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Microwave Occupancy Detectors: Are They Better or Worse than PIR Sensors?

Microwave Occupancy Detectors: Are They Better or Worse than PIR Sensors?

  2012-11-19         sparksdirect         Advice » Lighting Design Advice

We talked about motion sensors on the blog recently, but most of the items in our store are Passive Infra-Red (PIR) devices, which are great - but they're not all we have.

In the Sparks Direct store, you'll also find a couple of microwave motion detectors: a completely different technology.

How do They Work? 

A microwave motion detector emits pulses of specific microwave frequencies, then measures reflection off objects (like walls) when those waves return to the sensor. In this manner the whole area of detection is filled, and the reflections change when there is a moving object (like a person) in the area. It works very much like the radar guns used by police to catch speeding drivers in the act - these detectors 'sense' motion in terms of speed and size, as opposed to a PIR sensor which senses in terms of heat and light.


The reflection mechanism means that the line-of-sight problems of PIR sensors - the effect which means a sensor can't detect movement if it occurs around the corner - is reduced, and the microwaves emitted can penetrate most building materials.

This means that sensors integrated into light fittings can be protected against dust and moisture intake by design.

They are also affected by background temperature, in contrast with PIR sensors which often don't operate well above 35° Celsius.

Microwave occupancy detectors have a considerable lifespan of around 100,000 hours.

Any worries about microwave radiation should be curbed; the devices in our store are very low-power and pose no threat.


However, although these sensors detect motion through wood and most building materials, microwaves do not penetrate metals.

Metal objects act as a shield, which creates shadows or "dead zones" behind them. Also, everyday movements such as objects blowing in the wind can trigger a false alarm - even fluorescent lights, which emit detectable light particles, can activate the sensor erroneously.

For these reasons, we would recommend that a microwave sensor be paired with a PIR motion detector. The strengths of one device can account for the weaknesses of the other.

If you're interested in purchasing an advanced microwave detector, we stock the low-profile MWS6-PRM and the adjustable MW3SA-PRM, both from industry leader CP Electronics.