How anti-theft tags work - magnetostriction-----make money online

Anti-theft tags contain a tiny mechanical oscillator that is activated by a magnetic field.  The detectors at a shop's doorway can detect the oscillation magnetically. The tag can be activated and deactivated by magnetizing or demagnetizing it, respectively.

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Matthew Hill

Erik Saathoff

Max Loutzenheiser

(better credits titlescreen in the next video!)

today on Applied Science I'd like to

talk about these anti-theft tags you've

probably seen these little rectangular

tags that set the alarm off if you try

to exit the store before this is

deactivated and they actually work by

having a little tuning fork inside here

that's controlled by a magnet and so

I'll show with the scope what the

signals look like and also show under

the microscope the tuning fork moving

but first I'd like to give a big thanks

to my patreon subscribers you've

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very much there are a few different

techniques to make these anti-theft tags

but today I'm going to talk about these

hard plastic rectangular ones that use

magneto acoustic technology to work so

there's quite a bit of misinformation on

the internet about these and for a while

I believe that the way these worked is

they responded to a magnetic field and

then emitted sound you know hence the

name magneto acoustic but that's

actually not quite right so let's cut

one of these open and see what the deal

is here there's two really shiny thin

metal ribbons that are free-floating

inside here you saw I just cut the end

off and just dumped them out and so

we'll put these here and then the tag

itself has another metal strip that's

actually part of the adhesive that holds

the tag together in this particular tag

the metal is actually laminated inside

the plastic so there's the tag has sort

of a hard plastic top piece and then

this bottom part the actual adhesive

part has the metal strip laminated in

there so the thing that makes these work

is the principle of magneto striction

which is pretty cool so if you take a

piece of metal ferrous metal and put it

into a magnetic field it will actually

change shape slightly very slightly but

the point is that you can actually make

this thing oscillate make it vibrate

back and forth by just putting it into

an oscillating magnetic field so let's

take a look at that this tag works this

is still an active tag I know because I

exited the store and the alarm went off

of course I paid for the object but you

know sometimes they don't deactivate the

tags so let's put that in between these

two coils here and you can see there's

quite a signal on the scope so the

purple trace is the transmit and I'm

transmitting at about 58 kilohertz and

the yellow is the received trace and my

transmit coil is just about a hundred

turns of copper here and I'm using a

capacitor mostly just to keep DC out and

the receive is just a coil with no

matching capacitor at all just right

into the scope so clearly I haven't

spent all that much effort you know

setting up the electronics to be really

good but I was surprised at how much

signal I already got I mean that's quite

a huge difference

now this tag has been deactivated so

we'll put that one in and as you can see

almost no change at all I mean it's

clearly significantly different so the

the two tags are working as designed

such that this one is active and this

was inactive and we can detect them

electrically so what's actually going on

here the deal is that the transmit coil

is creating a very weak magnetic field

here I mean we're only putting a couple

volts or something through this coil so

the field being created here is very

weak but it is there and it's 58

kilohertz hence the acousto part of this

of this whole thing and if we put the

ribbon in here it will start to vibrate

also at 58 kilohertz and the exact size

of this ribbon is critical for how this

thing works so it's not really a tuning

fork it's more like just a piece of

metal so you know if you have a just a

flat piece of metal and you drop it it

has like a characteristic ring to it or

if I drop the smaller one it has a much

higher ring to it so the shape of the

ribbon is cut such that this ribbon will

resonate at 58 kilohertz so when we take

the active tag and put that between the

coils what's happening is that 58

kilohertz is exciting the ribbon and

it's ringing

it's an oscillator and then we're

actually picking up after the transfer

pulse is done we can see this thing

ringing down because the oscillator is

still going in this case we're seeing

even more power in so with the tag out

of the system it's you know lower

intensity once the oscillator gets going

it starts adding its own signal here

because the thing is mechanically

oscillating and adding more to it so the

trick is that this this essentially

tuning fork is is adding to the magnetic

system and the two archways on the

doorway exiting the store are just like

these two coils here one's a transmit

and ones that receive and it pulses the

58 kilohertz on and off and then looks

to see if there's this ring down so if

you have one of these tags buried in

your cart of stuff somewhere it will

start ringing as you go through and then

this thing listens to the ring down

magnetically so there's no sound being

transferred this from this back to the

system the only acoustic part of it is

