What is the difference between a human presence sensor and a human body sensor? I changed 4 of them before I stopped turning on the lights blindly.

The automatic light in my bathroom was originally installed for more than 40 yuan. Mijia body sensor 2 . It was good for the first few days after it was installed. The light came on when someone went in, and went off a minute after he came out. The problem comes after you actually use it: Less than two minutes after I sat on the toilet and checked my phone, the overhead light went out with a bang. I still held the phone in my arm and had to wave it again to turn it on again. The daughter-in-law said that the lamp was deliberately against her.

Later, I tried the study room, the hallway, and the door of the master bedroom. I changed four sensors one after another, and then I figured out one thing.: They both have "human body" in their names, but they are actually not the same thing at all. The human body sensor is to see if you are moving, and the human presence sensor is to determine if you are still there. . This article does not change the parameters, but writes down the locations, delays and misjudgments that I have stepped on.

Make it clear first: The human body sensor only recognizes big movements

The common "body sensor" is essentially pyroelectric infrared, or PIR in jargon. It is triggered by sensing the temperature difference between the human body and the background, so it is particularly sensitive to walking, waving, and passing by doors. But it has a fatal weakness: As long as your movement range becomes smaller, it will feel that you are gone . If you squat on the toilet, use your phone while sitting on the sofa, lie on the bed without turning over, or even just move your fingers while sitting on an office chair, it may not last.

My bathroom was originally set up to turn on the lights when it detects people, and turn off the lights 60 seconds after no one is around. At night, I was sitting on the toilet watching a video. As soon as 60 seconds passed, it would go off. I would light it up again when I waved my arm, and then it would go off again after another 60 seconds. Don't think it's a setting problem, this is the working logic of PIR. It's not that it's bad, it's that quiet people really can't be seen.

Such sensors also have benefits: Cheap and energy efficient. Installing a CR2450 can last for one or two years. It is enough to place it in doorways, corridors, and balconies where it will stay for no more than half a minute. I later moved it to the entrance, and it was just right to light it up when entering and exiting.

What are people looking at in sensors?

Later I changed to Xiaomi human sensor , the one around one hundred and thirty. In addition to infrared, it also has millimeter-wave radar, which can detect small fluctuations, such as the movement of the chest during breathing and fingers typing on the keyboard. So even if the person is sitting on the chair motionless, it will still think that the person is there and will not turn off the lights randomly.

I used the study room as a test: Set the lights to turn off for 5 minutes when no one is around. In the past, when I used PIR, stretching while writing a manuscript had to be counted as an activity, otherwise it would die out soon. After switching to the human sensor, I lay on the table looking up information and typing with one hand, and the light never went off once. The most intuitive difference is: PIR needs to "move", millimeter wave only needs "people to be there"” .

But millimeter waves are not everything. It needs to be powered all the time, USB or constant power, unlike the human body sensor, which just needs to be replaced with batteries. If installed poorly, it can penetrate glass, misjudge the room next door, or mistake a rotating fan for a person. More details later.

Actual measurement in four rooms: Only these two positions are the most valuable

The positions I finally fixed are these four. There are profits and losses. I tell people who want to buy to avoid pitfalls.

• bathroom ： Most worth changing. The PIR will definitely turn off the light in low-action scenes such as toilet and bathing. After a person is placed on the sensor, the light does not turn off in the bath for 30 consecutive minutes. Assume that no one is around for 10 minutes before turning it off again, and the accidental shutdown will basically disappear.
• study ： value. When you sit for a long time, work or read, the millimeter wave is obviously stable. Just don't face the window directly, as the figures outside the glass may accidentally touch it.
• Entrance/corridor ： No need to go expensive. People are passing by, and a PIR of more than ten yuan is enough. Just set the lights-off delay to 30 seconds.
• bedroom ： Be cautious. Millimeter waves are sensitive to turning over and shaking quilts, and sometimes light up by mistake in the middle of the night. Either install it higher and only cover the edge of the bed, or turn the brightness of the light to the lowest setting.

