Sensor apparatus for detecting the presence of a passenger on a seat, comprising: first and second rigid planar force translation plates (22, 24) disposed in said seat; first and second conductor plates (28, 30) separated by a dielectric substance and centrally affixed to inboard faces of said first and second force translation plates (22, 24), respectively; and a control circuit responsive to a gap capacitance between said first and second conductor plates (28, 30) for detecting the presence of said passenger when a weight of said passenger overcomes the bias force of said springs; characterized in that it further comprises: joining means (34, 40) for joining said force translation plates (22, 24) in a manner that maintains them substantially parallel to each other while permitting movement of either force translation plate relative to the other in a direction perpendicular to said force translation plates (22, 24); a set of springs disposed between said first and second force translation plates (22, 24) for exerting a bias force that biases said first and second force translation plates (22, 24) apart in said perpendicular direction; wherein said first conductor plate is formed on first circuit board affixed to the inboard face of said first force translation plate; and said second conductor plate and said control circuit are mounted on opposite sides of a second circuit board affixed to the inboard face of said second force translation plate. first and second rigid planar force translation plates (22, 24) disposed in said seat; first and second conductor plates (28, 30) separated by a dielectric substance and centrally affixed to inboard faces of said first and second force translation plates (22, 24), respectively; and a control circuit responsive to a gap capacitance between said first and second conductor plates (28, 30) for detecting the presence of said passenger when a weight of said passenger overcomes the bias force of said springs; characterized in that it further comprises: joining means (34, 40) for joining said force translation plates (22, 24) in a manner that maintains them substantially parallel to each other while permitting movement of either force translation plate relative to the other in a direction perpendicular to said force translation plates (22, 24); a set of springs disposed between said first and second force translation plates (22, 24) for exerting a bias force that biases said first and second force translation plates (22, 24) apart in said perpendicular direction; joining means (34, 40) for joining said force translation plates (22, 24) in a manner that maintains them substantially parallel to each other while permitting movement of either force translation plate relative to the other in a direction perpendicular to said force translation plates (22, 24); a set of springs disposed between said first and second force translation plates (22, 24) for exerting a bias force that biases said first and second force translation plates (22, 24) apart in said perpendicular direction; said first conductor plate is formed on first circuit board affixed to the inboard face of said first force translation plate; and said second conductor plate and said control circuit are mounted on opposite sides of a second circuit board affixed to the inboard face of said second force translation plate.