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At 04:27 PM 4/1/00 -0300, Mauricio Jancic wrote:
>Thank you for your answers, but, dont you have anything using infrared or
ultrasound?
>        Mauricio Jancic

Infrasonic works too.
Cheap. Condenser mic, filter, amplifier.
Fiddly to adjust, but reliably senses doors opening and the like.
Not "reliable", also senses wind rattles etc.
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