How to make flat coils

[pyroesp]

I've bought 0.26mm (40m) and 0.5mm (20m) thick copper wire.
I'm trying to make a flat coil the size of a 2€ coin, with or without a hole in the center (that's yet to be determined from testing), but I can't seem to either make a coil or not break the wire once I've made it.

I used a piece of wood plank, stuck a nail into it and started winding the copper wire around the nail, making bigger and bigger circles.
Because the wire wouldn't stay put I've tried hot glue, but it's a mess and there's too much hot glue on the thing.
I tried super glue, works great, but then I can't get the damn coil off of the wood plank. And when I somehow managed to unstick it, the wire broke and the coil was a mess.

So... how do you make a flat coil ?

The idea here is to make a coin detector for an arcade machine. A gap between 2 identical (or maybe 1:2 ratio) flat coils where the coin can go in between. One coil is the transmitter and the other is the receiver. Kind off like wireless power.
The receiver should see a drop in voltage, due to the coin blocking or attenuating some of the magnetic field lines. The voltage drop is determined by the type and size of coin put through the slot.
Then some sort of profiling type of program will be used to know which coin has been put through the slot (if the difference of voltage drop per coin is too low) and increase the life counter on the arcade. At least that's the idea.

[T3sl4co1l]

Just use a hoop shaped coil.  You'll get the strongest signal (difference between present vs. not) for coils about 75% the diameter of the coin.  Turns inside the outermost loop (i.e., making a flat pancake coil, where the inner diameter is anywhere from zero to 50% of OD) don't really matter.

You won't be able to get too many turns in there, I think, so you'll also want to do it at a fairly high frequency (some MHz?).

Tim

[MosherIV]

I used a piece of wood plank, stuck a nail into it and started winding the copper wire around the nail, making bigger and bigger circles.
Because the wire wouldn't stay put I've tried hot glue, but it's a mess and there's too much hot glue on the thing.
I tried super glue, works great, but then I can't get the damn coil off of the wood plank. And when I somehow managed to unstick it, the wire broke and the coil was a mess.

So... how do you make a flat coil ?

The idea here is to make a coin detector for an arcade machine. A gap between 2 identical (or maybe 1:2 ratio) flat coils where the coin can go in between. One coil is the transmitter and the other is the receiver. Kind off like wireless power.
The receiver should see a drop in voltage, due to the coin blocking or attenuating some of the magnetic field lines. The voltage drop is determined by the type and size of coin put through the slot.
Then some sort of profiling type of program will be used to know which coin has been put through the slot (if the difference of voltage drop per coin is too low) and increase the life counter on the arcade. At least that's the idea. 

Instead of sticking the coil to wood, you could stick it to some plastic backing sheet eg OHP (over head projector) film.

Some kind of coin detector.....
A coin will increase the magnetic coupling between the 2 coils not reduce it. This is because air has very low magnetic couoling. Most metals/alloys have bettwen than air megnetic coupling.
What the coil measures will be more than just voltage, it will be an AC waveform. Both the amplitude and freq will potentially be affected the the magentic coupling.

Try looking up simple metal detector projects to understand what I mean.

Good luck 

[pyroesp]

Just use a hoop shaped coil.  You'll get the strongest signal (difference between present vs. not) for coils about 75% the diameter of the coin.  Turns inside the outermost loop (i.e., making a flat pancake coil, where the inner diameter is anywhere from zero to 50% of OD) don't really matter.

You won't be able to get too many turns in there, I think, so you'll also want to do it at a fairly high frequency (some MHz?).

Tim

Something like this wireless power transmitter module ?


I thought of using an LC circuit with a resonant frequency around 100kHz ~ 500kHz, maybe. I don't think I'll need to go +MHz. Still need to make a few coils and test what's best for the application.

Instead of sticking the coil to wood, you could stick it to some plastic backing sheet eg OHP (over head projector) film.

Some kind of coin detector.....
A coin will increase the magnetic coupling between the 2 coils not reduce it. This is because air has very low magnetic couoling. Most metals/alloys have bettwen than air megnetic coupling.
What the coil measures will be more than just voltage, it will be an AC waveform. Both the amplitude and freq will potentially be affected the the magentic coupling.

Try looking up simple metal detector projects to understand what I mean.

