Hey guys I am new to Smartthings. My door jams have a deep recess and I don`t think the multipurpose sensor will work. I was thinking about mounting the motions sensor to the back of the door. Does anyone know if it will trigger an alert if the door is opend?
Most motion sensors read heat, not actual motion. If mounted to the door, the multi will report the door is in motion even without the magnet.
Just get the aeotec recessed sensor.
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As others have said, the motion sensors actually detect tiny changes in heat passing across the field of the sensor will not trigger in this situation. Those usually have “PIR” (passive infrared) somewhere in the product description. So those are likely not going to work.
there are two alternatives if you want to catch the movement of the door, not the movement of a person towards the door.
One) contact sensors. These can be put on either side of the door. This is a two-piece device, a magnet and a sensor. When the two pieces are very close together, they report “closed,” when they are a few inches apart, they report “open.” So you put one piece on the door itself and the other piece on the door frame. This is probably the most common type of door sensor because it solves exactly the issue you mentioned, catching the door from either side. Notice that it’s not really a motion sensor, but it is reporting the motion of the door as that is what makes magnet move away from the sensor.
The recessed sensor which was already mentioned is of this type. It’s just designed to go inside the door instead of on top of it.
Two) acceleration sensors. These are true motion/tilt sensors, not heat sensors but they are detecting movement of the sensor itself, not something moving towards it. There is an accelerometer inside which detects movement along all three axes: X, Y, and Z. This is a one piece device ( although there may be a second magnet piece if the same device is also a contact sensor). Opening the door would be movement along the x-axis.
These work fine for this purpose, it’s just that they are more complex devices than contact sensors and typically cost about twice as much. But they can definitely solve the problem if there’s a specific issue with how much room there is to place the sensor but you can’t drill inside the door.
So if you’re asking if the smartthings multisensor (which has an accelerometer and a contact sensor) Will catch the motion of the door if it is mounted on the backside of the door, then yes it will. And you won’t even need to mount the smaller magnet piece, as that is only needed for the contact sensor function. . 
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The important factor here is whether you want to know when the door is [Open Or Closed ] or [opening/closing (without knowing which)]
A motion or accelerometer solution will likely only bear the opening closing state. The contact sensor only gives an open or closed state.
In the OP he says he wants an alert if the door is opening, imo it will be difficult for the op to ascertain Only the opening from an accelerometer or motion. In simple automations you would likely have repeat alerts on the door closing (you could probably limit the alerts with more complex automations achieveable with CoRE)
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