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From: paul@sdd.hp.com (Paul K Johnson)
Newsgroups: sci.electronics
Subject: Re: Electronic Stud Finders
Message-ID: <1990Oct31.223138.26355@sdd.hp.com>
Date: 31 Oct 90 22:31:38 GMT
References: <4110@radio.oakhill.UUCP> <533@chiton.ucsd.edu>
Organization: Hewlett-Packard, San Diego Division
Lines: 21

In article <533@chiton.ucsd.edu> cdl@chiton (Carl Lowenstein) writes:
>In article <4110@radio.oakhill.UUCP> charlie@oakhill.UUCP (Charlie Thompson) writes:
>>How do those electronic stud finders work?  They seem
>>to be able to distinguish fingers from thin air as well.
>>They are apparently electrostatic or capactive from what
>>I can tell. 
>
>They are acoustic.  Sonar, so to speak.
>
Egads, no smileys!?!??!?!??!???!!!??

They measure the dielectric constant of the volume immediately below the
sensor.  This is easily done by measuring the capacitance between two
metal plates.  The dielectric constant of air is far different from
wood, and the fact that there is some drywall in the way just offsets
the two measurements and otherwise doesn't interfere.

paul johnson

Internet: paul@sdd.hp.com
UUCP    : {hplabs|hpfcla|ucsd}!hp-sdd!paul