After the winter break, I brought a projector I got from old high school's recycling pile back to my dorm with me. I made a screen for the projector from some blackout fabric, and motorized the raising and lowering of the screen with a small gear motor and some other parts I found.
The upper and lower limits of the screen are controlled by two limit switches and a wing nut on a threaded rod. One end of the rod is pressed onto the motor shaft, while the other is tightly threaded into the bit of hex stock that couples the screw to the steel rod the screen is wound around. The DPDT toggle switch forms a mechanical H-bridge to reverse the direction of the motor.
The opposite side of the screen is supported by an aluminum bracket with a bearing pressed into it.