How does a burglar alarm actually work?
Through a control panel receiving signals from sensors around the property and deciding, according to how it is programmed, whether to sound and whether to notify anyone — and what happens after it sounds is the part that determines whether it is useful.
The sensors:
Passive infrared detectors, which see changes in infrared heat across a pattern of zones — so they detect movement across their field better than movement towards them. Pet-tolerant versions ignore heat sources below a certain height and mass.
Magnetic contacts on doors and windows, detecting opening.
Vibration and glass-break sensors.
Dual-technology detectors combining infrared with microwave, which must both trigger — substantially reducing false alarms.
The grades of response, which is the real distinction:
Bells-only. Sounds a siren locally. Nobody is obliged to do anything, and the deterrent value rests entirely on noise and on the visible box.
Monitored, where activation notifies an alarm receiving centre, which verifies and then contacts keyholders and, where criteria are met, the police. Police response is conditional in many jurisdictions on the system being professionally installed to a standard, registered with a unique reference, and not having generated excessive false alarms — which can result in response being withdrawn.
Smart or self-monitored, which notifies your phone and leaves you to decide. Cheap, flexible and only as good as your availability.
Why false alarms matter so much. They are overwhelmingly caused by user error, pets, insects, moving objects like balloons and curtains near heat sources, and poor installation. Repeated false alarms lose police response and train neighbours to ignore the siren — which converts the system into noise pollution.
What actually deters, according to research on burglary: visible occupancy, good lighting, secure doors and windows to a recognised standard, and gravel or thorny planting. Alarms feature in offender interviews as a deterrent, and an unverified bells-only alarm is the weakest form.
Cameras are for evidence and verification, not prevention, and carry data protection duties if they capture beyond your boundary.