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The Driveway Sentry Mk.2

Here's an improved version of the Driveway Sentry we described in the November 2004 issue of SILICON CHIP. It detects vehicles like cars, trucks, tractors or other farm machinery moving along a driveway or through a gateway. When movement is detected, it switches on a mains-powered or battery-powered lighting system and activates an optional piezo buzzer alarm for a preset period ranging from 2-25 seconds.

By Jim Rowe

Unlike other motion-sensing systems that use light, heat or ultrasonic sound waves to detect motion, the Driveway Sentry Mk.2 operates by sensing small changes in the Earth’s magnetic field – the same magnetic field that’s 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 original Driveway Sentry detected them and that’s how this new Mk.2 version works too. The original unit used a special high-sensitivity shielded remote sensor unit (no longer available) but this new version simply uses a loop of sensing cable buried under the driveway.

Main Features

The Driveway Sentry Mk.2 detects moving vehicles by sensing the small temporary changes in the Earth’s magnetic field caused by this motion. It detects the changes using a rectangular sensor loop which is buried under the driveway, or concealed with two opposite ends of the loop in the expansion gaps in the driveway itself.

Exit Delay: allows the system to be switched to non-sensing “sleep mode” for a period of about five minutes, to allow the owner’s vehicle to exit from the property without activating the Driveway Sentry. At the end of the Exit Delay, the system returns to its movement sensing mode.

Test Button: allows the system to be manually triggered into “movement detected” alarm mode without having to drive a vehicle over the remote sensing loop. This makes system adjustment easier and more convenient.

Piezo Buzzer: produces a high-pitched sound to attract your attention when movement is detected. This sound can be disabled if you prefer the system to respond silently.

Relay Contacts: includes an SPST relay with mains-rated contacts. The relay is activated when the system detects movement, allowing the unit to be connected to control mains lighting or other equipment such as a high-powered siren.

Alarm Duration Control: allows the duration of the system’s “movement detected” alarm mode to be adjusted between a minimum of two seconds and a maximum of about 25 seconds.

Sensitivity Adjustment: allows the sensitivity of the Driveway Sentry to be adjusted over a wide range, so it can be set for reliable vehicle detection without being too sensitive and susceptible to false alarms.

Low Power Consumption: unit operates from 12V DC power (normally a plugpack), with a low current drain: <25mA in Exit Delay mode (<300mW), <15mA in armed mode (<180mW) and <100mA in alarm (movement detected) mode (<1.2W). This means that the system can also be operated from a 12V SLA battery and/or solar power in rural and other remote situations.

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