Passenger detection system

A passenger detection system including a single antenna electrode mounted on a surface of a passenger seat in an automobile. An oscillator generates an AC current having a predetermined voltage amplitude that is transmitted to the antenna electrode to produce a minute electric field around the antenna electrode. An amplitude control circuit is used to control the voltage amplitude of the output signal from the oscillator. The electrical characteristics of an object on the passenger seat alter the current and phase of the signal in the antenna electrode. A current detection circuit detects the amount of current flowing from the oscillator to the antenna electrode. In addition, a phase differential detection circuit detects a phase differential between the output signal from the oscillator and the signal on the antenna electrode. A control circuit detects the presence or absence of a passenger seated in the seat based on a current signal output from the current detection circuit and a phase differential signal output from the phase differential detection circuit. A control unit including the oscillator, the amplitude control circuit, the current detection circuit, the phase differential detection circuit and the control circuit, is contained in a single housing that is mounted under the passenger seat.