The Magnet Has a Reach: Fit the Door Contact to Your Own Frame Before You Buy

A door contact alarm is a small pair of pieces, one on the door and one on the frame, that signals when the door moves away from the frame. It works only when those two pieces sit close enough together to read as closed on the door you actually have. Before you buy one, the question is about your own frame, not the box.

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Measure the gap while the door is shut

Close the door the way it normally closes, without pushing it hard, and measure the space between the door edge and the frame where the sensor will sit. Check the top, the middle, and the bottom, because doors often sag and the gap at the top can be wider than the gap at the bottom. Use the widest reading as your starting number. Then compare it with the gap rating printed on the package. If your reading is larger than the package allows, that model will not hold reliably, and pressing the door harder will not fix it.

Check which way the door swings

Stand where Mom would be standing when she tries to leave, and look at the hinge side. Some sensors are made for the latch side only, and the mounting spot may need to move if the door opens toward the room. Note whether a deadbolt or a knob sits near the frame, since either can block the spot you planned. Decide the mounting side before you open the box.

Look at what the frame is made of

Wood, vinyl, and metal frames take adhesive and screws differently. A metal frame may need a spacer for a magnetic sensor to read correctly, and the package will say whether it expects one. If the door is a storm door, decide whether the alarm belongs on the inner door, the storm door, or both, because each one opens on its own. If she uses more than one door, such as a patio slider or a door to the garage, each one needs its own check.

Test it the way she will open it

Once the sensor is mounted, open the door slowly, then quickly, and listen from the chair or hallway where you would be sitting. If the alert is too faint to hear from there, the sensor works on the door but not in the room that matters. Close the door and watch the status light, if the model has one, to confirm it resets. Run this test on a day when you are home and not rushed, and not at dusk.

Name the owner before you need the alarm

An alarm is a signal, not a lock. It tells you the door moved, but it does not hold the door shut, and a sensor that has drifted out of line can go quiet when you need it most. Decide now who checks the gap each week, who changes the battery, and who responds when the alert sounds. Write a name next to each job so no one is guessing at dusk.