The ornamental design for a sensor for a signallamp, as shown and described.
FIG. 1 is a front, top and left side perspective view of a sensor for a signal lamp showing our new design;
FIG. 2 is a rear, bottom and right side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a right side view thereof;
FIG. 8 is a left side view thereof; and,
FIG. 9 is another front view of the design, shown in a position of use and attached to a signal lamp.
The features shown in broken lines in the drawings depict environmental subject matter only and form no part of the claimed design. The oblique line shading shown in FIGS. 2 and 7 indicates translucent surfaces.