The ornamental design for a portion of a light and motion detector assembly, as shown and described.
FIG. 1 is a perspective view of a portion of a light and motion detector assembly showing our design shown, with the broken line showing for illustrative purposes only and forming no part of the claimed design;
FIG. 2 is a front elevation view thereof shown in FIG. 1;
FIG. 3 is a back elevation view thereof shown in FIG. 1;
FIG. 4 is a left side elevation view thereof shown in FIG. 1; and,
FIG. 5 is a right side elevation view thereof shown in FIG. 1.