When I moved into my home it had only a remote bell and howell wireless doorbell. It didn't work worth beans. I decided that I'd install two HF Driveway Alert system, one for the front door and one for the back. I built two wooden boxes to house the motion sensors and attached them to the soffit over the front and back door. The hole in the box that the motion sensor was installed was cut to narrow sensed area to above the door and the walkway. False triggers are kept to a minimum.
The receivers were put in different parts of the house so that you could tell if the front or back alerts were activated.
So far, for the last 3 1/2 years they've worked great. Total cost was $18 (bought them on sale),some scrap wood, left over paint, screws and a couple of hours labor. I'd do it again faced with the same issue. I like it, the sensor usually catch people as the come up the walk and I can meet them at the door before they knock. The look on their faces when I'm at the door before the are was well worth the $18. I like it also because it will tell me if anyone's prowling around. Cats and dogs walk by and it beeps once.. but people are a different matter.