On 15/10/13 12:59, Jeff Liebermann wrote:
On Mon, 14 Oct 2013 10:10:57 -0700 (PDT), Frank Krygowski
wrote:

So: Anyone know the easiest way to determine the exact location
of such a buried coil?

The compass, stud finder, and divining rod will not work. What you
need is a coil resonant to the operating frequency, some kind of
detector, and an indicator. The problem is that the frequencies used
vary from 10 Khz to as high as 200 Khz. That means either a broadband
detector, or a tunable coil.
http://en.wikipedia.org/wiki/Induction_loop

If you have a smartphone of some flavor, you can probably use one of
the oscilloscope apps to act as the amplifier, detector, and display.
Something like this:
https://play.google.com/store/apps/details?id=com.nfx.noscpro&hl=en
There are also various "ghost finder" and "EMF detector" apps that are
basically LF (low frequency) signal detectors. For example:
https://play.google.com/store/apps/details?id=com.superphunlabs.emf&hl=en
https://play.google.com/store/apps/details?id=com.codebros.emffree&hl=en
They use the phones magnetic sensor as a LF pickup coil. I have no
idea of the operating frequency range or sensitivity, but methinks
it's worth trying.

Otherwise, I would build a resonant pickup coil, setup a tuning system
(switched caps and tuning capacitor), mount it on a wooden stick, and
plug the coil into the microphone input. Then use an oscilloscope
application to view the signal. If the signal is low, add a battery
powered audio amp. If you want, I can throw something together (time
permitting) and see what it produces.



You will have fun listening to 200kHz signals with a microphone input
circuit and A/D. Often the maximum sample rate is 192kHz for high end
audio A/D converters. I don't know what phone circuits are capable of,
but I'd be surprised if it was that high.

--
JS