I'm looking to make a simple metal detector with an dual coil differential operational mode through a comparator where a material placed in between the opposing magnetic fields triggers hysterisis response in relation to the inductance of the object.
Any ideas on how to optimize this design?

(The inspiration piece)

 

i see picture of two things (coils? speakers?) but no design. does current design work and how well? what kind of improvement would you like?

 

I just had the idea, there is no prototype, the picture is of an audio stereo speaker array.

 

the picture is of an audio stereo speaker array.

What is your intended use for those?

 

What is your intended use for those?

+OO- dual coil differential operation

 

The voice coils of speakers are (a) too small for general field use and (b) embedded in magnetic screening material. Way different from the usual metal detector coils. I doubt they'd be efficient.

 

You want a pulse induced metal detector circuit? PI metal detector is basically pulses a
transmit coil send, at the end of each pulse a search coil senses a corresponding residual signal from metal object.

 

I was thinking a constant signal potential across the coils where the difference in current flow i.e. hysterisis charactistics between the two opposing magnetic directions will result in identification of materials in the field domain.

 

If anyone could give insight into the circuitry to drive this it would be greatly appreciated. I was looking at the LM393 running in phase at full forward bias (potential of coils) to a simple piezo buzzer driver. With this setup different materials should present unique identifying tones and I'd like to utilize this characteristic in the design.

 

Hello,

There are many metal detector designs on the chemelec site.
Halfway down the following page you will find several types:
http://chemelec.com/Projects/Projects.htm

Bertus

 

If anyone could give insight into the circuitry to drive this it would be greatly appreciated. I was looking at the LM393 running in phase at full forward bias (potential of coils) to a simple piezo buzzer driver. With this setup different materials should present unique identifying tones and I'd like to utilize this characteristic in the design.

I can make absolutely no sense out of this.

I was looking at the LM393 running in phase

Running in phase with what? There must be two signals to be in or out phase.

at full forward bias (potential of coils)

This is nothing but word salad. I cannot even guess what it might mean.

to a simple piezo buzzer driver. With this setup different materials should present unique identifying tones

Distinguishing different metals requires some serious signal processing. You cannot simply connect a speaker and have them sound different

 

I can make absolutely no sense out of this.

Running in phase with what? There must be two signals to be in or out phase.

This is nothing but word salad. I cannot even guess what it might mean.

Distinguishing different metals requires some serious signal processing. You cannot simply connect a speaker and have them sound different

Well, genius, what would you do?

 

I would not expect anything simple to do what you want. I have read about this and there is quite a but of controversy over how well discriminating metal detectors work. Even very sophisticated ones. Anything for sale for less than 1000’s of dollars that claims to do this is almost certainly bull——.

 

I would not expect anything simple to do what you want. I have read about this and there is quite a but of controversy over how well discriminating metal detectors work. Even very sophisticated ones. Anything for sale for less than 1000’s of dollars that claims to do this is almost certainly bull——.

So you have no clue what I'm talking about and the underlying physical principles of +OO-
Can you do me a favor and share this thread with EE's so I can better understand how the circuit can operate?

 

what circuit? can you please post schematics of what you are thinking of?

 

You appear to want an induction balance type of detector. Unless the voice coils are removed from those speakers you have shown, any inductance change due to metal in the vicinity will be vastly outweighed by the fixed inductance due to the powerful speaker magnets.

 

If anyone could give insight into the circuitry to drive this it would be greatly appreciated. I was looking at the LM393 running in phase at full forward bias (potential of coils) to a simple piezo buzzer driver. With this setup different materials should present unique identifying tones

No, they shouldn't.

Nothing you can do with one LM393 can do this. What is your justification for this statement / idea / whatever?

Under a set of clearly defined conditions, a much more complex electronic circuit can do a subset of what you want. But not in a "simple" device that you wave over the ground.

Remember that a metal detector does not detect metal. It detects *changes* in the magnetic properties of the environment within a close proximity. If a sample of something is brought within that environment, of a known size, shape, distance, and specific gravity, then something way more complex could make a guess at a determination. I'm not a trained metallurgist, but I've dome some work in this field and it is way harder than a couple of speakers and a piezo element.

ak

 

The only resemblance of what you described to a metal detector is that they both have two coils.

 

Trying the suggestion that Alec_t made on post #16. The circuit being an induction balance detector of metal.
In the picture below a low frequency signal from a buzzer is fed into an inductive capacitance known as an LC resonant circuit.
The coupled inductors are indeed audio coils.

Alecander Graham Bell, Metal Detector, 1891 Stock Photo - Alamy

Further inquiry into operation of that circuit itself was commented on page 24 of the archive below.
A bullet was placed between the 2 coils and the condenser was adjusted by tone. When the bullet was removed from between the coils
the inductive value of the coils was reduced resulting in the signal having artifacts with an increase of a faint higher pitch.
Upon the electrical experiments to determine the location of the bullet in the body of the late President Garfield; and upon a successful form of induction balance for the painless detection of metallic masses in the human body : Bell, Alexander Graham, 1847-1922 : Free Download, Borrow, and Streaming : Internet Archive

One Circuit that uses inductive balance takes advantage of the very high impedance of the jfet op amp at that stage without distortion.
Induction Balance Metal Detector - Code Advice Needed. - Using Arduino / Programming Questions - Arduino Forum
I recommend that the development of good sampling use an oscilloscope first, then let the microcontroller software in the identifying process.
Take for example a TDR circuit. the circuit can be inexpensive but the oscilloscope makes the circuit perform. A good place to start.
There are imaging circuits that store data and enhance the file in this case sorting the electromagnetic data to refine those aberrations.
Giving higher definition to those faint signals of interest is facilitated in part by some analog conditioning.

If we examine a common BH curve of a ferrite core there is sufficient familiarity to discuss hysteresis. The hysteresis of a rusty metal beer cap
is easier to detect than a shiny silver dollar. This is better shown on an oscilloscope, when someone asks questions of such a nature
hopefully they will do something useful, that our time not be wasted.

 

Trying the suggestion that Alec_t made on post #16. The circuit being an induction balance detector of metal.

Alexander Graham Bell, Metal Detector, 1891 Stock Photo - Alamy

With BI operation the signal coil is coupled to the receiver coil looking at a out of phase differential comparison. Here with my idea two magnetic circuits are isolated looking at in phase comparison of changes in uniformity between magnetic directions.