TechStuff Classic: How Metal Detectors Works

now I'm in a much nicer studio. But we sat down to talk about how metal detectors work. What is the operating principle behind metal detectors, what's the science behind it. So you're about to enjoy this classic episode of tech Stuff how metal detectors work, And I'll talk to you again in just a moment. So today we are going to talk about this. You know what, This was a topic that I don't know that anyone has ever requested. Um. It came to me actually as I was looking through

stuff in my basement and went metal detectors. How should talking about We've never talked about metal detective Actually, we've sort of talked about metal detectors, because metal detectors use a lot of the technology we talked about before, especially the relationship of magnetism to electricity. Yeah, um, but yeah, I mean it's something that we run into very often, especially if we travel and walk through the metal detectors

at the airport. If we're not getting back scattered or uh, you know, dropping stuff that you can't find, losing your wedding ring and the malts outside. Yikes, that's good stuff. I've done that. I still have it, so there you go. I've never lost my wedding ring. All right, Well it was only temporary. So anyway, metal detectors talk about what

they are. We're mostly going to be talking about the kind that you would you would care you with you you know, the portable metal detectors that you would use when looking for say, loose change on the beach, or you know, searching for that rare civil war uh bullet or a gun that might be part of you know, in an area that you knew the battle happened here, so there may very well be stuff here. Yeah. We we should definitely talk about some of the applications of

the technology. Um, and I would like to point out that we have an excellent article on how stuff works dot com if you're interested in learning more, um, how metal detectors work. Yeah, it should be pretty easy for people to find, written by Jeff Tyson, and it is a very good article and very uh simple to follow, easy to understand. I would say this is this is one of those really great how stuff works articles. Yeah.

You know, by the time you get through it and you're like, oh, yeah, no, that totally makes sense, especially if you have a little bit of grounding in the way electromagnetism works and conductors and inducers. Right, So we'll we'll cover that. I mean, we've talked about an in previous podcast, but we'll we'll do a quick brush up so that we can explain it. Um, so you wanna, let's let's talk about the the Jeff does a great job at breaking down what the typical portable metal detector,

the four main parts of one, what they would be. Yeah, I mean, if you've ever seen people walking around with a metal detector, you you sort of know what they look like. They have a round dish part that's used close to the ground, right, that's a that's called the coil um then yes, And then you have the stabilizer um, which is the part that you actually hold onto, that holds onto europe, that that you see on your arm because you know, frankly, having used a metal detector, it's

not the lightest thing, right. This this helps, This helps distribute the weight somewhat so that you're not you don't feel like you're carrying around a you know, ten pound brick the whole time, because without that then you would get very tired very quickly. Yeah. And the the ten pound the aforementioned ten pound brick, which probably isn't ten pounds, but it certainly can feel that way after a while. That's the control box, uh, And you've got all your

your electronics in there. And and then the the connecting bit, yes, the long skinny part that's the shaft. Yes, it connects the coil at the base to the part that you're holding, and the of course the control box. So those are your four basic parts of your basic metal detector. Yes. And then you know you have your headphones because this is one of our episodes of stuff that beeps and

stuff what beats? Stuff? What beeps and it it does in fact deep Yeah, there's usually some sort of audible alert that you have in fact detected metal and now and more sophisticated UH, metal detectors will tell you a lot more often with uh, well sometimes with with an audible tone, but also with a visual uh, some kind of visual analysis. We can get into that to in a minute. So there are three main types of metal detectors, the ones at work, the three three main approaches to

to metal detection. Let's say, Uh, there's UM, there's the very low frequency which is probably the most um, the most popular or the most common type of MEL detector, at least for the portable metal detectors. UM. Yeah. When we get into this, you'll you'll see that each of these different types has its own strengths and weaknesses. And you might say, well, the VLF detector sounds great, why would you use the others? We'll get into that. Yeah.

The other two are pulse induction and beat frequency oscillation. Yeah. I I knew a beat frequency oscillator once. He was awesome. Anyway, I think there was something else I guess, So, yeah, that's the beat boxer frequency oscillator. That the uh, the VLF metal detector is the type that I have. UM, and uh the the basic differences, uh, and these is you know, you'll see those really in the coils. To

be honest, because the VLF has two different coils. One transmits um and and the other is basically an antenna to pick up the signals, so they're shielded from one another, so it doesn't pick up because otherwise you turned it on, you would just have the receiver saying, oh my gosh, there's metal everywhere. Holy crap, you're rich, even up in the air. It's it's literally everywhere. Yeah, that would that would not be very useful. And taking this back to