Good luck 

You are correct and I'm ashamed of having said the opposite. I'll blame it on me being tired as I've had to wake up early (4am, it's 8pm here now) .
So the coin is an easier medium for the magnetic fields to flow through and go to the receiver coil, thus increasing how much voltage is being induced. (Makes total sense)

I might have some OHP left somewhere, going to try that some other day.

[T3sl4co1l]

Yes, just that shape -- but if you want a lower operating frequency, try for >50 turns, probably using even thinner wire.

A coin will increase the magnetic coupling between the 2 coils not reduce it. This is because air has very low magnetic couoling. Most metals/alloys have bettwen than air megnetic coupling.

No, it will block.  Very few coins are ferromagnetic at these frequencies, and probably none that are the size and value OP is looking for.  Those are usually a bronze or cupronickel alloy, silvery or golden in color, nonmagnetic, and reasonably conductive (thus, blocking AC fields effectively).

Even at low frequencies (10s of kHz perhaps), I doubt most magnetic coins* would increase transmission.  They're simply too large, and eddy currents (especially from the plating) will block more than transmit.  You're also blocking the field anyway, if the arrangement is two coils, smaller than a plate, on either side of it; this is true even if the plate is ferrite.  (But many arrangements can be contrived which result in more or less field, depending on material.  For example, coils that are on equal centers but rotated perpendicular, against a diagonal plate.)

*That I'm aware of: some very rare US cents (1943ish); most countries' smallest denomination, cents or nickels (usually copper plated for corrosion resistance and appearance); and moderate age Canadian coins (5-25c pieces were high nickel).  I haven't looked at an exhaustive list in a long time, so look it up to be sure!

It occurs to me, if you need great sensitivity, and many degrees of freedom, to identify not just coin presence, but size and material as well: you could set up a two port network analyzer, basically, and measure reflected and transmitted power and phase.  Phase will vary with resistivity and permeability (skin depth) and thickness.  Even better if it's done over a wide frequency range, so the material thickness becomes apparent (skin effect is only an exponential cutoff in an infinite solid; in finite materials, waves echo back and forth, forming weak standing waves).

Tim

[derGoldstein]

The goal is to detect a coin going through a slot using induction, correct?
Why not use just one coil, and "measure" its inductance? Form a coil the shape of an oblong race track:



Design it so that it's just wide enough to allow a coin to fall through. Now, use a frequency generator and create an LC circuit using your coil and a capacitor of a particular value. With just air inside the coil you'll get a specific, constant frequency output. Once a conductor is dropped through it, the frequency will change, and you can detect and log that change. Every coin will be made of a different alloy and have a particular mass, and you can make a list of the frequency changes of each type of coin.

Circuits:
http://electronics.stackexchange.com/questions/61858/whats-the-best-easiest-way-to-measure-an-unkown-inductance-without-an-rlc-meter
http://www.doc-diy.net/electronics/l_meter/
http://electronics-diy.com/lc_meter.php

[pyroesp]

Well, guess I'll just have to experiment and see what happens, 'cause I'm a bit confused now...
I just want to detect 50c, 1EUR and 2EUR coins. I'll post some results whenever I finish this little project. I might make a new post on my website.

It occurs to me, if you need great sensitivity, and many degrees of freedom, to identify not just coin presence, but size and material as well: you could set up a two port network analyzer, basically, and measure reflected and transmitted power and phase.  Phase will vary with resistivity and permeability (skin depth) and thickness.  Even better if it's done over a wide frequency range, so the material thickness becomes apparent (skin effect is only an exponential cutoff in an infinite solid; in finite materials, waves echo back and forth, forming weak standing waves).

Tim

Very interesting, but I don't think I will (hope I won't) need something like that.

The goal is to detect a coin going through a slot using induction, correct?
Why not use just one coil, and "measure" its inductance? Form a coil the shape of an oblong race track:



Design it so that it's just wide enough to allow a coin to fall through. Now, use a frequency generator and create an LC circuit using your coil and a capacitor of a particular value. With just air inside the coil you'll get a specific, constant frequency output. Once a conductor is dropped through it, the frequency will change, and you can detect and log that change. Every coin will be made of a different alloy and have a particular mass, and you can make a list of the frequency changes of each type of coin.

