In order to tell his home automation system that he’s home, [Jim] mounted a RF transmitter in each of his cars. When the car is on, the transmitter is powered up. The house picks up the transmitter signal when the car arrives or departs. With that information, he was able to set up some stateful rules that can be activated when people arrive or depart. Some people prefer to use APRS and read vehicle location from the transmitted GPS coordinates, but this is a bit cheaper and doesn’t transmit your position to the entire world all the time. The useful range is about 100 feet, so this can work even if you have to park in the street.