the store where I bought it. It's a detector detector. This does not do me any good. It's a detected or it's a detected or where's perry? So the the the transmitter coil um it's it think of a circle, all right. It's the circular part at the base of the male detector. And if you were to take it apart, which I do not recommend you do, you would see that there is a warrant point your warranty and it

may not work anymore. You will have a there's a coil of wire that goes around the perimeter of this circle. Like it's it's wrapped around. So the circle itself is let's say it's like a quarter of an inch thick, and you've got this coil of wire that's that's coiled around this quarter of an inch all the way around

the perimeter of this circle. When you run electricity through this coil of wire, what happens besides the fact that electrons are moving through because of the nature of electricity and magnetism, Well, when you run electricity around a coil like that, it generates a magnetic field. That's right, It does generate a magnetic field, and it generates a magnetic field perpendicular to the direction of the electricity. It's just

the nature of electromagnetism. It's amazing, it's awesome. That's what how electro magnets work and UH dynamo's work in UH and sort of using this reverse um. So you've got electricity flowing through these coils, and that generates the magnetic field. Well, it doesn't just the electricity. Electricity doesn't just flow in one direction, no, it it goes back and forth. It alternates very very very fast, almost as fast as I waffle back and forth on what I want to eat

for lunch. And that means that the magnetic field also alternates. The direction of the magnetic field changes. Uh And if you think of it in terms of the disc, the the coil disc being towards the ground, you can think of it as the magnetic field occasionally is pushing against or pulling the ground in general. That just think of

it that way. That's the direction of the magnetic field while the electricity is going around and around in a circle, back and forth and in the various directions, and that alternation is happening hundreds of times per second. So you've got this magnetic field that's that's just oscillating essentially right there against the ground. When it comes to contact with something that is metal, well, the metal will start to that magnetic field will cause a a weaker magnetic field

to generate around the metal object. Yeah. Basically induces a magnetic field in a metal object in the ground. So let's say, uh, um, you drop a screw and when you're trying to build something outside and you're trying to find it with your metal detector so that you don't step on it with a bare foot, and you know, need a trip to the hospital and a technis shot. Um. So as soon as you run the coil of the field and you're not trying to touch the ground with it,

you really just need it close to the ground. Um, it will find the screw. The screw will basically start inducing a It will metal detect detector will generate a field, and that will cause the screw to generate its own a weaker magnetic Yeah, yeah, considerably weaker, but it will the metal detector, Uh, the antenna coil and the transmitter coil will say, oh, look there's something right there that

is generating a magnetic field. It's right here. Chris and I have a lot more interesting things to say about metal detectors, but first let's take a quick break to thank our sponsor. So, if you have a coil of wire and you introduce it into a fluctuating magnetic field, that will induce the flow of electricity within the wire. Just as electricity and the wire will create generate a magnetic field, a magnetic field will induce electricity to run

through wire fluctuating magnetic field. It can't just be just a magnetic field. So when the weak magnetic field from the mental object hits that that the wire in the the the the receiver coil that generates electricity which then alerts the control box and that that's what. Then when the control box gets an electric signal from the receiver coil, that says beep, something is here, stuff, what beeps? Yep? Stuff? What beeps? So yeah, that's the basic uh way that

a VLF metal detector works. It shoots out this magnetic field. The polarization is alternating the same as the the flow of electricity around the the the transmitter coil, and then the receiver coil picks up any magnetic fields generated by metal objects that are under the surface of the ground. It's pretty cool and generally it will go more or less about a foot. Yeah, you're not gonna find you know,

the Earth's core. Yea with a metal detect and the depth of what the depth of the The effectiveness of this metal detector is also going to be dependent on other factors, like the nature of the metal object itself, because some some metal objects are going to generate a stronger magnetic field than others, like iron. Uh. It will

also depend on the composition of the soil. Yes, so the soil has a lot of elements in it that would create a magnetic field, you're gonna get a lot of interference, yeah, or or an environment where uh that that would conduct electricity more readily, Like if you were trying to look for something and say saltwater, you might have more difficulty than if you were looking, you know,

on on a different type of soil. UM. Now there is something to uh called phase shifting, Yes, and that's how it's able to tell what sort of metal, what sort of metal it is, UM. And it really has to do with the the inductance and the resistance of the object that you're looking for. Now, the inductance is um the objects ability to conduct electricity UM. And if it's inductive, it will be able to to conduct electricity easily,

