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
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
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.