Ask HN: Is it really possible to use a WiFi signal as a motion sensor?

"Guardingo transforms the WiFi that you already have at home in a security system by analysing the radio waves. Radio waves are like waves in water, but they are invisible instead. Imagine a pond. The water in a pond is usually still. When you drop a stone in a pond, you can see ripples all around. If you can see the ripples, you can also prove that something happened to the pond. When a person/stone enters in the house/pond, there are ripples in the radio/water waves. Guardingo can see these ripples." Source: http://guardingo.net/about.html