A friend of mine's house has a plate like this on one of the exterior walls. What is it and what purpose does it serve?
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That is a small photocell. They work just like a switch by turning on/off via dusk to dawn. They require moderate visibility, range in maximum wattage, and generally are only for tungsten lighting loads. Though newer photocell switches are CFL/LED compatible.
Also, they usually direct wire to high voltage and need a permanent power source unless the owner is okay using a manual override switch.
Typically used for outdoor light automation. Best to use LED bulbs though as dusk to dawn on-time can burn through a incandescents life cycle.