Abstract: A counter-balanced pressure sensor for sensing a package carried by a belt-driven carrying roller conveyor includes a pivot support with a sensing roller attached. The sensing roller is deflected downward by the passage of the package on the conveyor, causing the pivot support to pivot downward, causing a feeler arm attached to the pivot support to contact a sensitive whisker valve mounted beneath the conveyor top surface. The conveyor includes a plurality of zones, all but one of which has one of the pressure sensors. When the pressure sensors in one or more succeeding zones detect the presence of a package in each of their associated zones, the belt drive in a preceding zone is disconnected from the rollers in that zone, putting the preceding zone into an accumulation mode. Each pressure sensor is sensitivity adjustable by adjusting a threaded connector for a counter balance positioned opposite the sensing roller on the pivot support.