Door and window sensors: the dull device that does the most

Door and window contacts are the least interesting devices in a smart home and among the most useful. They are cheap, they last years on a coin cell, they almost never produce a false reading, and they answer a surprising number of questions that have nothing to do with security.

They are also the sensor most people under-buy, because a magnet on a frame is far less appealing in a shop than a camera. Here is what they actually give you.

How they work, in one paragraph

Nearly all of them are reed switches: a small sealed switch in the sensor body, held in one state by a magnet in a separate piece fixed to the moving part of the door or window. Separate the two and the switch changes state and the sensor transmits. There is no interpretation involved, which is why they are so reliable — unlike a motion sensor, there is nothing to misread. Open is open.

Some newer designs use a tilt or vibration sensor instead, which suits windows that pivot rather than slide, and a few combine both. The reed switch remains the default for good reason.

The security uses, which are the obvious ones

Perimeter detection. A contact on every external door and accessible window tells you the instant your perimeter is breached — before anyone is inside, which is earlier than a motion sensor can manage.

Confirmation, which halves your false alarms. A door opening alone is ambiguous. A door opening followed by interior motion while the house is in away mode is not. Pairing contacts with motion sensors gives you both speed and certainty.

The unlocked door problem. A meaningful share of burglaries involve no forced entry at all. A simple check when the last person leaves — is anything still open? — addresses one of the commonest real-world failures, and it is the single most valuable automation you can build from these sensors.

Tamper detection. Better sensors report if the housing is opened or removed, which matters if someone works out what they are.

The uses nobody mentions in the marketing

This is where they earn their keep, and why we suggest buying more than you think you need.

Not heating the street. A contact on a window, linked to the radiator valve or heating zone in that room, pauses heating while the window is open. In a house with a heat pump, where efficiency depends on steady low-temperature operation, this is worth real money over a winter.

Ventilation and damp. Knowing how long windows are actually opened, combined with a humidity reading, tells you whether a room with a mould problem is being ventilated as much as everyone believes it is. It usually is not.

Fridge and freezer doors. A contact on a freezer door that alerts after two minutes has saved a great many people the contents of their freezer.

Medicine cabinets, gun cabinets, cleaning cupboards, garden gates. Anywhere you want to know about access without installing a lock.

Post and parcel boxes. A contact in an external letterbox tells you post has arrived without a camera and without a subscription.

Lighting that is actually instant. A contact on a wardrobe, pantry or cupboard door switches a light with no detection delay at all, because the trigger is mechanical rather than inferred. Motion sensors always lag slightly; a reed switch does not.

Occupancy inference. The front door opening followed by no interior motion means somebody left. The same door followed by motion means somebody arrived. Two cheap sensors give you a reasonably reliable arrival and departure signal without any presence detection at all.

Choosing and placing them

Gap tolerance is the specification that decides whether the sensor works on your particular door. It is the maximum distance between sensor and magnet at which the sensor still reads as closed — typically somewhere between 10 and 25 mm. A door that sits proud of its frame, or a window with a deep rebate, needs a generous tolerance or a packing shim. Check this figure before buying for an awkward opening.

Size matters more than you expect. On a slim uPVC window frame there may be very little flat surface available. Measure the frame before buying a chunky sensor.

Battery type and life. Most run two to five years on a coin cell. If you are fitting fifteen of them, standardising on one battery type is a small kindness to your future self.

Protocol. These are the definitive case for a low-power mesh protocol like Zigbee, Z-Wave or Thread rather than Wi-Fi. A Wi-Fi contact sensor burns through batteries and adds fifteen devices to your router for no benefit. Mesh sensors also relay through mains-powered devices, so range across a house is rarely an issue.

Placement. Sensor on the fixed frame, magnet on the moving part — the sensor is the heavier component and the one with the battery, so it belongs where it is not being swung about. Keep both above hand height on external doors where you would rather they were not noticed, and away from metal frames, which can affect radio range.

Where they fall short

Honesty about limits, because they are not a complete answer.

They detect opening, not breaking. A window smashed rather than opened leaves the contact closed. If that is a real concern for a particular window, add a glass-break or vibration sensor rather than assuming the contact covers it.

They tell you nothing about who. That is fine — it is why you pair them with other sensors rather than expecting them to be a system.

And a contact on a door that is opened forty times a day will report all forty. Design your automations around the events you care about rather than every state change, or you will mute the notifications entirely within a week.

Five automations worth building on day one

Hardware on a frame does nothing by itself. These five take a few minutes each and cover most of the value.

The leaving check. When the last person leaves, if any external door or window is open, send one notification naming which one. Not a list of everything — the specific opening, so the answer is obvious.

Heating pause. Window open in a room for more than two minutes, close the radiator valve in that room. Window closed, restore it. Two minutes of delay stops it reacting to a quick airing.

Freezer watch. Freezer door open for more than three minutes, notify loudly. This is the one people are most grateful for.

Night perimeter. After everyone is in bed, any external door or window opening turns on the landing light at low brightness and notifies. Low brightness matters: it should orient you, not blind you.

Arrival lighting. Front door opens after dark, hall light on for three minutes. Instant, because the trigger is mechanical, and it works before you have found a switch with your hands full.

Notice that only two of the five are about security. That is the honest ratio of how these sensors get used once they are in.

A sensible buying order

1. Every external door first. This is the highest-value coverage and usually only two or three sensors.
2. Ground floor and accessible windows next, prioritising the least overlooked ones.
3. One or two interior motion sensors, so you can build confirmation rules.
4. Then the non-security ones — freezer, letterbox, the window in the room with the damp problem, the wardrobe you always fumble in.
5. Build the leaving check before anything else. It is five minutes of work and it is the one that will actually prevent something.

At the price of these things, buying a few spare is entirely rational. You will find uses for them.