PIR Motion Sensor vs Presence Detector – Zigbee Presence Detection Sensor Setup

This little guy is a zigbee presence detection sensor – specifically the SonOff SNZB-06P – and it is completely different from this, a PIR motion sensor – specifically an Aqara Zigbee motion and light sensor. Both do detect motion, both even have ambient light sensors, but they work completely differently, and I’d argue one is considerably better than the other. Let me explain how these work, why you might want either, and how to set this one up.

I’ll start with the regular motion sensor – you’re likely familiar with these, whether in this sort of smart-home format, or more commercial ones for bathrooms and cupboards, basically you move, they normally turn on a light. It’s pretty simple. The way they work is really cool. You’ll likely want to watch Alec’s video for more detail, but in short the PIR in the name stands for passive infrared, and basically in here is a little chip with two sets of crystals. This semi-transparent bit of plastic is actually a lens that focuses infrared light onto that sensor. As those crystals heat up – both from ambient heat and from a hot bod like yourself – they emit a tiny amount of energy, which the chip can detect and here’s the neat thing. Only one of those sensors is exposed to the infrared light itself, so if both crystals heat up, then it’s an ambient temperature change, but if only the exposed one heats up, then it knows it’s you walking by. 

The downsides to this are mostly in the latency between when it first “sees” you, and when it actually triggers. If you’ve ever noticed that it takes some time between you first walking in the room and the light coming on, that’s because the crystal has to heat up. The other catch is to do with motion – once you are, say sat on the sofa, sitting still, the sensor acclimatises to the new heat level. Motion is what’s needed, hence the name motion sensor. That means even if you have a motion sensor like this say in your living room to turn your lights on when it’s dark enough, if you have it set to turn them off again, it’s going to leave you in the dark once you sit still. That isn’t ideal.

So, what’s different with this, a presence detector? Well, despite this looking pretty similar with it’s own clear dome lens looking thing, this works completely differently. This uses radio waves – this particular one uses 5.8GHz waves, although some use more like 24GHz or 60GHz – to detect even the tiniest motion. This is literally radar for your home. There are a few benefits, as well as drawbacks, with this design though. The benefits are clear, there is basically no latency between it detecting motion and it reporting that to Home Assistant, so it’s much faster and more responsive. The other major benefit is that because it’s detecting presence – well really it’s detecting even the movement of your chest from breathing – your lights won’t turn themselves off when you’re sat watching the TV. The drawbacks are mostly to do with false detections, as due to this being so sensitive, even something like a fan being left on, or even a gust of wind through the window moving the curtains, that can be enough to trigger it. Luckily most sensors offer sensitivity calibration so you can normally tune that sort of thing out. 

As for how to set this one up, it’s remarkably simple, assuming you’ve already got Home Assistant and Zigbee set up – videos in the cards above if you want to know more – you just pair it like you would any other Zigbee device, then set up an automation for when the occupation state becomes occupied, and another for when it becomes clear. It’s spot on for accuracy, although I would mention that the built in light sensor is pretty annoying. It’s a fixed value that just changes between “light” and “dark”, and you can’t control how light or dark it has to be to trigger that, and for me it triggers way too late. I’d like to be able to make it turn my lights on when it’s lighter than it’s set for, but that isn’t possible at least with this sensor. There are plenty of other sensors on the market, including the Aqara one which even does mutli-person and multi-zone detection which is incredible, although you do pay for that privilege. This Sonoff one was under £30, whereas the Aqara one is more like £80. Perhaps you get what you pay for though… 

Anyway, if you want to be able to walk into a room and have your lights turn on, I’d recommend a presence sensor like this over a motion sensor. They are just as easy to use, but are, in my experience, more reliable, and more useful especially for rooms you plan on continuously occupying. This Sonoff one does work well, even if the light sensor isn’t quite tuned to my taste, and does even have a setting to maintain the presence setting for up to 60 seconds – as in it’ll keep the lights on for up to 60 seconds after you leave the room. That’s handy. Anyway, I’ll link this sensor in the description if you want to grab one yourself, and if you haven’t already, check out the whole smart home series I’ve got up already – there’s plenty to choose from in there!