the fact that it's using this ribbon

that mechanically oscillates at about 58

kilohertz and it's easier and cheaper

basically to do it this way than to

build for example a inductor capacitor

network that would oscillate at a

similar frequency I mean this is just a

piece of metal it costs very little this

is about 25 micron thick and it's it's

just it's it actually is a special alloy

but it's not that exotic it's mostly

iron and you know making these tags is

super cheap so you might be wondering

how you actually turn them on and off so

the next trick if we had this tag that

oscillated and you know you go through

the doors and it goes off that works but

how do we turn the tag on and off the

other trick this other bit of metal in

the tag is actually a magnet a very

small weak magnet and one of the tricks

with this oscillating strip of metal is

that in no field like in a field where

it's it's normally nothing let's say

just Earth's magnetic field and then

we're transmitting this 58 kilohertz AC

magnetic field onto it it doesn't

actually want to oscillate all that much

because it's characteristics have been

tuned such that it needs to have like a

a magnet nearby for it to have this 58

kilohertz resonance

the reason for this is that the response

curve looks something like this so if we

have increasing magnetic field strength

this way an increasing mechanical output

this way the curve has this kind of S

shape to it so if we're in almost no

magnetic field if we put an AC on top of

this the AC field is kind of going back

and forth like this but we're not

getting much mechanical movement out of

it because down near zero this curve is

very flat but if we add a magnet to it

let's say we're now positioned over here

because we're near a magnet and we add

that same AC to it now we're kind of

moving up and down the curve like this

because we're in a very steep region of

the curve and so the material that

ribbon is tailored such that when it's

in almost no magnetic field it doesn't

respond very well to this AC but when

it's in a static magnetic field then it

responds really well and then of course

the material saturates and so if we keep

putting more and more magnetic field on

it it eventually doesn't move anymore

and we'll see this under the microscope

in in just a minute so the trick is that

this little cheap magnet in here it's

basically just a thin strip of another

iron compound when it's magnetized the

strip is active because it's in that

steep region of the slope of the of the

response curve there so to activate a

tag you put a magnet near it which

magnetizes this you know permanently and

then the tag is active and to deactivate

one of these tags you demagnetize the

strip then it's back down to that

shallow area of the curve and it won't

respond it won't oscillate it's it 58

kilohertz so we just saw that this tag

is working you can see a pretty big

response there I'm going to demagnetize

it by putting it into this open frame

motor from which I've removed the rotor

so I'll switch this on and just kind of

put the tag through it carefully without

shocking myself hopefully and let's see

how it responds now it's pretty much

gone we don't see the same response at

all so this is demagnetized and we're

not getting the response let's see if we

can bring it back to life by putting

this magnet near it so I'll put the

magnet here

and re magnetized the strip that's in

there hopefully and we'll see if we're

back in business and we are it's not

quite as much signal as we had when we

started but it's pretty close so you saw

from the graph that the exact level of

magnetization of the strip that's in

here is actually important too so I came

in here with a really big neodymium

magnet and actually caused one of these

not to work anymore

because I over magnetized it or

something so if the metal ribbon in this

tag is actually moving enough to

mechanically oscillate and cause this

whole system to work I figured I ought

to be able to see it and incorrectly one

of these sources of information on the

internet said that the ribbon actually

moves a thousandth of an inch now this

this ribbon is only you know a little

more than an inch long and so one

thousandth change in its length is

actually quite a lot and think that you

know those the archways that create the

magnetic field that actually move this

thing are a good you know six feet apart

at the store so making this thing move a

thousandth of an inch from these coils

that are six feet away it sounds pretty

unrealistic and sure enough yeah it's

not even close to true so I looked up

the alloy that this ribbon is made of

and I'll put links to all this in the

description and it says that the

saturation magneto striction values

about 20 parts per million so in other

words if you had a meter long strip of

this ribbon and you saturated it you put

it in a magnetic field big enough to

cause it to shrink as much as it Everett

will it's going to move 20 micron and if

this is only a couple centimeters big

then you have to divide that by 50 and

so you know we're getting really small

here hundreds of nanometers change this

thing is going to change hundreds of

nanometers even when it's in a very

strong magnetic field so I tried various

ways to measure this and of course was

not it's not easy to measure a couple

hundred micron or a couple hundred

nanometers so even putting this into the

microscope and looking at it with the

highest optical power is not going to