Circuits:
http://electronics.stackexchange.com/questions/61858/whats-the-best-easiest-way-to-measure-an-unkown-inductance-without-an-rlc-meter
http://www.doc-diy.net/electronics/l_meter/
http://electronics-diy.com/lc_meter.php

That's also a viable solution I didn't think about. Could be interesting to test this too, may be better than 2 coils as tx & rx.
This will need a redesign of the coin slot thing I've made in openscad. I'll post a picture after work if I don't forget about it .

[MosherIV]

Fair enough T3sl4co1l, I must admit I am no expert in the subject.
I know you can induce currents in non ferric metals, I worked in the NDT industry and there are companies that make Eddie current devices for testing metals, works for Aluminium if I remember correctly.

FYI the UK 1p and 2p coins are now copper plated steel, previously they were pure copper (or some high copper alloy). the £1 coin is the cupronickel alloy, silvery or golden in color, nonmagnetic that you mentioned. Not sure what all the other coins are.

I'm a bit confused now

Sorry. Yes, experiment.

FYI, if your need a coin mechanism, try this :
https://www.sparkfun.com/products/11636
They do a simpler/cheaper version. If you are experimenting for the sake of it - fair enough   

[pyroesp]

FYI, if your need a coin mechanism, try this :
https://www.sparkfun.com/products/11636
They do a simpler/cheaper version. If you are experimenting for the sake of it - fair enough   

I had seen one of those before but could only accept one coin if I remember correctly, obviously not the same as this one.
I'm still going to try and make it myself, for the sake of learning.

If it doesn't work then I might buy one of those things.

[AF6LJ]

Well, guess I'll just have to experiment and see what happens, 'cause I'm a bit confused now...
I just want to detect 50c, 1EUR and 2EUR coins. I'll post some results whenever I finish this little project. I might make a new post on my website.

It occurs to me, if you need great sensitivity, and many degrees of freedom, to identify not just coin presence, but size and material as well: you could set up a two port network analyzer, basically, and measure reflected and transmitted power and phase.  Phase will vary with resistivity and permeability (skin depth) and thickness.  Even better if it's done over a wide frequency range, so the material thickness becomes apparent (skin effect is only an exponential cutoff in an infinite solid; in finite materials, waves echo back and forth, forming weak standing waves).

Tim

Very interesting, but I don't think I will (hope I won't) need something like that.

The goal is to detect a coin going through a slot using induction, correct?
Why not use just one coil, and "measure" its inductance? Form a coil the shape of an oblong race track:



Design it so that it's just wide enough to allow a coin to fall through. Now, use a frequency generator and create an LC circuit using your coil and a capacitor of a particular value. With just air inside the coil you'll get a specific, constant frequency output. Once a conductor is dropped through it, the frequency will change, and you can detect and log that change. Every coin will be made of a different alloy and have a particular mass, and you can make a list of the frequency changes of each type of coin.

Circuits:
http://electronics.stackexchange.com/questions/61858/whats-the-best-easiest-way-to-measure-an-unkown-inductance-without-an-rlc-meter
http://www.doc-diy.net/electronics/l_meter/
http://electronics-diy.com/lc_meter.php

That's also a viable solution I didn't think about. Could be interesting to test this too, may be better than 2 coils as tx & rx.
This will need a redesign of the coin slot thing I've made in openscad. I'll post a picture after work if I don't forget about it .

I use to work on this stuff a long time ago....
This technique is old and well refined.
Any time you want to detect a coin say your 50 cent piece, to do it well you need sets of coils...
One that reacts to the metal content that is smaller than the actual coin.
The second set is larger than the coin / coins to be detected, because Size matters.

Back in the late seventies I was working on equipment that used NRI GMBH and Tiltman Langley coin validators. The former used two coils and a custom IC that was the front end of the coil validator, followed by a bunch of 4000 series CMOS.
The latter used a photo sensor array to detect coin size and a coil to measure material content. This was made of no custom ICs and lots of mixed analogue and digital logic.

You also are going to need a supply of 50 cent coins for tuning, because believe it or not there is an amount of variation between coins...

Oh and one more thing, neither system used flat coils, they both used coils wound on a form that set in half of a powdered iron pot core.

