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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.
||Nice! + Could also be applied to computer screens, which need not be so bright at night.