The RCWL-0516 microwave doppler radio board is a very cheap sensor for home automation. It responds to even slight motion anywhere within a considerable range (~10m) and it works through walls and other obstacles. It's not really "doppler" but it does detect changes in the interference between a weak transmitted microwave signal and the returned signal.
The downside to these properties is that it's no longer telling you about occupancy in a single room and it's susceptible to random triggers for no apparent reason. I tried the various hacks to reduce the sensitivity but it still triggers occasionally when it should not.
Given these properties I was forced to come up with a new approach in my home automation system that can handle unreliable sensors like this and sensors that span multiple rooms. My probabilistic model solved both these problems. By assigning a low probability to the radar sensor type and by associating each radar sensor with a percentage coverage in multiple rooms I can incorporate the useful data it provides whilst ignoring the spurious triggers.
In effect the radar sensor can confirm that someone is still in a room and it can detect someone has moved to a room (after multiple triggers each one increasing the likelihood of a move), but it cannot on a single trigger determine that someone has entered a room, nor even which room is being triggered.
The sensors are cheap and worth including in your home automation plans. Some commercial sensors combine technology like this with PIR to provide greater false rejection for both sensors which seems like a smart approach.
I've been working on home automation for over 15 years and I'm close to achieving my goal which is a house that understands where everyone is at all times, can predict where you are going next and can control lighting, heating and other systems without you having to do or say anything. That's a true "smart home".
An overview of the many sensors I've experimented with for home automation including my favorite under-floor strain gauge, through all the usual PIR, beam and contact sensors to some more esoteric devices like an 8x8 thermal camera.
Bluetooth sensing for home automation is a great proxy for people counting as it can detect and locate each cellphone in the house. iBeacons attached to tools, cars and pets can provide a 'find my anything' feature too.
Having at least one light sensor is critical for any home automation system that controls lightng. Lights need to be turned on when it's dark not at specific times of day, especially here in Seattle when it can be dark and cloudy at any time of day.
An if-this-then-that style rules machine is insufficient for lighting control. This state machine accomplishes 90% of the correct behavior for a light that is controlled automatically and manually in a home automation system.