How Do Door and Window Sensors Work? | The Magnetic Contact Explained

Door and window sensors are two-piece devices. One half mounts on the frame, the other on the moving door or window. When closed, a magnet inside one piece sits pressed against the sensor in the other. When you open the door, that magnet pulls away, which trips the sensor and logs an open event. That basic mechanism protects millions of homes, and understanding it helps you install, test, and troubleshoot your own system.

The Two-Piece Design: Magnet Meets Reed Switch

Most door and window sensors use a reed switch—a pair of thin metal contacts inside a glass capsule. When the magnet from the second piece comes near, the contacts snap together and close the circuit. The system reads that as closed. Slide the door open and the magnet moves away, the contacts spring apart, and the system reads open.

The two halves are deliberately small—about the size of two stacked matchbooks—so they blend into the frame. Wireless models then send that open or closed state to a hub, alarm panel, or your phone over Zigbee, Z-Wave, or Wi-Fi.

What Makes One Sensor Different From Another?

Not all sensors are equal. Some are strictly magnetic contacts; others pack in extras like a temperature probe. The protocols they speak vary too, and that governs which hub or app you’ll need. The table below compares four current models to show the range of features and requirements.

Before you buy any sensor, confirm it matches your system’s protocol. A Zigbee sensor like the Aqara needs a compatible hub. A Z-Wave sensor like the Fibaro must match your panel’s regional frequency—908.4 MHz in the US versus 868.4 MHz in Europe. A Wi-Fi sensor like the Deltaco needs a 2.4 GHz network; it will not connect to a 5 GHz-only router. And a Netatmo sensor will only pair with Netatmo’s own camera, so it won’t work with a generic alarm panel.

Placement and Installation Tips That Prevent False Readings

Mounting distance decides success or failure. The larger sensor and its smaller magnet must stay aligned and close. For example, the Deltaco SH-WS02 lists a maximum installation gap of 15 mm, while alarm provider Cove recommends its sensors sit within ¾ inch of each other. Exceed that gap and the contacts never close, so the system reports an open door even when it’s shut.

Pairing steps vary by brand, but the routine is similar. With an Aqara, you open the Aqara Home app, pair through the hub, and follow the prompt. With a Lidl Smart Window or Door Sensor, you insert the battery, open the Lidl Home app, select Sensors, and press the metal pin for 5 seconds until the green LED flashes quickly. Always mount the magnet on the moving part and the sensor on the frame so the two align when the door closes.

Do They Trigger Alarms by Themselves?

No. A sensor only reports state. Whether that turns into a siren depends on your alarm panel’s programming and arming status. A door contact on a fully armed system will usually trigger an alarm, but on a partially armed system, a first-floor door might be protected while an upstairs window is not. Check your panel’s configuration to know which contacts actually sound the siren.

Testing is simple. Program the device into your panel, place the magnet next to the sensor, and then pull it away. Look at the alarm panel to confirm the status flips from closed to open. If it doesn’t, the two parts are likely mounted too far apart—shift the magnet closer and test again.

If you’re weighing models, spec sheets on gap limits, radio frequencies, and hub requirements get tedious fast. For a shorter route, our tested roundup of top door and window sensors compares the practical options side by side so you can focus on installation instead of research.