Nightime clock dimmer

Some clocks are smart enough to dim when you turn out the lights. This is a $10 item that dims the other digital clocks at night. 1) polarized film layer 2) liquid crystal layer preoriented at 90 degrees to the film so it is normally in light-blocking mode 3) cheapo solar cells designed in series to generate a high enough voltage to disorient the liquid crystal when a light is on.

The cost would all be in the solar cell - similar to the kind used in sidewalk lights.