MLA Citation: Bloomfield, Louis A. "Question 954"
How Everything Works 19 Jan 2018. 19 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=954>.
954. How does a foghorn turn on and off? — M, Brant Rock, MA
Although I am not certain, I would guess that most automatic foghorns detect the fog optically. They either send light from a source to a detector and turn on the foghorn when the detector fails to see the light or they send light into their surroundings and turn the foghorn on when they see excessive reflection of that light.

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