari Photo

There are two primary technologies used in detecting the presence of people, pyro-electric / passive infra-red (PIR) and ultrasonic. Alessandro Mastellari takes a look at how they differ and how they work.

Our environment and our society are very rapidly becoming more connected and, as a result, smarter. This connected technology is changing and enhancing the way that we live and work, bringing greater productivity and efficiency to our homes and workplaces.

At the heart of this revolution is the Internet of Things (IoT) – a network of connected devices that already comprises billions of devices, and shows no signs of slowing as new devices and applications arrive incessantly.

While the applications for this smart connected technology are many and varied – and growing, some of the most established and most common are in the area of building energy management (BEM) and security. With energy costs rising and environmental consideration high on almost every agenda these days, making good use of energy is an increasingly important topic.

Along with the ability to manage efficient use of energy, most smart building systems provide a high level of automation which delivers convenience to building occupants. However, to be truly useful, these systems need the ability to detect the presence (or absence) of humans. With this ability, they can deliver automated ‘on demand’ energy management, turning lights on and controlling the environment when it is needed due to people being present.

There are two primary technologies used in detecting the presence of people, pyro-electric / passive infra-red (PIR) and ultrasonic. Both work quite differently and have different benefits, depending on the application.

All objects with a temperature above absolute zero emit heat energy in the form of radiation – this is known as Wien’s Law. PIR sensors work on the basis of detecting changes in this infrared (IR) radiation thereby detecting the presence of a human – or any other warm, moving object.


A block diagram of an IoT connected human sensor using a PIR as the main sensing element

PIR sensors typically have two slots in them, each of which is capable of detecting IR radiation. As a warm object (human) passes in front of one detector, a positive pulse is generated and, as it moves past the second detector window, a negative pulse is generated. This represents movement and, using relatively simple analogue electronics based upon a pair of operational amplifiers, a signal is generated to signify the presence of a moving warm body. Often an optical IR filter is fitted to the front of the sensor to limit the wavelengths to those of interest – the IR energy from a human body is around 10µm, for example.

The area that a PIR detector is able to cover is a function of its placement in the room as well as the lens that is typically fitted over the sensing element. In many cases this will be a Fresnel type lens (often made from semi-opaque plastic) that concentrates IR radiation from a wide area on to the sensor. Read this Engineer's Insight post for more on PIR sensors and how they are now being adapted to new applications.

An alternative approach is to use ultrasonic transducers to detect people present within a building. This uses sound waves at a frequency higher than humans are able to hear – typically in the range 30kHz to 10MHz. Transducers comprise a pair of devices, one is a transmitter and one is a receiver. A sound pulse is sent out at a given frequency and, as it bounces off objects in its path, it is reflected and captured by the receiver. In an empty room, the reflections will come from the opposite wall and the time taken for the reflection to be received will be proportional to the distance between the transducer and the wall. When a human enters the room then the pulses will reflect from them and, as they are closer than the wall, the time taken to receive the reflections will be less.

A diagram of a room showing how an ultrasonic sensor can be used to detect movement
Ultrasonic sensors can be used for room-level sensing and
individual robotic appliances

 

These types of systems are known as ‘Time of Flight’ (ToF) systems as, for a given medium such as air, the sound waves travel at a constant speed – meaning that the object distance can be detected by knowing the time taken for the sound pulse to be received.

Both PIR and ultrasonic detection can be used in standalone systems and ‘connected’ (IoT) systems, principally to detect the presence of humans – but there are other applications as well.

Lighting control is a key part of many BEM systems that sense the presence of people and control lighting so that it is only on when needed. PIR sensors can be used for this, but they do require the person to be moving. On the other hand, ultrasonic sensors can scan an empty room and then know when one or more people are present. As part of a more sophisticated BEM, this information can be used to control and automate heating, ventilation and air conditioning (HVAC) systems to save energy and minimise environmental impact.

PIR sensors are very commonly used in security or intrusion detection systems in both domestic and commercial applications. As they are placed away from the potential point of entry (door or window) they will detect any intruder before they are able to reach the sensor and tamper with it.

