Understanding and Building Metal Detector CIRCUITS - Opamps, Oscillators, Transmitters, Receivers ..

[jlsilicon]

I want to open this Thread up - for the discussion to understand and refine Circuits (and Parts and Tweaking) used to build Metal Detectors.

Reason being that I have been working on a few Detectors using simple circuits, such as:
* NAND and and Opamp and Transistor Oscillators,
* Opamp and 2n3819 FET Receivers,
* Analysis using Arduino,
.. etc,
- with some success.
But, I reach a loose end on improving / refining / expanding these Projects.

I also attempted a few kits
- but also have a hard time Tweaking (improving) the circuits and results.

So, in this Thread, I
would like to discuss some simple commonly used circuits, their common problem, replacement parts, and fixes.

Consider this a FAQ Thread.
And, hope to get a lot of advice and solutions for me and others.

[kt315]

seems new try to write Inside The Metal Detector =) Part 2, Appendix.

[jlsilicon]

One task, is using the Opamp.
I like to use the Lf347 that seems to have generally easy and best results.
I find generally, would set the Feedback to the OpAmp -Neg Input with a 100K resistor.

Assume connecting the Lf347 to a receiving Coil - and other end of coil to Gnd.

Coils, I have seen may vary: from 10 winds to 200 winds,
with a diameter of 15cm (7in) - (maybe up to 30cm (12in)),
and may vary from 1mH-10mH.

If we first assume an input Freq from 1khz - 100khz,
then how would we setup the Opamp Filter and Bias ?

I am assuming that this is simply connected to an Oscilliscope or an Aud Amp and Speaker.

[Silver Dollar]

Well the problem is that in general a metal detector is a tuned system from the coil
all the way to the speaker. Slight changes can affect the balance of the circuit.

I had subbed out some 1 meg gain resistors in my Bandito and put the 1 meg's back
in but I lost 2" of air test depth! I found the original 1 Meg's and re installed them and
got the 2" back. Now all of these are 1% 25ppm resistors so they should have been pretty
close in value.

I have also subbed better opamps into a circuit and had it work worst. So there is definatly
some magic going on in there.

The best way to improve a Metal detector is to redesign sections. A balanced transmitter,
center tapped RX coil and differential input can improve a metal detector. See my attempt
here; http://www.geotech1.com/forums/showt...-Quad-Sampling

[Gwil]

"If we first assume an input Freq from 1khz - 100khz, then how would we setup the Opamp Filter and Bias ?"

Don't know whether you have a textbook that covers this sort of thing, but I'd recommend the classic "The Art of Electronics" by Horowitz and Hill. Some of the digital stuff is now out of date but the chapters on analogue can't be beaten. Not cheap but you may find it can be downloaded (free!) as a pdf file if you do some searching.

[Koala]

"I also attempted a few kits
- but also have a hard time Tweaking (improving) the circuits and results."

Do you mean kits like the IDX and TGS if so I don't think there much to be gained. Decades of development and refinement have gone into them.

More gain will be found through the coil size then through the electronics.

Tesoro in particular used the same basic design for years with minor changes to the filters and gain.

moving the pre amp into the coil is an obvious improvement. As the noise and losses from the coil cable are eliminated.