Extra Sensor for Original Guardline Driveway Alarm Extra sensor compatible with the original Driveway Alarm by Guardline. This sensor will NOT work with the Guardline 1/4 range driveway alarm . The 1/4 mile extra sensor must be purchased to work with the 1/4 mile range driveway alarm. A total of 16 sensors can be added to the driveway alarm set, offering complete coverage of your property.