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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?

exterior wall plate

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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.

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It's an old photovoltaic sensor, possibly for exterior light control.

  • +1- Just to add that a photovoltaic sensor is commonly used as a switch on a lighting circuit; turning lights on when no light is present (nighttime) and turning them off in the presence of light (daytime). – Jimmy Fix-it Apr 16 '17 at 13:45
  • Technicality: that's a photoconductive sensor, not photovoltaic. Probably the old cadmium-sulphide type. The color & squiggly line are giveaways. – brhans Apr 18 '17 at 4:58

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