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How Do Window and Door Sensors Work — And Are They Reliable Enough to Replace a Full Alarm System?
Inside the reed switch that guards every entry point, how sensors talk to your phone, what they genuinely protect against — and the honest answer to whether a set of sensors is all the security you actually need.
The Short Answer
Window and door sensors work using a reed switch — one of the oldest and most reliable electrical components in everyday use. A magnet sits on the moving part (the door or window); a glass-encased switch sits on the fixed frame. When they're close together, the magnetic field pulls two tiny metal contacts inside the switch into contact, completing a circuit. When the door opens and the magnet moves away, the contacts spring apart, the circuit breaks, and that event is reported as an open. Simple, durable, and remarkably hard to fool.
Can they replace a full alarm system? Not entirely — and understanding exactly why reveals the one gap you need to fill to make a sensor-based approach genuinely robust. Sensors cover entry points. They don't cover what's already inside, what comes through a wall or ceiling, or what happens when someone breaks glass rather than opening a frame. A sensor system with a motion detector added covers the vast majority of realistic intrusion scenarios at a fraction of the cost of a full alarm installation.
"A reed switch costs less than a coffee, contains no electronics, and has a design that's barely changed in sixty years. Its simplicity is the point — there's almost nothing to go wrong."
The Reed Switch — What's Actually Inside
The reed switch is one of the most elegant mechanisms in common use. Inside a small sealed glass tube — roughly the size of a grain of rice — sit two overlapping ferrous metal strips called reeds. They're springy, and in their natural resting state they don't touch. When a magnet comes close, the magnetic field passes through the glass and the reeds, magnetising them. Opposite poles form at the overlapping ends and the reeds attract each other, making contact and closing the circuit. Remove the magnet and the spring tension pulls the reeds apart again.
The glass seal matters: it keeps the contacts in a nitrogen or inert atmosphere, preventing corrosion and arcing that would degrade them over time. A quality reed switch can cycle — open and close — tens of millions of times without failure. The front door sensor you install today should still be working in fifteen years, which is more than can be said for most consumer electronics.
The two-piece form factor — one piece on the door, one on the frame — means the sensor tolerates the constant mechanical movement of opening and closing without any wear on electrical contacts. The electrical connection is made through the air via the magnetic field, not through physical contact between the two halves. This is why door sensors last so much longer than most electronic devices: the part that senses (the reed switch) never touches anything.
Types of Contact Sensors
The magnetic reed switch is the foundation, but the broader category of contact sensors includes several variants worth knowing:
What About Glass Break Sensors?
Glass break sensors are acoustically different from contact sensors — they listen for the specific frequency of shattering glass rather than detecting movement of a frame. They're a complement to reed switches, not a replacement: a reed switch on a window catches someone opening it; a glass break sensor catches someone smashing it. For complete window coverage, you need both, or you need a motion detector behind the window line that will catch an intruder who enters through broken glass regardless of how they got through.
How Sensors Communicate
A sensor that detects a door opening is useless unless it can tell something about it. Modern smart sensors communicate in three ways, and the choice affects battery life, range, and which hubs and systems they work with.
Z-Wave and Zigbee are the preferred protocols for dedicated security sensors. Both operate on frequencies below the congested 2.4 GHz Wi-Fi band — Z-Wave at 868 MHz in Europe, Zigbee at 2.4 GHz but with a much lighter footprint than Wi-Fi. Both form mesh networks where each sensor extends the range of the others, meaning a large property can achieve reliable coverage without repeaters. Battery life on Z-Wave and Zigbee sensors typically runs two to five years on a single CR2032 coin cell. The trade-off is that they require a hub: a SmartThings hub, a Home Assistant installation, or a system like Ring Alarm or SimpliSafe that includes a compatible base station.
Wi-Fi sensors connect directly to your router with no hub required, making them simpler to start with. The downside is battery consumption: the Wi-Fi radio draws significantly more power than Z-Wave or Zigbee, pushing battery life down to three to six months in active sensors. They're also subject to the same network congestion issues as any Wi-Fi device.
Matter over Thread is the emerging standard worth knowing about for new installations. Thread is a low-power mesh protocol similar to Zigbee, and Matter-certified contact sensors — appearing in increasing numbers from 2025 onward — will work across Apple Home, Google Home, Amazon Alexa, and SmartThings without an ecosystem-specific hub. If you're starting fresh, looking for the Matter logo future-proofs your sensor investment.
What Sensors Catch — And What They Miss
Sensors cover the frame. They don't cover the glass. An intruder who smashes a window rather than opening it — common in opportunistic break-ins — won't trigger a reed switch that never separated. They also can't detect someone who was already inside before the system was armed, or who enters through a structural gap that has no frame: an attic hatch with a sensor is covered; a weak loft wall without one is not.
The fix for both gaps is a single PIR motion detector positioned to cover the interior ground floor. One sensor, typically £15–£30, covers every possible entry route through its detection zone regardless of how the intruder got in. This is the upgrade that transforms a door-and-window sensor system into something genuinely comprehensive.
