1. A motion detector having a transducer which produces an electrical signal representative of the condition of a supervised area, said electrical signal comprising a background noise component and a portion indicative of motion desired to be detected when a source of motion is present in the supervised area, said detector including a processing arrangement for processing said electrical signal and determining whether or not the electrical signal is representative of background noise which is approximated by a known probability distribution function of a random signal source using at least three magnitude classification categories, said processing arrangement analysing a host of samples of said electrical signal using said magnitude classification categories to form a magnitude distribution of said host of samples over a known time segment and producing a signal indicative of detection of motion when said magnitude distribution of said host of samples is not statistically consistent with said known probability distribution function.