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Instead of being named after an inventor or some strange historical quirk (I’m talking to you bayonet light fittings*), the word PIR is actually an acronym that stands for Passive Infrared, and that gives us a clue into how they work.

They are called “passive” because they are essentially waiting passively for energy to come to the sensor.  PIR sensors don’t actively send out any energy for detection purposes, unlike active sonar that emits pulses of sounds and listens for returning echoes.  The advantage here is that PIR sensors don’t need much power to run (our Smart Lamp BULB sensor uses less than 0.1 Watts), and we all love efficiency here at Smart Lamp.

PIR sensors detect the infrared radiation emitted by the objects around it. Infrared light is a form of electromagnetic radiation that cannot be seen by the naked eye.  We warm-blooded humans give off this energy without even realising it, as do all objects above absolute zero temperature, emitting heat energy in the form of infrared radiation.

PIR sensors aren’t just detecting infrared alone, because then any inanimate object in the area could set them off, rendering them useless.  Technically what PIR sensors are detecting is the change in infrared radiation within its field of vision, and in that way, they detect the movement of objects. The sensor triggers the light or alarm that it is connected to when the gradient of change is higher than the predetermined value set by the manufacturer.

Have you noticed the multifaceted dome-shape of the Smart Lamp’s sensor?  Most PIR sensors focus the infrared radiation through the use of facets and Fresnel lenses, thus enabling the sensor to have a wider field of vision.

*Bayonet light fittings are a traditional standard in Britain and colonial countries.  Historically it is similar to the fastening used for quickly attaching bayonets to rifles, hence the name.

How Do PIR Motion Sensors Work?
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