Earth field detector using special core

[agdtensor]

Don Lancaster’sTech MusingsSeptember, 1996
More magnetometer info article-
You thus end up with a plain old coil whose inductance varies with the applied field strength. Put this in any suitable oscillator circuitry, and your output frequency should follow the strength and direction of the earth’s applied field. With proper design, as much as a 2:1 frequency change can be caused when you rotate the sensor through the compass points.
Whose permeability varies with field strength. By biasing as shown, a coil can be built so its inductance strongly varies with compass orientation.What is really unique here is that a
single ultra cheap solenoid winding over an ordinary core bar or rod acts both as a field sensor and the control bias setter. The sensing gets done by measuring the inductance. And the
biasing by inputting a dc current.

Has anyone built one?

[Ivars Kums]

Re: Earth field detector using special core

I kinda suspect that this method is used in Palm Navigator electronic compass made by Precision Navigation. Because it's outdated and works only with older Palm models, it can be found on eBay sometimes, usually for less than 15$.

[Agdtensor]

Re: Earth field detector using special core-w/7414 IC

I wanted to build one using the 7414 Ic-wondered if anyone had built one? I thought I would try a pair as a simple sensor like flux gate pairs. I want to enclose them in a waterproof PVC pipe with fins and tow it behind a sea kayak at special shallow New England shore line.-AGDTENSOR

[Phil Barnes]

Re: Earth field detector using special core

I have not been successful in obtaining the specified core. But I did some research on Metglas and found that it is used in anti-theft tags such as Walmart uses and are often enbedded in the product box where it cannot be removed without opening the box. I asked the salesman in the video department for a few and he gave me a couple after demagnetizing them so I could get them out of the store.

The Metglas core material must be removed from the sticky white plastic tag which is sometimes stuck on merchandise. There is a flat tapered core which is often magnetized to bias the Metglas strips. Remove all core material and discard the tapered magnetic strip.
The remaing strips are Metglas shiney core material and may be simply slid into a tight fitting solenoid. I have used the coils from radioshack reed relays. The reeds slip out easily and the metglas can be gently rolled and slid into the coil nylon bobbin.These are very sensitive sensors and oscillate at about 100KHz. Power consumption is extremely low and they will operate using a pair is 1.5 volt cells.

[Agdtensor]

Re: Earth field detector using special core

Phil-Do you have a circuit that I could build?I am trying to use a 7414 Ic with an internal relaxation oscillator or a 555 IC to create the oscillator. Thanks for the tip on the reed relay ---I have a few from my robotic project bench -AGD

[Phil Barnes]

Re: Earth field detector using special core

CMOS oscillator is on page 3.
http://www.tinaja.com/glib/muse104.pdf

[Agdtensor]

Earth field detector using special core-Circuit design

Phil- Have you built one of these circuits using the MetGlas strip from a security sensor and using the 7414 IC?-AGD

[Agdtensor]

Re: Phil-Earth field detector questions

Hi Phil-Your Metglas coil detector-(1) Did you connect this to a audio or visual meter display? (2) Do you have additional components added-such as an additional op-amp?(3) Did you combine two to make a gradiometer?-AGD

[Phil Barnes]

Re: Phil-Earth field detector questions

(1) Did you connect this to a audio or visual meter display?
Audio

(2) Do you have additional components added-such as an additional op-amp?
I used CMOS circuits as described by Don Lancasters article.

(3) Did you combine two to make a gradiometer
Yes. The magnetometer oscillators (2) operate at near 100KHz and two are operated in differential mode to produce an aduible tone (Beat frequency between the oscillators). This "tone" is subsonic when operated at maximum sensitivity. The sub-sonic output is used to gate a 1000 Hz audio oscillator which allows you to hear 1 or 2 Hz. as a beep, beep, beep . . .

[Phil Barnes]

Re: Earth field detector using special core-Circuit design

Please see answer to questions just before this entry.

It has been several years since I "played" with this and I would have to lookup my old lab notes.

[Agdtensor]

Re: Phil-Earth field detector questions-update

Phil- thanks for the info. With that low battery drain you should be able to search all day. Did you find any interesting below ground targets?-AGD

[Kianoosh Motallebi]

Re: Earth field detector using special core

anyone ever used a crt-screeb for field detection?