[Karen W. (OP)]

Well this year I purchased 3 metal detectors! Two for children and one pretty basic adult size metal detector. Today while my grandson was here we went out to use his child version metal detector which by the way works about as well as the very expensive adult one I bought for his brother at Christmas. While digging for his treasure he began asking me about how the meal detector worked and I not having ever thought about it immediately began mulling it over in my head, thinking surely there must be something to do with magnetism that helped it find metal, but because I really had no idea about the basic principles of their workings, I refrained from giving him my immediate thoughts and told him I'd have to refer to others who knew more about it then I do, So what lovely things can you tell me about the inner workings of the metal detectors? if you can enlighten us on how they work, including any details my 7 year old Grandson might like to know? Like how they detect different types of metals etc,,, ? He is autistic and very bright and inquisitive!
Whilst giving me many of his own analogies, He was extremely excited to find his two little treasures .. LOL And old metal bottle cap off perhaps an old alka-seltzer bottle along with a large Bolt! He exclaimed That surely any pirates would be very frustrated by the people who have made metal detectors and by us for finding their treasures so easily!

[vhfpmr]

When a current is passed through an electrical conductor a magnetic field is created around that conductor (see the blue field lines in the diagram below), and if the current alternates, so does the field, ie: it moves. However, if a conductor is placed in a moving magnetic field, a voltage is induced in that conductor which opposes the flow of current that created it (known as the back-EMF). This is self-inductance, and it means that an AC voltage applied to a coil (inductor) will produce a lower current than is produced by a DC voltage of the same magnitude in the same coil.

If we now place a second coil where it shares some of the magnetic field produced by the first, a voltage will also be induced in that one too, and if the ends of the coil are connected to complete a circuit, a current will flow. This is mutual inductance, and is how a transformer works: two coils (a primary and a secondary) sharing a common magnetic field.

The important bit is that the current in the secondary also generates a magnetic field just like in the primary, but this field opposes the first, so that the back-EMF induced in the primary is now reduced, which amounts to a reduction in the primary inductance and a corresponding increase in the primary current. This is a change that can be detected by the control circuit, but the degree to which the secondary current is reflected in the primary depends on how closely the coils are coupled.

In a transformer the coupling is tight because they?re both wound on the same core, but with metal detectors the coupling is looser, which brings us to the next important factor: what affects the coupling? For maximum sensitivity, the plane of the loop needs to be perpendicular to the magnetic field, so that means horizontal and just above the road, as in diagram A or C below, but not B. Solid objects are just like multiple shorted loops, so the orientation will be less critical.

This applies to any material that will conduct electricity, but magnetic materials also have another effect: they will concentrate the magnetic field because their permeability is higher than that of non-magnetic ones. This increases the inductance. If you connect a coil to an instrument that measures inductance you can see that if you place a piece of ferrite in the coil the inductance increases, but a piece of brass reduces it. If the coil is excited with a VHF current, then counterintuitively something magnetic like steel reduces the inductance instead of increasing it, because the magnetic domains in the steel are too large to alternate as fast as the current in the coil, so it just appears like a non-magnetic material such as brass.


Traffic Light Coil 2.png (97.4 kB . 603x193 - viewed 187 times)

[alancalverd]

The fun bit is that some detectors deflect in the opposite direction depending on whether the target is ferromagnetic or not. This can save a lot of time if your grandson is interested in Saxon swords - he won't waste his time digging up Roman gold.

[Karen W. (OP)]

Well this is very interesting is there a visual display I can set up to show him from the batteries to the coils etc to sort of show him where the emf field stats and what happens like from point A to B..etc.so that it in the right order but visably like a chain reaction from the push of the button to power it up to the sounding of the beep when it finds metal?

[Zer0]

Perhaps Utube has a few animated videos on the Topic.

Might not be Kid Friendly in terms of Understanding, but an Adult with a lil bit of time could break it up into smaller digestible chunks.

A.T.B.

[vhfpmr]

The fun bit is that some detectors deflect in the opposite direction depending on whether the target is ferromagnetic or not. This can save a lot of time if your grandson is interested in Saxon swords - he won't waste his time digging up Roman gold.

My understanding comes from being a radio engineer rather than having ever used a metal detector, so I'm not familiar with the practical refinements they have. I imagine it would be possible to distinguish most metals from one another with a detector that switched between two or more frequencies.

That description I posted was edited from a file I drafted to explain to cyclists why there's no point in attaching magnets to their bikes in an attempt to trigger the traffic light sensors, and where the best position on the loop is.

Do you happen to know at what frequency iron transitions from ferromagnetic to diamagnetic? I've only checked it at VHF, because that's the band I worked at, which leaves quite a lot of room below it.

[Karen W. (OP)]

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Just how does a metal detector work?
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Just how does a metal detector work?
? on: 08/09/2023 11:14:33 ?
Well this year I purchased 3 metal detectors! Two for children and one pretty basic adult size metal detector. Today while my grandson was here we went out to use his child version metal detector which by the way works about as well as the very expensive adult one I bought for his brother at Christmas. While digging for his treasure he began asking me about how the meal detector worked and I not having ever thought about it immediately began mulling it over in my head, thinking surely there must be something to do with magnetism that helped it find metal, but because I really had no idea about the basic principles of their workings, I refrained from giving him my immediate thoughts and told him I'd have to refer to others who knew more about it then I do, So what lovely things can you tell me about the inner workings of the metal detectors? if you can enlighten us on how they work, including any details my 7 year old Grandson might like to know? Like how they detect different types of metals etc,,, ? He is autistic and very bright and inquisitive!
Whilst giving me many of his own analogies, He was extremely excited to find his two little treasures .. LOL And old metal bottle cap off perhaps an old alka-seltzer bottle along with a large Bolt! He exclaimed That surely any pirates would be very frustrated by the people who have made metal detectors and by us for finding their treasures so easily!

? Last Edit: 08/09/2023 11:24:01 by Karen W. ?
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Re: Just how does a metal detector work?
? Reply #1 on: 08/09/2023 12:50:19 ?
Quote from: Karen W. on 08/09/2023 11:14:33
surely there must be something to do with magnetism that helped it find metal
It has to do with electrical permeability which is related to magnetism. Magnetism would only find ferrous materials, so not say copper, but a metal detector is quite good at finding copper objects.

The idea is that it puts out an EM field which energizes (sets up eddy currents in) anything that allows the internal formation of magnetic lines of force (stainless steel being very bad at this). Then it shuts off and detects ('listens') for the currents created in the objects below. So the object needs to be able to pass electrical current, and it's mostly only metals that do this.

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That surely any pirates would be very frustrated by the people who have made metal detectors and by us for finding their treasures so easily!
A decent pirate would bury his treasure a little further than the range of most detectors, and they put it where people tend not to look for it. Metal detectors are great for coins and jewelry dropped in public places like parks and beaches where the grass and sand quickly hide small dropped objects.

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The following users thanked this post: Zer0

Thanks Zer0, I will check it out and see!