What makes a good PIR Presence Sensor?

In this blog, we explore the features that make a good PIR presence sensor for smart lighting control applications When you’re planning a lighting control scheme, you will inevitably need…

In this blog, we explore the features that make a good PIR presence sensor for smart lighting control applications

When you’re planning a lighting control scheme, you will inevitably need sensors somewhere in the system.  Whether it’s presence detection, light level or another parameter such as temperature, you want to be sure that the device you choose will provide accurate and relevant information.

You’ll also want to be sure that your chosen sensor can interoperate correctly with the other system components, whether it’s a wired DALI® system or a wireless solution such as BlueTooth® SIG or Casambi®.

So let’s start with the basics and look at how a presence detector works:

Most people are familiar with a PIR sensor.  At its heart, this consists of a lens which focuses infrared radiation (heat) onto a pyroelectric sensor (or pyro), which generates an electrical signal when it sees different amounts of infrared radiation across its sensing elements.

The electrical output from the pyro is tiny, so this is amplified and then an electronic circuit or processor decides whether the signal corresponds to a  ‘trigger’ of the detector, in which case it activates the output.

That’s the basics, now let’s look at the individual components in more detail…

Size matters

Let’s start with the lens.  Just as a top camera lens will show more detail than the lens on a cheap phone, so the quality of the lens on a PIR detector will determine its range and sensitivity.  Many manufacturers use very low-cost lenses from commodity suppliers, however more quality conscious manufacturers will use premium branded lenses from a UK or European optics manufacturer.

And just as camera lenses come in different types (wide-angle, telephoto) for different uses, so PIR lenses are designed for different applications, so that a sensor used in an office will have a shorter focal length than one used in a high-bay warehouse.

Lenses are designed to match the characteristics of a specific pyro, or a range of pyros.

When considering sensors for an office or classroom, the key thing to look for is the number of facets or elements on the lens. A PIR sensor will typically detect a signal when movement crosses between detection zones. The more lens facets, the more detection zones, so this determines how sensitive the detector will be to partial body movement – the sensor may detect someone walking into an office, but how well will it continue to detect that person when they’re sat at a desk typing, writing or talking on the phone?  As a rough rule of thumb, a larger lens will be able to incorporate more facets.

So the first rule is more lens segments or facets = better detection of partial body movement.    

Comparison of coverage patterns for two lenses

In recent years, there has been a tendency towards very small sensors that can integrate into luminaires, especially linear fittings.  While these are very discreet in terms of appearance, many have a limited detection range or a very limited number of detection zones, so you end up installing more of them to ensure adequate detection coverage within the space.

Feeling the heat …

Next, the pyro detector itself.  Early sensors used dual-element pyros, consisting of two sensing elements side by side.  These work well where the detection is in one plane only (for example, a corridor), but perform poorly in detecting movement from other directions.  Over the years, manufacturers developed quad-element pyros with four opposed sensing elements to overcome this problem, however in extensive testing at Calon we have found that the best performance can be obtained from so-called 360 degree pyros, which feature two intertwined sensing elements  (think ying and yang).

OK, now our lens has focused infrared on the pyro which has generated a signal – what next ?

Remember, the signal coming from the pyro is tiny – a few millivolts at most – so we need to boost it using an amplifier circuit.  The input circuit for a pyro amplifier is high impedance, so an electronic low-pass filter is needed to avoid pickup of radio frequency signals (e.g. mobile phones).  A good modern pyro will incorporate a simple filter to inhibit interference pickup through the pyro itself, and the amplifer will typically incorporate a low-pass filter to roll off the frequency response at an appropriate point to avoid radio frequency noise being fed into the trigger system.

Decision time – Real or false trigger ?

Conventional ‘dumb’ sensors typically amplify the signal coming from the pyro and compare this to a threshold to determine when to trigger.  This works, but it’s crude and it doesn’t distinguish between the many different things that can induce a signal into the pyro (for example, electrical interference or air movement).  This can lead to false triggering, in which sensors turn the lights on when there’s no-one in the room (for example, lights that trigger when the office heating comes on first thing in the morning).

One way to get around this is to set the trigger threshold quite high, but this limits the sensitivity of the detector and can delay a genuine trigger (no-one likes to walk halfway into a darkened room or corridor before the lights come on).  

Getting the balance right between good sensitivity and the avoidance of false triggers is one of the key challenges facing an engineer developing a new sensor.

Nowadays some sensors may incorporate a basic processing function to count the number of ‘hits’ on the pyro, so triggering only occurs when the pyro has sensed movement several times in a row.  These are commonly used on intruder detection systems to avoid false triggering, but are of limited usefulness in lighting control scenarios.  

Some pyro manufacturers build the detection circuits and logic into the pyro itself, to simplify the overall sensor design, but these can be very expensive and involve design compromises.

With the advent of high performance, low-power processors, it has become feasible to perform digital signal processing on the signal coming from the pyro, to look at the shape of the waveform in order to determine when to trigger.  

Over recent years, Calon has developed TrueMovement™ technology, which utilises advanced digital signal processing techniques to distinguish between real body movement (including partial body movement) and electrical noise, air movement, etc.  This allows Calon SmartSensor™ products to combine excellent sensitivity with class-leading rejection of false triggers and has been proven over thousands of installations.

PIR Presence Sensor

TrueMovement looks at the shape of the amplified signal coming from the pyro and performs three separate digital signal processing actions to compare the characteristics of the waveform against the known responses for a ‘real’ person moving – covering both full and partial body movements.

Talking to the world

So now our sensor has decided that it’s a real trigger, what next?

Traditional sensors activated a relay to switch mains power to the light fittings, or provided a simple volt-free contact output to a wired lighting control system.  Today, sensors are likely to be deployed in a networked system, be it a wired DALI system, or a wireless BlueTooth SIG or Casambi network.  For these applications, compliance to the network standards is a must to ensure interoperability with other system components.

For BlueTooth, use of the NLC (Networked Lighting Control) stack ensures compliance to the BlueTooth SIG standards for lighting control.  DALI devices follow the specifications set out in the IEC 62386 family of standards, and are certified compliant (“DALI 2” certified) by the DALI Alliance (DiiA).

Summing up

So to summarise, what make a good sensor?

  • Good optical design featuring an approriate lens for the application – this means a lens with many facets for office / classroom applications;
  • The right pyro detector matched to the lens;
  • Careful design of the amplifier electronics to eliminate interference pickup;
  • A detection system which can process the resulting signal to accurately distinguish between real and false triggers;
  • Network communications which conform to open standards, allowing interoperability with the widest range of other system components.

In practice, the sensor that you choose to suit your application or project will be influenced by many factors. We hope this helps to explain some of the factors to consider when choosing the right solution.

Calon is a UK-based company who develop, manufacture and supply class-leading components and solutions for wired, wireless and hybrid lighting control systems.  

You can contact Calon at hello@caloncontrols.com or visit our contact us page.