Mobile robot contact sensing can be useful for navigation and manipulation with small robots. In this work, we present a low-cost, 40 gram force-torque sensor for a 10 cm hexapedal millirobot. A planar array of photointerrupters, rigidly attached to the structure of the robot, is used to measure the six-axis movement of a shell attached to the robot with springs. The sensors measure the intensity of infrared light reflected off of a surface on the underside of the shell, which has been specially designed to enable the resolution of the forces and moments in the x-, y-, and z-axes applied to the shell. The sensor has a force sensitivity of 17 mN and torque sensitivity of 0.72 mN-m for a sampling rate of 100 Hz. The sensor can resolve a force equivalent to 2.4% of the combined robot and sensor weight of 700 mN.