. The photodiode will be collecting light from the LED (signal) and other sources like the sun, room lights, laptop screen, etc. (background). We can set the frequency of the LED signal (how rapidly it flashes) higher than these sources and then filter out the background frequencies. The sun provides DC light and light bulbs generally operate at 60 Hz or 120 Hz (50 or 100 Hz in some countries). Therefore, we should use frequencies above these values. Our choices are constrained at the high end as well. What is the maximum frequency we should operate the LED at to get a good signal at the photodiode