Adapting PIR sensor technology to new applications
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. The PIR sensor is usually the first choice for home security systems, as its ability to sense warm, moving objects such as people walking into a room is coupled with simplicity and cost effectiveness.
There are many other applications that can make use of the ability to sense people or animals to improve energy efficiency, convenience and safety but without the high additional cost of using advanced image processing to handle images from a camera. Used in the outdoor environment you will want a system to discriminate between people and animals, so image processing makes sense. But, in an office setting, it is a safe assumption that the PIR sensor will only pick up people. So the sensors can be used to start air conditioning or turn on the lights.
Vending machine designers, for example, are now incorporating PIR sensors into their products so that their displays only light up when someone is standing in front of the unit or maybe waving their hand in front of a panel – which saves on operating costs. Thanks to the advent of the internet of things (IoT), PIR sensors can drive multiple applications by sending status updates to smart servers. Why not use the output from local security cameras to wake the display panels in a meeting room or the printer in an office cubicle during working hours?
It is important to pick the right kind of PIR sensor. The various offerings on the market use the same core architecture but there are increasingly important differences in design and construction. The IR radiation typically enters the sensor housing through one or more windows at the front and lands on a set of panels made from a pyroelectric material – one that generates an electric current when exposed to IR energy. The pyroelectric materials vary from inorganic gallium nitride and caesium nitrate to carbon-based polyvinyl fluorides and phenylpyridines.
A typical PIR sensor in operation
In a typical PIR motion sensor there are two rectangular windows that allow IR to penetrate. Behind each window is a sensor with electrodes attached in such a way that one sensor provides a positive output signal and the other a negative output signal. If no object is detected, both sensors receive the same amount of infrared radiation and their signals cancel out. But if a warm body passes into the view of one of the detection sources, its value deviates from its paired sensor, causing the sensor electronics to register change in output.
A Fresnel lens in front of the window area can be used to increase the sensing range of the sensor and detection angles and detection patterns to be matched to the requirements of the respective application. Various sensor layouts, many of which use more than two pyroelectric elements arranged in a grid, provide the ability to tune performance for different applications. For example, the lens and array configuration in Panasonic’s AMN series is optimised to detect motion within a narrow field of view, suiting the product to vending machine and light switch applications compared to the general-purpose EKMB and EKMC families.
Panasonic's EKMB and EKMC PIR sensors
To improve usability, the PIR sensor will often include an amplifier. One example is the IRA-S210ST01 from Murata. This three-port device includes a low-noise JFET that converts the electrode outputs to a convenient output level. Manufacturers will generally offer choice between logic and analogue outputs. In general, the analogue output will offer more design flexibility and provide the ability to apply filtering circuits or components to improve performance under noisy conditions. Logic outputs provide an easier design path for simpler applications.
Sensor manufacturers have in recent years improved performance under the noise generated by smart technologies. For example, the electromagnetic emissions from phones can reach into the detection range of some sensors, triggering them when no one is there. In Panasonic’s PIR sensors, there is an amplifier/comparator circuit inside the TO-5 metal that rejects the high-frequency interference caused by wireless devices (shown right).
As the application breadth for PIR sensors widens, features such as sensitivity become important. Panasonic’s sensors incorporate a novel slit design for the pyroelectric elements that form the core of the sensor. The separated sensing areas prevent thermal crosstalk between the single sensing elements. This makes detection more reliable even if the temperature difference between the background and the target object is small.
These design improvements mean PIR sensors are moving into a wider range of applications than ever, and make suitable choices for the growing range of IoT-compatible products and services that are now emerging. If you need some advice on PIR sensors or would like to discuss your design in more detail, get in touch with our team of technical specialists by clicking the Ask an Expert button to the right of this post.