1 b alternately separate well known emitter and receiver pairs can be used in either case the optical sensors 3 a 3 b and 3 c are arranged in such a way that they are responsive to a light beam interruption the general geometric arrangement of the cart detector sensor cluster 3 and retroreflective targets 6 with respect to the checkout lane is shown in fig1 a more detailed view showing the positions of the three sensors 3 a 3 b and 3 c relative to the check out aisle floor 1 c and the relevant structural elements of the shopping cart 2 is shown in fig3 the sequence of sensor responses generated by a passing shopping cart 2 may be understood by reference to fig4 a through fig4 e note that the relevant structural elements of the shopping cart 2 involved in interrupting the three light beams corresponding to each of the three sensors 3 a 3 b and 3 c are the cart frame side rail 2 a and the cart front wheel 2 b a state transition diagram indicating the logical processing of the signal received from the three sensors 3 a 3 b and 3 c is shown in fig5 this diagram represents the function of the cart detector finite state machine fsm which in the preferred embodiment is implemented in a field programmable gate array fpga finite state machines are logical processing constructs well known to those skilled in the art of fpga applications design note that in a state transition diagram such as fig5 every bubble represents a u201c state u201d of the logic system it is only possible.