Connected robots are becoming more popular in both domestic and commercial applications. In the home, simple robots can vacuum rooms unattended while similar technology is increasingly being used for robotic lawn mowers. Objects in the path of these robots are an issue and ultrasonic transducers are commonly used to detect these obstructions and change the direction of travel.

In industrial applications such as large factories and warehouses, robots are used to move goods from one place to another. While these automated guided vehicles (AGVs) may be intended to run in defined paths that should be free of objects, almost all AGVs use ultrasonic transducers to detect other AGVs and objects that have inadvertently been left in their path.

The applications of this non-contact distance sensing technology are many and varied. For example, it is common to embed ultrasonic sensors in tanks to measure the level of liquids. This is equally applicable to domestic heating oil tanks as it is to large tanks of chemicals in process industries. In the connected world of the IoT, the level information could be used to drive a system that manages auto-replenishment.

This post only scratches the surface when it comes to the capabilities of PIR and ultrasonic, and there are a number of important factors to consider in the design process. For a technical deep-dive on how to approach sensor selection when designing motion detection systems, register for our latest webinar with Murata. Alternatively, get in touch with one of our technical specialists by clicking the Ask an Expert button.

Want more like this? Sign up for our newsletters

About Author

Alessandro Mastellari Photo
Alessandro Mastellari

Alessandro has over 20 years experience in the electronics industry, spanning product management and...

PIR and ultrasonic sensors: What's the difference and how do they work? | Engineers' Insight | Avnet Abacus

Display portlet menu
Contact our product specialists

Ask an expert

Have a question? Our regional technical specialists are on hand to help.

PIR and ultrasonic sensors: What's the difference and how do they work? | Engineers' Insight | Avnet Abacus

