Motion sensors

5 min read
Chosing motion sensor for smart home. What the difference? What to pay attention to when choosing wireless motion sensor?

Motion sensor is very important part of any smart home. It can be used for human presence detection and to perform certain actions when motion detected or there is no motion in several time interval.

Motion Sensor

Typically, motion sensors are not only detecting movement (or triggering events "No motion detected in N minutes"), also they can measure brightness in the room.

There are several motion sensors compatible with Mi Home, which look almost the same (a cylinder with a diameter of about 30mm and a height of 30-35mm), but at the same time they have differences that can be very important when used.

Let's compare, for example, this Xiaomi motion sensor and almost the same sensor Aqara:

Here is Xiaomi motion sensor:

Xiaomi motion sensor Xiaomi motion sensor, bottom look

And here is Aqara motion sensor and its specifications:

Aqara motion sensor Aqara motion sensor specs

Both sensors has almost the same stand, but in case of Aqara the stand can replace the bottom cover, while for Xiaomi the stand mounting to the sensor in the same way as to surface, with double-sided tape.

At the top part of Aqara sensor you can see the light sensor window, but Xiaomi sensor also can detect ambient light.

First and most important difference for these sensors is a communication interface. Xiaomi motion sensor uses Bluetooth, while Aqara uses Zigbee. Therefore, your gateway must be compatible with that wireless connection protocol. Usually smart home gateway supported both protocols (like, for example, this one), but it makes sense to check that before purchase. Both sensors uses the same battery CR2450, but it looks like Zigbee sensor can work longer time with the same conditions. Battery life is approximately 1-2 years.

Next very important difference are events which supported by Mi Home application for each sensor. Here is the list of events supported by Xiaomi motion sensor:

Xiaomi events

And here are events, supported by Aqara sensor:

Aqara events

As you can see, Aqara motion sensor can use events "no motion within 10 - 20 - 30 minutes",and it can be very useful (for example, in the evening, when you watch TV, it is unlikely to be convenient if the background light goes out every 2 or 5 minutes, but during 20-30 minutes of watching a movie, the sensor will surely at least once detect movement and will not turn off the light).

But Xiaomi's motion sensor has separate events "light became bright", "it became dark", "someone moves and bright light", and a mysterious "time limit for non-human movement" event (when aliens from other planet moving in your flat?). Please post in the comments below if you know how to use this event.

Another difference is how these motion sensors are presented in the Mi Home app. When you tap on the image of the Aqara sensor, it simply takes you to the recent events log. But the Xiaomi sensor is displayed differently:

Mi plugin

If you click on the dots in the upper right corner, you will open the device information:

Mi settings

Here you can change the timeout for "non-human movement" in the range from 0.5 to 60 minutes, turn on the blue indicator that will light up in the motion sensor each time it is triggered, see the battery level and signal strength. Touching the battery indicator will open the battery replacement instructions:

Mi battery

It is also important to know when planning usage scenarios that motion sensors are polled approximately every 2 minutes. This means that if the sensor has just detected motion, it will be delayed the next time it is triggered. If you hang a sensor above your bathroom door and expect it to turn the light on the first time it detects motion, and turn it off the second time (when a person leaves), this scenario will not work correctly. The sensor will trigger when a person enters, but will not trigger when a person leaves if less than 2 minutes have passed. Therefore, it is often convenient to use such motion sensor in pair with the door/window sensor.

You can change the behavior of Xiaomi motion sensor by switching it to test mode. To do this, you need to open the cover and solder a jumper. Then the sensor will be polled every 10 seconds. This will slightly reduce the battery life, but you also need to know how to use a soldering iron.

Comments:

Aleksandr, 2024-02-10:
Maybe "Time limit for non-human motion" means what some motion was detected, but it does not look like a human? For example, a pet movement, or a light change in the room?