but it's slow to react to changes in the electrical current. UM. If it's resistive, it doesn't conduct electricity is easily, but it is quick to react to those those changes. And a sophisticated metal detector will be able to tell you, well, that's I think that's gold or based upon that phase shift, because the face shift for a particular element is going

to remain constant, you know. That's that's why if it didn't remain constant, then this wouldn't matter at all because it wouldn't you know, we wouldn't know, you know, It's like we just know something is down there, we wouldn't know have any way of guessing what it is. But because it's an inherent property of this particular material and it's really dependent upon the molecular composition of whatever that material is, UM, then we can make start making guesses

based on that phase shift. UM. And it's yeah, the face shift, you know, with it being that difference in timing between the transmitters frequency uh and the frequency of the magnetic wave that's generated by the object. Um. You know, it's these are sort of ranges, right like if it if it's a certain range there, then the male detector can give a best guess as to what that material maybe.

So there's never necessarily a guarantee that whatever it is you've detected is specifically going to be you know, aluminum versus goal or whatever. But it's a it's an indicator, so that uh, it gives you at least the idea of you know, is that you measure risk versus reward, the risk of exhausting yourself digging up useless junk versus the reward of possibly finding something that is of true value.

So that's really the whole purpose of this anyway. It's just you know, to kind of tune in on that beep and make it more meaningful so that you know, you don't waste your time digging up uh little pull tabs from soda cans. Yeah. Now, yeah, if you have a higher inductance, it's going to create a larger phase shift, and lower inductance has a smaller phase shift. Now, Uh, metal detectors use a phase demodulator to read this, and uh,

what that creates is discrimination. Yeah, yeah, I know it's terrible, but oh no, not that. Within metal detectors, it's okay, Yeah, because the thing is a good metal detector will allow you to filter out stuff that you don't want because the metal detector, if you said it at a certain setting and say, you know, I'm really looking for metals

that fall within this range, yeah, perhaps gold. I've actually seen uh, metal detector booths and they'll have like this giant jar of class rings that people have lost, which is sort of a you know reminder that you know, hey, you you can find stuff if you just take your metal to of course, people look at you funny, but yeah, that way. That way. Let's say that you're out there

looking for something specific. You can you know, meld detectors have settings where you can, uh you can tell the meld detector to ignore anything outside of a particular range of frequencies. So in other words, you're saying, you might say, I'm only interested in finding silver. Let's say, so what I'll do is I'll set my meld detector so that it'll it'll only uh activate, it will only alert me if something that generates a magnetic frequency similar to that

of silver is encountered by the metal detector. So in other words, what you might you might pass over a gold deposit and nothing happens. But as soon as you go over that that silver half dollar but ink, and then you dig it up and you like, hey, look I'm rich. I Meanwhile, the gold guy behind you was like, sucker. Well that's that. That can be called notching. Basically, you set it for a if you imagine a scale, uh

you know, he's one notch on that scale. Or some of the more sophisticated metal detectors will allow you to set more than one notch r So that way you could look for both silver on gold and you'd say, ha ha, Mr smarty gold pants. Um, now that's that's the VLF. The post induction metal detector a little bit different. Yeah, it uses only a single coil, or it can use multiple coils working at the same time. Yeah, but they're these are these are coils that haven't that don't have

that shielding. It's not like you have a transmitter coil and then a receiver coil, and all of these coils are generating very quick bursts of electricity, which of course creates a very sharp magnetic wave. And uh so it's just a really quick spike and then what will happen is that magnetic wave if it encounters a metal object again, that metal object will have to will generate its own magnetic wave, and the metal detector will pick up on that. And it's all it's kind of like an echo, right,

It's kind of like an echo, right, thank you. So if you think about it that way, it's it's like sending out a signal and waiting for something to bounce back. Yeah, but it's very fast. I mean the average is about a hundred pulses per second, which is yeah, fairly rapid, pretty quick, and then you're looking for the reflected pulse. Yes, which is what the the object that is under the ground or wherever. Uh, that's the the the pulse that it generates in response to the one that came from

the MEL detector. Yeah. Now it uses a these these types of metal detectors use a sampling circuit basically trying to determine whether it's actually metal that you're finding or something else, some other type of magnetic field that is interfering with this signal. And it's basically judging that based on the amount of time it takes, uh too to reflect. Um. Yeah, that's another thing that we should point out is that MEL detectors can and encounter lots of different kinds of interference.

