A system for detecting position of a light beam propagating along a beam path utilizes a single variable gain preamplifier in a time shared mode for each of the outputs of a four quadrant position sensors. A detector mounted in the beam path and having a reference point, generates a plurality of signals indicating deviation from the reference in plural directions. A multiplexer is connected to the detector, and combines the plural signals into a time multiplexed output. A variable gain preamplifier normalizes the signals in the time multiplexed output to generate the normalized output. The normalized output is then supplied to a logic circuit for processing the normalized output to generate a position signal indicating the position of the light beam.