I HIGHLY recommend Mighty Mule. Bought one for my mom's place - she is somewhat isolated on 6 acres with a long gravel drive. Then bought one for us. It works by detecting large magnetic field disturbances - just place the coil assembly parallel to the driveway near the edge (a few inches buried). My wheelbarrow will actually set this thing off. Can be purchased at big box stores or on-line.
I've replaced the transmitter batteries once in three years.
Wireless Driveway Alarm by Mighty Mule - FM231
We had the motion sensor type detector first. Got TONS of false alarms - deer, wind, tree branches, sunlight, ghosts? - you name it.