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[on:]

Hello everyone this is my first time here! I am interested in electronics, talked about getting an arduino for years, and recently bought one for myself!

I need all your help in guiding my first project in the right direction!

The main goal of my first project is to keep silverware out of the trash at work. I work at a restaurant, and a lot of silverware gets thrown away on a daily basis by accident.

My idea is to tune some type of metal detection to pick up when a silverware passes by into the trash can. If I can get the arduino to recognize the silverware passing as a certain numeric value, I can use that value to tell the arduino that it's time to sound a buzzer and flash some lights. And yeah basically the idea is a metal detector fit over a trash can lid.

So here's the problem. I did a bunch of research on how metal detectors work, and came up with a bunch of information. I found this project which makes me believe this is feasible:
I've been reading this article too http://www.deeptech-bg.com/Tumanski.pdf which has a lot of great information, I just don't know how to interpret this into something I can use for my project. I don't want to make something more complicated than I have to, so any insight you may have would be greatly appreciated!!!

I don't have time at the moment, but I have done some more research, and can explain a little more about what I think I know about VLF, and EMF detection, which may help you help me more. So i'll post more tomorrow, and hopefully get some feedback in the meantime.

Thanks!  

[Reply #1 on:]

Just found this also:
http://electronicdesign.com/components/consider-all-factors-when-selecting-proper-inductive-proximity-sensor

This article definitely helps me realize I have to utilize the right sensor and design. I have a specific goal of detecting one type of object, so it should be pretty easy  . I just have to understand the principles on how these types of sensors work, and go from there. Any insight would be appreciated!

[Reply #2 on:]

That video does not show a metal detector.

To detect metal, you need some extra hardware.
Can you make a big coil over the trash can ? Just wind a long wire a number of times (5 to 100) around the top of the trash can. You need a circuit that puts a RF signal on the wire and detects a disturbance.

Since the metal is going through the coil, it will be easily recognized.

This is a normal metal detector circuit, connected to the Arduino.
http://dzlsevilgeniuslair.blogspot.nl/2012/06/metal-detector-for-bikes-and-cars.html

This creates a frequency in a coil, and the Arduino could detect a change in the frequency.
http://www.instructables.com/answers/Can-I-Connect-A-Metal-Detector-Coil-Directly-To-A-/

I think it would be possible to let the Arduino create a pulse for the coil, and detect the received pulse length. But I have not seen such a circuit yet.

The alternative is a compass and detect the disturbance of the earth magnetic field. But that would probably not work in your situation. Search for "HMC5883L metal detector" if you want to know more.

[Reply #3 on:]

I've seen a documentary showing a big restaurant doing this with a really, really big magnet to separate utensils from the food as it was thrown into the waste bin.

[Reply #4 on:]

That's what I was thinking - a big ass electromagnet to collect up any metal thrown in.  Silverware, rings, loose change, iphones, ...

[Reply #5 on:]

That's what I was thinking - a big ass electromagnet to collect up any metal thrown in.  Silverware, rings, loose change, iphones, ...

Not all metals are ferrous, stainless steel flatware for example would pass right through even a big ass magnet.

Lefty
 

[Reply #6 on:]

If an electric current can be induced, a reaction with a magnetic field can be made.

[Reply #7 on:]

I imagine that people going to the trouble of installing a system like that would ensure that the cutlery they used was suitable for it.

[Reply #8 on:]

That's what I was thinking - a big ass electromagnet to collect up any metal thrown in.  Silverware, rings, loose change, iphones, ...

Not too good for those of us with pacemakers!
And if the 'silverware' is Sterling Silver and the rings 24ct gold, how would that work?
I don't know about US loose change, but here in the UK, coinage is non-ferrous.

[Reply #9 on:]

Eddy current magnets:

http://www.nipponmagnetics.com/eddy-current-magnets-c-4.html

"Eddy current separators are useful for separation of aluminum, brass, copper, magnesium and other highly conductive but not necessarily magnetic attractive materials. "

[Reply #10 on:]

Thanks a lot CrossRoads... now how am I going to sleep? I can't believe I went through college physics and never came across this! Makes perfect sense, yet magical!

[Reply #11 on:]

Bah!  Silverware is a walk in the park... http://www.ru.nl/hfml/research/levitation/diamagnetic/

[Reply #12 on:]

In the food packaging industry, they utilize metal detectors when bagging food stuff.

These detectors can detect the tiniest amount of metal.  (they use these detectors to detect any pieces of metal that may have chipped/broken off their machinery that got into the packaging.)

I think this is what you need. 

[Reply #13 on:]

Thanks everyone! I knew I was going to get the feedback that pointed me in the right direction!

I really like these circuits:








The first circuit pictured links to an explanation of the circuit, but it's in Danish and didn't translate well enough for me to understand. But it looks like it uses an op amp, capacitors, and a transistor to create an oscillation through the coil that would produce a low radio frequency. How low would the frequency be? I'm sure you can calculate it somehow based on the values of the components but I have no idea how to do that.

If you click the link on the second circuit, it leads to a page that explains the circuit in good detail, but you'll notice the picture here has an added mosfet before output to the arduino. I think the purpose of the mosfet is to amplify the small current signal that comes from a disturbance (eddy currents skimming the surface of a conductive material silverware) in the electromagnetic field. PLEASE CORRECT ME IF I AM WRONG OR MISSING SOMETHING

I am thinking I will build the second circuit and experiment with different coil sizes and shapes, AND a variable capacitor for c1. I'll experiment and hopefully I can find the right combo that will give the arduino the best possible picture of what silverware will register as.

I think it would be possible to let the Arduino create a pulse for the coil, and detect the received pulse length. But I have not seen such a circuit yet.

I don't understand how the arduino is put together or how it works in detail, or what could really even be done with it, but that seems possible. Some one use your huge brain to figure this one out! haha

[Reply #14 on:]

The first picture can be connected to an analog input, and the Arduino should detect a change of the analog value.

In the second picture the fet is used to interface with the microcontroller.
That way some kind of open collector compatible output is created.
But the Schmitt-trigger output of the second 40106 gate is already a digital signal. You can tie that output to the Arduino pin, without the fet.
The Arduino sketch should be a frequency counter, which is a lot harder than the first picture.
Since you don't want to detect a coin at 5 meters deep, the frequency counter solution should work for you.

The frequency range of a metal detector could start at 1kHz, up to the MW range 150kHz ... 1500kHz.
So if you make your circuit somewhere between 5kHz to 10kHz, that seems okay to me.

The frequency determines what kind of metal is detected best. With a metal detector that uses two coils (a transmitter and receiver coil), copper and silver is detected best with 3kHz and gold with 20khz to 70kHz. That is why the expensive metal detectors have multiple frequencies.