>I've been in a public restroom that has an automatic air freshener.
>Next time i'm there i'll look at how it knows to actuate.  It sprays
>a bit of smell good stuff occasionally, but I don't know what it
>uses for sensory input.

I suspect you will find it uses a normal IR or microwave motion sensor, or
possibly a light beam sensor. Similar devices seem to be used here to
determine when urinals should be flushed.

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