Not just the composition of the soil. But let's say that you happen to cross over where in elector called cable has been buried. That could cause a problem. Your metal detector might think that, hey, look there's gold in them our hills, and truth, what you've hit is the power line, which you don't want to dig that up. No, it could result in everything from death, which is in the metal detection biz we call a bad thing, or it could result in or and or it could result

in an entire region losing electricity. That's also a bad thing. That's that's a very good point. I mean, these these metal detectures are not only incredibly useful, they can be a lot of fun. A lot of hobbyists love them. But you do have to be very very careful. Um, and it's good to know your the territory. To quote a certain musical, but yeah, I mean, you need to know the territory. You gotta know where you're you're digging because you you really could get hurt. Hey, guys, it's

Jonathan from the present day, butting in here. I was to want to talk to you for a second because my my, my AD detector is going off. Looks like we've got a sponsor read So we have an integrator on these on these pulse induction metal detectives, and that's reading the signals from the sampling circuit and converting them to d C. That's not current, yeah, not the district of Columbia. Yeah, not not a comic book publisher either. And that's feeding into the audio circuit and that is

what beats. Yeah, that's the stuff what beats. The pulse indicator. The thing is, uh, these are very good at detecting metal, but they're not very good at discriminating between different types of metal. UM. So you're not going to get the level of precision that you would get with a VLF metal detector. But you know, they also you've got to remember that these different types of detectors, while some of them may work in conditions that others do not, there's

there's also a price range here as well. Oh yes, yes, now, yeah, the beat frequency oscillator metal detectors can be uh fairly affordable. UM. They also used two coils, but one is in the search head and the other one is actually in the control box. UM. They're connected to an oscillator that as pulsing UM many many times per second, thousands of times per second UM. And and the and the frequency of the one that's inside the control box and the one

that's inside the actual detector are slightly offset. They're not in sync or backstreet voice. Oh you got the other one I was gonna do so yeah, well, ironically enough

to our our current part of the conversation. They create radio waves UM, and then the receiver in the control box UH picks that up and plays tones to you know when you're you're coming into range of metal, right, because what's happening is that when the metal encounters this uh this this this electromagnetic field that is going to alter that radio wave. And so what's really listening for

our changes in the sound that you're hearing. The change in the sound indicates that you have encountered something that is generating this magnetic field. Because of the one you're generating, they know what they're going through. Um. So yeah, I mean detectors killed the radio star. But these these uh, like I said, these are a lot cheaper to manufacture.

And therefore, if you are uh interested in getting your feet wet uh in the metal detector, the thing is a hobby, you might consider this, but they're they're far less accurate as in terms of being able to give you any kind of discrimination or any really any real control. Um, they actually use these are the types of metal detectors that you the handheld detectors that they use the airport. Um, you know if you walk through the scanner and and then yeah, that's the type they use in the one.

Because they don't really need to know whether it's gold or iron. They just want to know what it is that you've got now once once, once something beeps, then they can say, all right, well let's see what that is. They don't, you know, necessarily need to know immediately. Yeah. And the metal detectors, the frames at the airport, they work on essentially the same principles we've talked about here.

They're generating electromagnetic fields, which of course has brought up some concerns with people who may have uh devices like medical devices that could react to electromagnetic fields. That's why you'll occasionally hear about UH folks like that having to go through like a pat down security as opposed to a metal detector, just because of the concern that it

could interfere with a medical device. Right right now, with with all of these um, what you're gonna find depends on the type of metal detector, the type of metal that you're looking for, about how big it is, the kind of makeup of the soil, um any kind of interference from other objects, and the halo, which is basically you know, if you have a piece of metal in the soil, it's going to read a little bit differently, it's going to it's going to register is larger than

it actually is. Yeah, it's it's because those interesting things. Yeah, they I could see your halo. I was I was waiting. I was trying so hard not to go into that. We've done so many songs references in this podcast. It's crazy. This might be our like our most intense the funny thing references per minute other than the first one. None of them have been metal. That's true, that's true. So but yeah, no, that this is really the halo effect

is really an interesting thing. It's just this the fact that certain kinds of metal objects, uh, can actually increase the conductivity of the soil that's around on them. And the longer that object, those particular objects have been in the ground, the greater that conductivity change can be. So then yeah, you might be like, hey, there's a there's a Sherman tank down there or something, and it just turns out that there's another object that's been down there

and has affected the conductivity of the soil. But it's pretty phenomenal. If you have a Sherman tank buried in your backyard, please give me a call. I'm I'm interested to well not anymore. Yeah, I mean, well, you think

about it, it it makes sense. If you have let's say, let's say the aforementioned screw and it's been there for a couple of years, it's let's say iron and it's beginning to rust, it's beginning to disintegrate, and you can change the conduct conductivity depending on the Again, it also depends rain, and you know, it really depends upon the