Display portlet menu
Related Articles
The passive infrared sensor (PIR) seems so familiar that it is easy to miss the many applications that exist for it and the subtleties of engineering that guide the best ways in which to use it.
Adapting PIR sensor technology to new applications
By Martin Keenan   -   October 25, 2019
The passive infrared sensor (PIR) seems so familiar that it is easy to miss the many applications that exist for it and the subtleties of engineering that guide the best ways in which to use it.
How can MEMS pressure sensors help you keep fit? Well, they are now commonly being integrated into wearable activity monitors that help consumers track their daily activities and thereby monitor their fitness levels.
Accurate altitude measurements help keep you fit
By Mathias Goebel   -   October 25, 2019
How can MEMS pressure sensors help you keep fit? Well, they are now commonly being integrated into wearable activity monitors that help consumers track their daily activities and thereby monitor their fitness levels.
As the world becomes increasingly smart and connected, batteries can be undesirable in some of the use cases. Kinetic energy harvesting is an alternative that can help achieve longer operating lifetimes, relieve ownership costs, and ideally improve r
A quick guide to powering smart objects with kinetic energy harvesting
By Alessandro Mastellari   -   October 25, 2019
As the world becomes increasingly smart and connected, batteries can be undesirable in some of the use cases. Kinetic energy harvesting is an alternative that can help achieve longer operating lifetimes, relieve ownership costs, and ideally improve r
Man controlling home devices via his watch
Let your home do the work so you don't have to
By Martin Keenan   -   October 25, 2019
Today, we need to interact with connected products in the home deliberately. But it does not have to stay this way.
A growing pre-occupation with leading healthier life styles means that many of us are becoming much more sensitive to the environment around us, and in particular air quality.
How sensitivity to our environment is creating a growing market for air quality sensors
By Giovanna Monari   -   October 25, 2019
A growing pre-occupation with leading healthier life styles means that many of us are becoming much more sensitive to the environment around us, and in particular air quality.
When Murata purchased Finnish MEMS specialist VTI Technologies and renamed it Murata Electronics Oy at the start of 2012, it gained the company’s unique 3D MEMS technology which has outstanding performance and reliability in harsh environments.
Second Generation MEMS Enables Impressive Performance for Automotive and Industrial IMUs
By Adam Chidley   -   October 25, 2019
When Murata purchased Finnish MEMS specialist VTI Technologies and renamed it Murata Electronics Oy at the start of 2012, it gained the company’s unique 3D MEMS technology which has outstanding performance and reliability in harsh environments.
Giving machines the power of vision gives them the ability to interact with people in a much more natural way, but integrating this capability was previously a complex and difficult task.
I recognise that face: Machine vision with Omron's HVC
By Giovanna Monari   -   October 25, 2019
Giving machines the power of vision gives them the ability to interact with people in a much more natural way, but integrating this capability was previously a complex and difficult task.
Avnet Abacus helps engineers to develop smart flow meters for monitoring water and gas consumption. Alan Jermyn, VP Marketing, looks at what’s driving demand for smart meters, how they work and how they’re built.
Taking flow measurement to the next level
By Alan Jermyn   -   October 25, 2019
Avnet Abacus helps engineers to develop smart flow meters for monitoring water and gas consumption. Alan Jermyn, VP Marketing, looks at what’s driving demand for smart meters, how they work and how they’re built.
In an increasingly connected world, performance demands on antennas are greater than ever. These increased performance demands also have to be balanced with cost, and of course, the limits imposed by the laws of physics, which make antenna design a t
Customising and optimising antennas with materials science and manufacturing techniques
By Martin Keenan   -   October 25, 2019
In an increasingly connected world, performance demands on antennas are greater than ever. These increased performance demands also have to be balanced with cost, and of course, the limits imposed by the laws of physics, which make antenna design a t
Combining the data from multiple sensors can tell us a great deal more about the application environment than each sensor could on its own.
Sensor Fusion: The future of intelligent devices
By Martin Keenan   -   October 25, 2019
Combining the data from multiple sensors can tell us a great deal more about the application environment than each sensor could on its own.
How are the latest trends in sensing technology enabling the Industrial IoT, and how do you make the right choice for your installation?
Maximise your IIoT installation success by choosing the right sensors
October 25, 2019
How are the latest trends in sensing technology enabling the Industrial IoT, and how do you make the right choice for your installation?
When faced with the need to make design decisions, system engineers are challenged with multiple options, any of which can have a major impact on the effectiveness of the wireless performance.There are many considerations when selecting RF antennas f
Key design considerations for selecting the right RF antenna
October 25, 2019
When faced with the need to make design decisions, system engineers are challenged with multiple options, any of which can have a major impact on the effectiveness of the wireless performance.There are many considerations when selecting RF antennas f
The Internet of Things (IoT) combines pervasive connectivity to control devices remotely with Big Data applications in The Cloud capable of organising vast quantities of information and extracting information hidden in the data to enable new services
The internet of medical things (IoMT) bringing better healthcare for all
By Martin Keenan   -   October 25, 2019
