I use a device called the "cartel CP-2". It is a buried sensor that sees moving metal. It has a pair for power, which can be roughly 8 to 31 volts AC or DC. There is another pair that gives contact closure for about 2 seconds when moving metal is within 10' of the sensor.
I like these because you can't tell they are there, don't trigger on wildlife (or people)- just on moving metal. You can mow, plow, etc. and not worry about hitting the sensor. While the sensor is a bit pricy, the other system costs (trenches, conduit/pipe, connection posts, etc) bring the total installed cost into the $300 range. I also like wired connections- no batteries, no interference. Only down side is the protection for the wiring, both physical and electrical.
Here are some links:
HomeTech Solutions: Cartel CP-2 Probe System 100ft
and
http://www.worthingtondistribution.com/solutions/Manuals/Cartell/CP-2%20Install.pdf
This is a low level sensor, and there are other models that have electronics that do more stuff. But if you have an alarm panel, this is all you need. The contacts will go into a zone, and then the alarm system can beep or announce or do something. These systems can also turn on outside lights on contact closure. One person I helped with one of these just took the contacts and put them to a dedicated doorbell so they got a sound when a car (or tractor) was in the driveway.
If this sort of thing looks like what you want, here are some important safety and usage tips:
I put a 3" 10' PVC pipe about 12" deep with one capped end right up against the driveway, and the other about 4' out from the edge. Take carful measurements to the end of the pipe. This lets you put the sensor close to your driveway, but if it fails you can get to it without disturbing your driveway.
Get the 100' version. Run a 3/4" black PE to a post and connection point, preferably within 50' or less of the sensor. You can then come up into the post/box and connect the sensor to the cable(s) going to your house. You will also need to put some transient zener protection diodes at this point, one on the power supply and one on the relay wires. I've been involved with about 7 or these, and it seems that if you have more than 100' of wire, the units get popped by lightning in a few years. Note that with the black PE pipe and the 3" PVC you can replace a sensor fairly easily. The wire on the blown sensor becomes the pull wire for the new one, and the sensor gets pushed into the 3" pipe. The pipes also provides some better protection agains near by lightning strikes. I don't put any kind of wire in the ground without having it in a conduit or pipe.
When you come into your house with the wires, you'll want to put a three terminal gas tube protector on there, just like _any_ other low voltage wire coming into the house (telco). Try to come in where everything else comes in- single area of entry and a common protective ground are good stuff. I would highly advise a secondary fuse connection before you connect to your equipment, if you go this route I can discuss this more.
I have two of these sensors since my (homebrew) security system can deal with them and tell me if someone is arriving or departing. You can start with one, but if the idea of arriving/departing sounds interesting, while you're tearing up the ground put in the trenches and pipes for a 2nd sensor at least 25' away from the 1st. Take careful measurements and cove the pipe ends so you can dig it all up later to add the 2nd sensor. Use the same post/box for the connections back to the house. I have a 900' driveway, and have the sensors at 50' off the road and 60' from the house. This gives me time to turn on the monitor for the security cameras when someone comes down the driveway (or go look out a window).
The linked to pdf has installation notes. In general, stay 15' away from power lines and 50' away from roads to avoid interference and false triggers.
Standard disclaimers, YMMV, this works for me, etc.
Pete
