Unlike other sensing systems that use light, heat or ultrasonic sound waves to detect motion, the Driveway Sentry operates by sensing small changes in the Earth’s magnetic field – the same magnetic field that is sensed by a compass.

Since cars, trucks and similar vehicles contain a significant amount of ferrous metal (iron, steel etc), they inevitably produce small temporary changes in the Earth’s magnetic field when they move into or through an area.

That’s how the Driveway Sentry detects them, using a special high-sensitivity shielded remote sensor unit.

Because it doesn’t generate any sensing fields of its own, the Driveway Sensory produces no environmental pollution of any kind; it’s quite ‘clean’.

He's the Sentry you don't have to pay, feed or even be nice to... but he's ready and rearing to go 24 hours a day! This photo shows the control box and behind that, the sensor which is buried in the ground alongside your driveway.

Also, because it only senses moving iron and steel objects like vehicles, it is much more selective than other kinds of sensor.

This makes it immune to false alarms from birds, dogs, cats, sheep, cattle and other animals, falling tree branches, rain and snow, people walking past and so on.

At the same time it can be used to detect the movement of vehicles which contain very little ferrous metal – like aluminium trailers, boats and caravans - simply by attaching a strong magnet to the underside of their frame. The magnet ensures that if they’re moved past the Driveway Sentry’s remote sensor unit, the Earth’s magnetic field will be disturbed locally and the system will activate.