composition of the soil itself as well. So, yeah, if there's a lot of iron content in the soil, that's gonna cause some issues when you're when you're doing mild section, you know, you're just you're gonna get a lot of interference and you just have to factor that in. Doesn't mean that the metal detector is going to be useless. It just means that you have to kind of adjust your expectations and listen for bigger changes in UH in the soil content UH and not and kind of kind

of ignore the noise, separate the signal from the noise. Yeah, I would argue that it probably takes some personal discrimination too. You really need to get out there in practice if you're going to start using doing metal detecting as a hobby, UM and maybe even trying it with things that you would that you know what they are. You know, some coins, a ring, a screw driver, you know something that you hammer, hammers, you know, sherman tanks, Uh, stuff that you do just

you know, tend to run across in your general area. Well, you know, you never know what you're gonna find when you're out in a field somewhere, and if you can identify what they are, you can get a sense of what different metals sound like. How get a feel for how your metal detector works, how sensitive it is, and

how sensitive it is. Um, Yeah, it can be a lot of fun to uh you were talking about before, like going to uh battlefields course, I imagine probably all the major battlefields have been picked pretty clean at this point, you would think. But it's it's amazing what you can

stumble across. It's again, it's one of those things where it may be it may require a phenomenal amount of luck, sure experience, there's no such thing as luck, but it may require a phenomenal a series of coincidences in order for you to stumble across the right section to find something significant. But really you never know. You just it's it's amazing to me how something that you would think, oh, well, this has got so much traffic. There's no way there's

anything of significance can be found there. And yet like five years later you'll find that the find of the injury. How did that happen? Yeah, and you don't know what's around in places that you know might be a value. I mean again, people find I'm still searching for black beards, treasure, all kinds of stuff out in time. I go to the airlinas out in normal places. Maybe you should actually go to the everyday beach instead of Finland. That's probably

part of the problem. Yeah, you know, Charleston is so nice. I just like to know, spend some time there enough. Yeah, people even use them for prospecting, you know, and uh and for coin shooting. Now, if you do want to use your mail detective for prospecting, there are some things you need to remember. First, you have to grow a bear of at least a foot in length that needs to be gray and grizzly. He's gonne punchy, ladies and gentlemen. Second, you need to be able to say things like consarn

it with a straight face. And then third, the most important thing about being a prospector is you have to develop your crazy prospect or dance when you strike gold. Very important. Yeah, coin shooting is the practice of looking for coins after an outdoor event like a wedding or or like stock you go to you go to some big outdoor conference and you take a mail to actor

and you start looking for change. The thing is that if you live in an area where this sort of stuff goes on in a regular basis, you could presumably pay off that mail detector after you know, a few visits to these these areas, once the concert is over or a game or whatever. Um, you know. And and because we're usually talking about change, you know, some some nickels or dimes or quarters or whatever. But whatever money you use, because you know, it's not all just United

States folks. Well, if you find euros, you're rich, because I think a euro is equivalent to like seventeen dollars at this point. Um, that's a gross exaggeration, but the point being that, you know, yeah, you could conceivably pay off your mail detect there's costs after a few visits, you know. Um. But although a lot of people just enjoy the this as a hobby, like it's just it's

a way to get outside. You're enjoying your joining time outdoors and you're finding stuff which just brings you a level of of of satisfaction, right at least that whole treasure hunting aspect. You know when I when we discover something, there's an element of satisfaction there that you can't put

a monetary value on. Who Yeah, yeah, you're just like, hey, look I found some money woo, And yeah, there's there's an enjoyment level there that goes beyond oh and it's worth you know, seventy five cents um, although the five cents always nice to have seventy five cents more than you had, but when you showed up at the field. So yeah, I keep wondering, actually, why the value of the giant jar of class rings. You'd figure they'd cash that in, Yeah, I mean for more than just selling

metal detectors. Maybe that's more lucrative than selling off some of those classroom you have, prying out all the little jewels and melting that stuff down and then just hey, you know you can yourself bar a gold. It's worth its weight in gold. All right, guys, hope you enjoyed that classic episode of tech Stuff How Metal detect there's

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Tor has been working hard on that website for a while, so I know she's very excited that you guys are gonna get a chance to interact on it. She's just given me a big smile. She's dancing. Look at her, go gosh, sorry, wow, she just did the floss. Now she's doing the hand give. It's amazing. Thanks Torry. Well, guys, that's all for me. I'll talk to you again really soon for more on this and thousands of other topics. Is it how stuff works dot com