Abstract: The capacitance of a shielded capacitive load cell is determined so as to minimize the effect of stray or parasitic capacitance between the load cell and other objects including the shield. The load cell conductors are coupled across input and output terminals of an operational amplifier that is tied to a reference voltage. A constant current is applied to the load cell, and the resulting rate of change in voltage at the amplifier output is measured as a representation of the load cell capacitance. In a vehicle seat sensor application including an electromagnetic interference shield between the load cell and the seating surface, the amplifier output is coupled to the load cell electrode furthest from the shield, the amplifier maintains the other load cell electrode at a virtual reference voltage, and the shield is tied to the reference voltage.

Abstract: A printer system is associated with a taximeter for calculating the taxi fare, a seat switch for detecting a passenger who occupies the taxi, and a timepiece. A printer driver functions to drive the printer to record a drive condition of a taxi and time information each time when the drive condition is changed, for example, from "for hire" wherein the taxi is not occupied by the passenger and no fare is computed to "occupied" wherein the taxi is occupied by the passenger and the taxi fare is computed through the use of a predetermined tariff.