cut it and then putting it in the

scanning electron microscope it's not

going to work either because the

magnetic field is going to affect the

electron beam so this is the apparatus

that I came up with to detect its

movement and I'll take it apart here

since I've already collected the data

I made a combination I basically just

took two of the ribbons and put them

together so that I had more total length

to work with here so you know if this is

going to move 400 nanometer if I put two

together in theory it should do 800

since it's it's more length and I have

them inside this coil and inside the

coil there are two very small permanent

magnets fairly weak a ceramic magnet

just like this one but a bit smaller and

they're oriented such that the field is

going straight up and down like out of

the table so when the ribbon is above it

the field is going perpendicular through

the ribbon or perpendicular to its face

and then I have a coil of wire here that

I can control with a button and when the

coil is on the magnetic field is going

parallel to the table surface like this

solenoid I then have a spring pulling

this arm down and the ribbon connects to

this pivot point on the arm and then a

fixed point here so basically the whole

idea is that this guy will move or it's

under constant spring tension and then

if the ribbon shrinks a little bit in

length it makes you know I get a lever

effect out of it and so it's about 20 to

1 I'll put the exact numbers in the

description in order to see the movement

I put a very fine wire coming off the

end of this this is actually a carbon

fiber rod it's just a nonmetallic

lightweight very stiff thing and then I

put a thin wire coming off the end and

then use the highest power objective

that I have so that I could see the wire

basically moving back and forth in the

microscopes field of view so I got video

of that and here you can see the wire

moving so you can see it sort of click

back and forth and that is me pressing

the button on and off when the field is

perpendicular through the ribbon face

the length is a little bit longer and

when I turn the solenoid on and the

field is parallel to the ribbon the the

length increases a little bit and you

can see why here when the magnetic

domains are aligned this way so the

field is going perpendicular through the

ribbon the total length is a little bit

less

and then when all the domains line up in

this direction to lengthen a little bit

this property of magneto striction can

be tailored and so there are some

materials that have very high

coefficients like a thousand times

higher than what we're looking at now

but the fact is that they use weird

materials like terbium and whatever make

this trip too expensive and so in order

to keep this thing very cheap they're

using a fairly standard like iron nickel

alloy I'll put a link to it in the

description to get an idea of how much

this thing is actually moving to see if

we're in the realm of reasonable stuff

here I also looked at a cd-rom with the

microscope with the same setup same

camera same objective and the track

spacing on a cd-rom is 1.6 micron so I

made this composite image that shows the

wire moving over the cd-rom so you can

get an idea of how much it's actually

going so I'll figure out the math and

then put it in the description there but

I have a feeling that we're talking

about movement around the order of one

or two hundred nanometers for this strip

length also just to show how sensitive

this setup is the thermal effects you

know just a piece of metal expands when

it gets warm and so this is me just

breathing on the strip and you can see

that the effect of just a tiny amount of

breath touching it I wasn't right on top

of it either is just a little bit is way

bigger than the effect caused by magneto

striction and then just for kicks I got

some freeze spray out and hit it with

that and you can see it just the pointer

went off the screen because it was just

a huge change in comparison

finally there's actually another effect

so this is just showing about the same

volume but a different size change

because of the magnetic field and this

is sort of a classic magneto restriction

effect however for some materials

there's also a very very slight volume

change so besides the strip keeping the

same volume and becoming longer just

putting it into a magnetic field will

cause the volume to decrease a very very

small amount however I read on the

internet so I don't know if it's true or

not if the earth didn't have a magnetic

field it would actually be about 10

centimeters greater in diameter because

they you know it's an iron core and all

the magnetic field created by the earth

itself is actually

causing it to be a little bit smaller

because of these effects so let me know

if you think that's true or not and I

will see you next time byeonline earn money website, online jobs to earn money, best online income site, top 10 online money earning sites, easy income online, easy online earning, earn money online from home, make money online legit, earn money online free fast and easy, online earning websites list, genuine online money earning sites, online work to earn money, online surveys to earn money, earn money through internet, best online income, earn money online data entry, easy ways to make money online, best online earning websites, top websites to earn money, online typing jobs for students to earn money, earn skrill money online,