I picked up Mighty Mule driveway alarms from Lowes. They were on sale for $55 in a special display in the lighting section.
It's magnetic and is designed to detect vehicles. It's wireless. You trench the sensor which is connected to to the transmitter which sits on top a PVC pipe. Detection range is 3-12 feet adjustable.
The receiver can be up to 400 feet from the trany according to the mfg. Mine is about 50 feet.
Out of the box, it was useless as it just sounds a beeper and turns on the 'visitor' LED.
I modded it.
The beeper is DC powered so I measured the activated voltage (12 VDC) and found it was in the input range of a X-10 Powerflash. I soldered 2 wires across the beeper, cut a slot in the enclosure to pass the wire and connected the Powerflash. The beeper volume control changes the voltage; I keep mine on full. The HS2 event associated with it switches a camera, flashes outside lights and causes a property-wide visitor announcement.
There are dip switches in the trany and rcvr to change the code so you and your neighbors don't trigger each other or in my case, to define which drive a vehicle is moving in.