The Internet of Things (IoT) combines pervasive connectivity to control devices remotely with Big Data applications in The Cloud capable of organising vast quantities of information and extracting information hidden in the data to enable new services
Industry 4.0 and the digitisation of the manufacturing sector will have repercussions for all sectors of industrial electronics.
Evolving component technologies will make Industry 4.0 viable
By Martin Keenan   -   October 25, 2019
Industry 4.0 and the digitisation of the manufacturing sector will have repercussions for all sectors of industrial electronics.
With more and more applications combining data from multiple sensors, Alessandro Mastellari looks at how sensor fusion is being applied with pressure sensors.
How can combining different types of sensors result in more accurate data?
By Alessandro Mastellari   -   May 9, 2019
With more and more applications combining data from multiple sensors, Alessandro Mastellari looks at how sensor fusion is being applied with pressure sensors.
To give the electronic systems we design the ability to detect changes to what is around them we need analogue sensors. But the processing that sits behind those sensors is in almost every case going to be in the digital domain.
Pressure sensors: Why you should consider going digital
By Martin Keenan   -   May 9, 2019
To give the electronic systems we design the ability to detect changes to what is around them we need analogue sensors. But the processing that sits behind those sensors is in almost every case going to be in the digital domain.
In the second installment of our pressure sensor series Alessandro Mastellari explores how manufacturers are developing smaller and smaller devices.
Pressure sensor miniaturisation: How smaller sensors are enabling better healthcare
By Alessandro Mastellari   -   May 8, 2019
In the second installment of our pressure sensor series Alessandro Mastellari explores how manufacturers are developing smaller and smaller devices.
Pressure sensors became to automotive systems in the early 1990s, but as the rate of adoption of these components increases Martin Keenan investigates why.
What the growing pressure sensor market means for electronics
By Martin Keenan   -   May 8, 2019
Pressure sensors became to automotive systems in the early 1990s, but as the rate of adoption of these components increases Martin Keenan investigates why.
Recent advances in wireless technology have enabled the use of mesh network topologies in home and building control applications, but what are your options, and can you combine them?
A quick guide to multi-standard mesh networking
By Alessandro Mastellari   -   April 11, 2019
Recent advances in wireless technology have enabled the use of mesh network topologies in home and building control applications, but what are your options, and can you combine them?
To mark International Women’s Day, we spoke to two female engineers at different points in their careers – one from the UK and one from Germany – to find out from them what their take is on being a woman in engineering
Women in engineering – where are we now?
By Lisa Rees   -   March 7, 2019
To mark International Women’s Day, we spoke to two female engineers at different points in their careers – one from the UK and one from Germany – to find out from them what their take is on being a woman in engineering and to learn more about t
And so electronica 2018 is over. In our final post, we look back on the best bits of day three.
From humanoids to holograms and humanity projects - How Avnet showcased the future at electronica
November 16, 2018
And so electronica 2018 is over. In our final post, we look back on the best bits of day three.
Another day at electronica 2018 has passed and it was arguably even busier than the first. A constant stream of customers and suppliers kept the atmosphere on the Avnet Abacus stand buzzing, and our schedule was packed full of great speakers and demo
electronica 2018 day two highlights
November 16, 2018
Another day at electronica 2018 has passed and it was arguably even busier than the first. A constant stream of customers and suppliers kept the atmosphere on the Avnet Abacus stand buzzing, and our schedule was packed full of great speakers and demo
On a crisp morning at Messe Munich there was an excited buzz around the exhibition centre as electronica 2018 opened its doors. Read what happened on day one.
electronica 2018 day one highlights
November 13, 2018
On a crisp morning at Messe Munich there was an excited buzz around the exhibition centre as electronica 2018 opened its doors. Read what happened on day one.
As electronica 2018 prepares to open its doors, here are three reasons you should visit Avnet Abacus on stand C5 101 this week.
3 reasons to visit Avnet Abacus at electronica 2018
November 12, 2018
As electronica 2018 prepares to open its doors, here are three reasons you should visit Avnet Abacus on stand C5 101 this week.
The new era of home security won’t just be an alarm that rings when a lock is picked or a window broken. With the rise of new technology comes the added need to protect a home’s hardware and its software.
Security and surveillance: Two sides of the coin in smart homes
By Martin Keenan   -   July 9, 2018
The new era of home security won’t just be an alarm that rings when a lock is picked or a window broken. With the rise of new technology comes the added need to protect a home’s hardware and its software.
Wireless technology is endemic in modern society; broadly speaking we rely on cellular technology to stay connected when we’re travelling, and probably WiFi when we’re not.
Choosing the right wireless technology for your IoT end-device
By Adam Chidley   -   May 4, 2018
Wireless communications will connect the IoT, but it’s not quite as straightforward as adding a Wi-Fi access point to your home broadband. Avnet Abacus’ Adam Chidley takes a look at low power WANs and why they will be essential in connecting thin
Just how smart can a smart home get? Tomorrow’s home is a true a brainiac, making decisions for you before you even think to tell it to.
From enabled to proactive: The evolution of home automation
April 3, 2018
Just how smart can a smart home get? Tomorrow’s home is a true a brainiac, making decisions for you before you even think to tell it to.

PIR and ultrasonic sensors: What's the difference and how do they work? | Engineers' Insight | Avnet Abacus

Display portlet menu
Related Events

No related events found