Where to Place Your Sensors
Always start with exterior doors. A ground-floor front and back door sensor covers the two most commonly used entry points in residential break-ins. Ground-floor windows come next — every window accessible from the ground without a ladder is a realistic entry route. Upper-floor windows, while technically accessible with a ladder, are a lower priority unless your property has specific features (a flat-roof extension, a garage roof) that make them easy to reach. An attic hatch with a sensor is worth adding if yours opens from inside a room; an inaccessible hatch in a ceiling void can typically be skipped.
Garage doors need a tilt sensor rather than a standard reed switch — the up-and-over movement doesn't produce a clean gap between magnet and switch. The door from the garage into the house is often left unsecured on the assumption the garage door itself is enough; in practice, a sensor on the internal access door is one of the most overlooked and highest-value additions you can make.
Smart Sensors vs. Standalone Sensors
Standalone sensors — a simple alarm that beeps on the sensor itself when opened — are cheap and need no hub, but they're limited to making noise at the point of entry. A smart sensor, connected to a hub or directly to your phone, can do everything a standalone does and then some: log every open and close event with a timestamp, send a push notification the moment a window opens while you're away, integrate with a camera to trigger recording on entry, or feed into an automation that flashes every smart light in the house as a deterrent siren.
The practical gap between smart and standalone narrows at the low end: a Zigbee sensor at £12 paired with a free Home Assistant installation on an old Raspberry Pi gives you a full event log, notifications, and automation capability at a total cost comparable to a dedicated standalone unit. The meaningful additional cost is the hub itself — which many households already have if they're using any other smart home devices.
Can They Replace a Full Alarm System?
A sensor-on-every-door-and-window setup, combined with a PIR motion detector and connected to a hub with a loud siren output, covers the overwhelming majority of realistic residential intrusion scenarios. Statistics on residential burglary consistently show that most opportunistic break-ins happen through ground-floor doors and windows — the exact points that door and window sensors cover most directly.
What a sensor-based system doesn't provide out of the box: professional monitoring with automatic police dispatch, cellular backup if your internet goes down, NSI or SSAIB certification for home insurance purposes, a dedicated panel with battery backup, or the deterrence signal of a branded external alarm box. For those requirements, a full alarm system with a monitoring contract is genuinely the right answer.
For everyone else — a homeowner who wants reliable alerts, a complete event log, and a siren that triggers on any unauthorised entry — a well-implemented sensor system is a completely legitimate security layer. The honest framing is that it's not a replacement for professional monitoring; it's a capable alternative to it, at a fraction of the ongoing cost.
| Sensors + PIR + App | Full Alarm System | |
|---|---|---|
| Entry point detection | ✓ Complete | ✓ Complete |
| Motion detection | ✓ With PIR added | ✓ Included |
| Smartphone alerts | ✓ Instant | ✓ Instant |
| Siren / deterrent | ✓ Via hub or smart speaker | ✓ Dedicated 110dB siren |
| Professional monitoring | ✗ Self-monitor only | ✓ 24/7 human response |
| Cellular backup | ✗ Internet dependent | ✓ Dual-path standard |
| Battery backup (panel) | ✗ Needs UPS | ✓ Built-in |
| Insurance certification | ✗ Not SSAIB/NSI rated | ✓ Certifiable |
| 3-year cost | £100–£350 one-time | £700–£2,100 total |
The Best Sensors in 2026
| Sensor | Protocol | Best for | Battery life | Price (each) |
|---|---|---|---|---|
| Aqara Door & Window Sensor P2 | Matter / Thread | Best overall — any ecosystem | ~2 years | ~£18 |
| Eve Door & Window | Matter / Thread | Best for Apple HomeKit | ~2 years | ~£35 |
| Samsung SmartThings Multipurpose | Zigbee | Best for SmartThings — includes tilt sensor | ~1.5 years | ~£22 |
| Ring Alarm Contact Sensor (2nd Gen) | Z-Wave (proprietary) | Best for Ring Alarm system | ~3 years | ~£20 |
| Sonoff SNZB-04P | Zigbee | Best budget — HomeAssistant / Zigbee2MQTT | ~2 years | ~£9 |
| SimpliSafe Entry Sensor | RF proprietary | Best for SimpliSafe system | ~5 years | ~£19 |
Quick Picks by Use Case
The Bottom Line
Door and window sensors are among the most reliable components in any smart home — a sixty-year-old mechanism that requires almost no maintenance, costs under £20 per opening, and runs for years on a coin cell. They cover exactly what they promise: every time a door or window moves, you know about it instantly.
As a standalone system, sensors leave two meaningful gaps: motion inside the perimeter and glass smash attacks. Fill those with a single £20 PIR motion detector and you have coverage of every realistic residential intrusion scenario. What you don't have is professional monitoring, cellular backup, or insurance certification — and if any of those matter for your situation, a full alarm system with a monitoring contract remains the right answer. For everyone else, a well-deployed sensor network is a capable, cost-effective, and genuinely reliable security foundation.
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