Electronic – Best way to detect when I’m home

I'm working on a home automation setup and I'm trying to find the best way for the system to know when I'm home. A motion sensor won't work because my wife or my dogs may be home when I'm not. The two things I almost always carry with me are my keys and my cell phone and I liked this solution which pings the phone to see if it has joined the home wifi network. The only problem is that my phone goes to sleep very quickly when not used. This would make it good at detecting when I get home (if I just wake the phone for a few seconds on entering), but not when I leave. My other thought would be a bluetooth tag on my keys with a reader near the door, but how could it tell arriving from leaving? Anyone have any suggestions?