Patent ID: 7843575

Claim:
An apparatus for measuring each distance from a plurality of sensors to a related movable surface associated with each said sensor, the apparatus having: a key decoder having a plurality of analog inputs and one or more control signals associated with each said analog input; a reader having a plurality of n-way multiplexers, each said n-way multiplexer having n said sensors, each said sensor comprising an optical emitter oriented towards one of said movable surfaces and a photo-sensor oriented to receive reflected optical energy from the same said surface, each said optical emitter of said n-way multiplexer operative to a unique said key decoder control signal such that each said n-way multiplexer has only one said optical emitter and an associated said photo-transistor enabled during a particular interval of time; said key decoder causing a particular said n-way multiplexer optical emitter to be enabled for a first duration T 2 sufficient to measure from an associated said photo-sensor a LghtCmp plus ambient value for a particular movable surface, after which interval particular said n-way multiplexer optical emitter is disabled for a second duration T 3 sufficient to measure an ambient value from same said photo-sensor, thereafter subtracting said ambient value to form said LghtCmp value, said key decoder deriving said distance from said LghtCmp value.