Every human body is surrounded by an electric field – a stronger field than many people would expect. With some simple test equipment, I was able to measure this field up to a metre away.

The phenomenon of capacitance is entirely dependent on the existence of electric fields. If a human body should approach one plate of a capacitor, the body’s electric field can inter-react with that of the capacitor and can cause the capacitance to increase. Again, this may easily be detected.

Further, any number of metal objects may be attached to one plate of a capacitor, for example a sheet of aluminium foil or even a set of window security bars. These then become an extension of that plate.

In our Body Detector circuit, we call these the sensor. In this design it is attached to the positive plate of a capacitor.

While in theory the Body Detector is dependent on the electric field which surrounds the human body, in effect it’s as if an invisible field surrounds the sensor – somewhat like the "invisible" defence shields seen in the latest Star Wars movie.

The principle employed here is different to alarms which detect EMF-induced eddy currents in the body. Because the Body Detector is based on the principle of body capacitance it has a high degree of immunity to AC fields, as well as being able to function well out of range of such fields.