A door-open sensor tells you when a specific door moves. It is an alert, not a lock, and it does not replace a check-in or a person at the house. It is worth installing when a door is the exit Dad uses at odd hours and you need to know when it opens. Install it in order, because most of the trouble happens in the order.
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Survey the door first
Note the door type: hinged, sliding, or a French pair. Check the frame material, the depth of the jamb, and how far the door sits from the frame when closed. Read the product manual for the largest gap it can detect and the mounting surface it expects. Note where the nearest power or network point is, if the product needs one. Write all of it down before you open the box.
Check the exit path before you mount
Confirm the sensor, the bracket, and any wire stay clear of the handle, the deadbolt, the swing, and the path to the door. If this is a required exit in the house, ask your local fire or building office whether any device is allowed on that door before you mount anything. Do not mount a device that makes the door harder to open from the inside.
Mount the two halves in order
Set the sensor on the frame and the magnet on the door, using the alignment mark in the manual. Mark both with a pencil, check with a level, then fix them with the adhesive or screws the product calls for. If the unit is wired, keep wire out of the swing path and secure it so it cannot be pulled loose. Test the open and closed states before you finish.
Set the alert route
Decide who gets the alert, in what order, and what each person does when it arrives. Add at least two people to the route and confirm that each one receives it. If the device needs the home network, place the hub where it reaches the door, and check the signal from the door itself, not from the sofa.
Test it three ways
Open the door in daylight while a family member watches the alert arrive. Open it again after the lights go down, to confirm the alert reaches the phone in the dark. Leave the door open for a while to see whether the alert repeats or goes silent. Write down what you saw and who got it, so the next person knows what a normal alert looks like.
Decide what Dad knows
Talk with Dad about the device before it goes up if you can. A sensor he finds later turns into an argument about trust. If he objects, hear the objection before anything goes on the door, and decide as a family whether the alert stays.
Next move: Survey the exit door tonight with a tape measure, the product manual, and a pencil, and get the fire or building answer before you buy anything.