The most unfair place I have ever placed it is next to the curtain box in the living room. At night, the curtains were blown by the air-conditioning wind, the lights came on repeatedly, and dozens of records appeared in the app overnight. Later, it moved 20 centimeters inward to avoid the window before it stopped.

Four pitfalls you should avoid when purchasing and installing

The first pitfall is just looking at the word "induction". Many merchants also write PIR as "human body sensing, motion detection". If you click on the parameters and there is no millimeter wave, presence, micro-motion, or breathing detection, then it is an old solution. You can tell the difference quickly by looking at the power supply before buying.: Most of the button batteries are PIR, and those with constant power may be millimeter wave.

The second pitfall is that the millimeter wave installation is too low or facing the door. Its coverage is at an angle. I initially attached the one in the study to the door frame, but it also triggered when people passed by the door. Later, I moved it to the corner of the ceiling and hit it diagonally towards the desk, so that there would be no false alarms outside the door. The installation height and orientation are more important than you think.

The third pitfall is that glass and metal will trick it. Millimeter waves can penetrate some glass, so don’t detect them through the balcony door. ； Metal cabinets and large mirrors will also reflect signals, changing the actual range. After installing it, stand in a position where you don't want to trigger it and try it for 10 minutes. It is more reliable than reading the manual yourself.

The fourth pitfall is that "no one" in automation does not mean shutting down immediately. Set a reasonable delay in the Mijia scene, don’t write “no one turns off the lights immediately”. People just go out to get something, and when they come back, the lights have to be turned on again, which is a bad experience. It takes 10 minutes to set up my bathroom and 5 minutes to set up my study, which is currently the most convenient.

How to cooperate with home mode and presence sensor

During the test, I also specifically counted the delay.: For the same "no one turns off the lights" automation, the PIR starts counting from the last big movement. If people sit there and don't move, it means the countdown has already started. ； On the millimeter wave side, you have to wait until you actually leave the room to start counting. So it's not that the setting is wrong, it's that the "presence/absence" reported by the sensor to the automation is originally different. After I figure this out, I won't be so angry when I look at the automated logs.

Many people think that with the presence sensor, they don’t need to worry about the home mode. Actually it is two things. The home mode solves the problem of "the moment you enter the door", the door lock or door sensor is triggered, and the lights and air conditioner are turned on together. ； The human presence sensor solves the problem of "not going out when a person is in the room". I initially put both of them into one automation, but the result was that when the door was opened and the light came on, there would still be logical conflicts after people sat on the sofa for a long time.

Later it was disassembled: The home mode is only responsible for the door entry action. After the light is turned on, it is left to the person to maintain the sensor. ； When leaving home, use the door lock to lift and lock it to trigger full closing. The middle section no longer relies on PIR guessing, which makes it much smoother. If the home mode happens not to be triggered, you can read the previous article first. How to set up Mijia home mode ； The automation is not executed to the point, I will separately record the troubleshooting sequence, that article Mijia automation is not executed to the point More complete.

Ending with a table: Buying by length of stay is more accurate than buying by room

one sentence notation: Use human body sensors when passing by, use human presence sensors when waiting . Don’t save dozens of dollars on places like bathrooms, study rooms, and bedrooms where you can sit for more than ten minutes. ； There is really no need to go expensive for the entrance balcony. Don’t rush to fix the sensor after you buy it. First use double-sided tape and stick it on the location where you intend to install it for one night. Then read the automation log the next day and move it if there are many false lights. This is more effective than reading reviews.

If there are more and more smart devices in the home, it will be more worry-free if both people and sensors are connected to the Zigbee gateway. This is my idea. What does Zigbee mean? Said in detail. If the sensor is not connected to the gateway and the WiFi is fully connected, the router will not be able to handle it at first. You can refer to How to handle dozens of smart home device routers at home .