If you remember the bell that goes off when you used to drive into a full-service gas station, or I suppose if you live in New Jersey or Oregon, well, I've got one. With far more technology.
I'm pretty sure the bell thing works because driving over the rubber tube compresses it, causing the air pressure in the tube to rise and ring the bell. I didn't do that.
I started with a Dakota Alert DCR-2500 driveway alarm. Here's the magnetic sensor before burying a foot into the ground next to my driveway, half way up my driveway and about 300 feet away from my house.
And the wireless transmitter unit. It's battery powered.
It transmits to this receiver unit in my house. Most normal people would stop here, because the unit can make a beep sound or doorbell chimes when a vehicle passes by. I did not stop there.
I connected it to my environmental monitoring unit. It's a Phidgets 8/8/8 analog input, digital input and digital output unit. It's connected by long-range USB-over-Cat.5 to a server in my house.
The Phidget server is written in Java, running in Apache Tomcat. It communicates over a HTTP-JSON interface to the another webapp that monitors all of the systems in my house. It generates alerts and daily reports of the comings and goings at my house.
I purchased (quite inexpensively, I might add) the gas station bell sound.
The color scheme is white on black because the app is displayed on the monitor that is mostly dedicated to TweetDeck, so I wanted to match the style.