Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!comp.vuw.ac.nz!canterbury.ac.nz!phys169
From: phys169@canterbury.ac.nz
Newsgroups: sci.electronics
Subject: Re: Electronic Stud Finders
Message-ID: <1990Oct31.144258.9595@canterbury.ac.nz>
Date: 31 Oct 90 01:42:56 GMT
References: <4110@radio.oakhill.UUCP>
Organization: University of Canterbury
Lines: 12

In article <4110@radio.oakhill.UUCP>, charlie@oakhill.UUCP (Charlie Thompson) writes:
> How do those electronic stud finders work?  

They're small metal detectors. There are many ways of making a metal detector,
but the best for this job is probably an air-core coil as part of a tuned
circuit, where there are 2 tuned circuits, one sensitive to nearby metal (and
fingers) in the base circuit, one screened off in the collector circuit. When
both are tuned to about the same frequency (because the inductance of one
changes when the nail is nearby), the transistor oscillates.  This is too
over-simplified perhaps, but you probably get the idea. Sensitive ones could
use crystals.
Mark Aitchison, Physics, University of Canterbury, New Zealand.