[pyroesp]

I will do some experiments, to determine the best coil shape/nb windings/sensitvity/and such.
In the mean time here are a few pics of the coin checker body in opescad (with a cross section of approx. 1/3 width gone):
The size of coin is customizable as is the slope angle and slope/fall length.
Already 3D printed it (very poorly though) and it seems to work fine .

[AF6LJ]

You could do this in a vertical format and drop the coin down a shute..
let it roll on its edge past the sense coils and use a solenoid to push a ramp so the rejects go into a separate container. Both the coin validators I mentioned above use that method. The rejected coins would end up in the change cup... the good ones get counted...

[pyroesp]

This thing just has to "feel" like an arcade coin thing. I don't think I'm going to add a container of sorts.

Also, the slope helps with the detection. Here's my thinking: The small slope means that the coin rolls slower, which means it's between the coils longer, which means more data can be read and analysed, which I hope will give me better accuracy.

[AF6LJ]

This thing just has to "feel" like an arcade coin thing. I don't think I'm going to add a container of sorts.

Also, the slope helps with the detection. Here's my thinking: The small slope means that the coin rolls slower, which means it's between the coils longer, which means more data can be read and analysed, which I hope will give me better accuracy.

The transit time for a coin in an NRI coin validator was less than half a second.
Reading a coin's material content doesn't take long.

Just as an aside...
The NRI rep who taught the class I took mentioned that the mix of materials that was proposed for the coinage to be used in the future EU were of an alloy content that would not only be easily traceable but would be costly in small batches, thereby limiting the possibility of counterfeiting. We had a future project wherein we would need to make the fare collection equipment we built compatible with the new Euro.

[derGoldstein]

I will do some experiments, to determine the best coil shape/nb windings/sensitvity/and such.
In the mean time here are a few pics of the coin checker body in opescad (with a cross section of approx. 1/3 width gone):

Slightly off-topic -- have you tried FreeCAD, and if so, how does it compare to openscad? I've been using freecad for parametric stuff mainly because it seems to be the most popular and has a lot of plugins, but I haven't tried openscad yet to compare the two.

[pyroesp]

I used it for a bit, didn't really like it. For what I wanted, it was over complicated and I didn't find it intuitive to use at all, all tough it might just be me not using it correctly.
To use openscad it helps to have a bit of programming know-how and use this cheatsheet : http://www.openscad.org/cheatsheet/index.html.
I was a bit skeptical at first, but when I first tried openscad I was really impressed.

I only use openscad now.
Just try it out, you might or might not like it. I sure do .

[Michael Lloyd]

Would winding it like this work?

[pyroesp]

I thought you had to make coils a certain way and not just twist and turn the copper wire, or is it ok to do whatever as long as the turns are in the same direction and somewhat same diameter?
Like my flat coils I tried to make are 1 layer 0.26mm thick copper wire, starting from a nail in the center of a wood plank, turning CCW (~35 times) and making sure it stays flat. No wire overlapping or such.

A few years back, I remember a friend making a solenoid actuator, which was powered on something like 400VDC (huge capacitor bank) to shoot some plastic balls, very cool and dangerous.
Anyways, he made sure the winding was in the same direction and never to cross 2 windings, except for when he made the next layers.

If winding it like that, doesn't affect the magnetic field (too much), then I might try making one like that.

[T3sl4co1l]

Always wrapping around in the same direction, is basically all that matters.

There is proximity effect, which depends on how close the wires are to each other, but this is only important for high efficiency or high Q coils.  There is also inter-layer capacitance, which depends on the order of turns (but not how organized they are; indeed, perfectly even and flat layers will have more capacitance than random-wound layers, because the wire fits together more closely!).  But that's also not important until higher frequencies.

Tim

[jonovid]

make a coil bobbin out of two circles of flat  clear plastic.your mold,  then wind on the wire. you will need varnish or epoxy resin to hold the wire till & when dry,   peel off the bobbin circles when dry  & remove the centre core.  make sure that all the sides of the bobbin are smooth so the clear plastic will peel off.  I know from experience varnish or epoxy resin will peel off a flat surface. but a bundle of wire is set hard inside the resin.  OR you can open an old HHD  hard drive as they have a flat coil in side them.