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Offline m_electTopic starter
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« on: February 13, 2010, 05:41:54 am »
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HI
any one can buld this metal detector
good lock

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Offline Cornelius
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« Reply #1 on: February 13, 2010, 08:15:54 am »
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Let me know where I can get the transformers  and I will build it .   Cornelius

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Offline GoldDigger1950
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« Reply #2 on: February 13, 2010, 03:55:40 pm »
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Quote:Posted by Cornelius
Let me know where I can get the transformers  and I will build it .   Cornelius


They are both hand wound on plastic bobbins and have a ferrite case around them. The parts can be purchased from Mouser here:

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Whatever you do, Cornelius, order one of their free catalogs. They sell current electronic parts with full size and specifications for each part. A fantastic reference whether you order the parts to hand wind your transformers or not.

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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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« Reply #3 on: February 13, 2010, 04:39:56 pm »
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Hi GD

I see you found a new compass  Smiley nice I have a question for you if I build an oscillator that runs at a frequency of lets say 16Khz (sine wave build using a dual Op-Amp) and I use a receiver build in an LC configuration will there be a compatibility problem between the two? I don?t see any reason why there should be and yet I seem to have a problem any ideas?

Regards Xavier


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Offline GoldDigger1950
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« Reply #4 on: February 15, 2010, 02:50:20 pm »
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Xavier, an oscillator alone wouldn't work but assuming you are feeding the output of the oscillator to a coil that matches the receiver coil, it should be compatible irrespective of how the signal is made. You need to get the signal "out there" by feeding it to an antenna, which is what the transmitter coil really is. It's an antenna radiating the 16khz into the free air space between it and the receiver coil. If all goes well, it should receive the signal and be a constant which will only vary when something gets in the way like metal. Hooking it up directly won't work either since the oscillator signal is an AC signal and must be isolated from the receiver by at the very least, a capacitor.

The coils in this case would be facing each other with a gap in between. All that fancy circuitry you see in most metal detector designs allows the same coil or coils situated in the same plane to transmit and receive in turns. They never transmit and receive at the same moment. If you did this, the transmit coil would swamp the receiver coil in the same plane. Thus, all those timing circuits and reverse diodes to keep the signals routed properly are necessary to allow the coil to transmit downward and the receiver to sense any residual eddy currents in the underground metals as they are transmitted upwards. Actually, they are transmitted in all directions but we're only interested in the ones that come back to the receiver coil.

This may be more than you wanted in an answer but the basic theory is this. You transmit a frequency of 16khz into the ground and then turn off the transmitter. As that signal propagates, it diminishes by the distance squared plus any soil attenuation. A very weak signal in the form of an eddy current is set up on or in metals in the earth. In the case of ferrous or magnetic metals like iron and nickel, eddy currents are set up inside the underground metal material and they become miniature, weak transmitters for just a few milliseconds. That transmitted signal is received by the receiver coil and interpreted as a coherent eddy current. When you irradiate non ferrous metals like gold and silver, the eddy current is set up on the surface like a bubble of magnetism which then collapses and creates a very weak signal which is again received but as a scattered eddy current.

Simple circuitry will interpret these two signals identically. Complex analysis of the return signal will allow what we know as discrimination. There are many factors and characteristics of the return signal that can be analyzed with the two primary ones being the timing of the return and the phase angle of the return. Very technical stuff, that, but in a nutshell and simplified, the ferrous metals return fast and with an aligned phase while non ferrous return later and with a phase shift. This is due to how the eddy currents are set up inside or outside of the metal.

For the terminally bored, this is almost over. For the technically minded, I greatly simplified this intentionally with the intention of allowing the average reader here to at least get a grip of the basics. I didn't even touch on ground balancing as that is very, very difficult to explain even to a technically minded person.

This is the end of my ramble. You may now return to your regularly scheduled forum topic reading.

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« Reply #5 on: February 15, 2010, 06:18:42 pm »
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 Grin Grin Nice I like the end part

Ok now here is what is happening to me I build a prototype VLF 9.5Khz it works and can detect up to 170mm (in the ground) for a small coin that would be about 20mm diameter but the dam transmitter was not stable so build one with a dual OP-Amp at the frequency of 9.5 Khz with the idea to do the testing with the original coil and if all worked well push it up to 16Khz and make a new coil but the thing is that I?m getting a signal from the receiver all the time just like if I was detecting metal. It could be the strength of the signal transmitted so I tuned it down but still the same I played with the frequency but still no change. All the circuitry is the same just a new transmitter.
GD do you have a diagram of a frequency analyzer that you could post?

Regards Xavier


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Offline GoldDigger1950
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« Reply #6 on: February 15, 2010, 07:36:40 pm »
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Quote:Posted by xavier
Grin Grin Nice I like the end part

Ok now here is what is happening to me I build a prototype VLF 9.5Khz it works and can detect up to 170mm (in the ground) for a small coin that would be about 20mm diameter but the dam transmitter was not stable so build one with a dual OP-Amp at the frequency of 9.5 Khz with the idea to do the testing with the original coil and if all worked well push it up to 16Khz and make a new coil but the thing is that I?m getting a signal from the receiver all the time just like if I was detecting metal. It could be the strength of the signal transmitted so I tuned it down but still the same I played with the frequency but still no change. All the circuitry is the same just a new transmitter.
GD do you have a diagram of a frequency analyzer that you could post?

Regards Xavier



Actually, that may be easier than you think. There are a number of DIY projects that use the sound card in your PC for an audio frequency analyzer known as a Spectrum Analyzer. Do a search on those terms and you'll find a few simple circuits that make use of what you probably already have. Don't try to use it above 50khz, though. Your sound card has limits.

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Use Google to search using the following terms for more: make diy audio spectrum analyzer

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« Reply #7 on: February 16, 2010, 02:54:43 am »
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Hi GD

Got the PC scope works quite well apart for the fact that you are limited to 20Khz the best one that I have found is Zeitnitz you can get it at Christian@Zeitnitz.de it has a free registration but I would like to build one that I could take with me when I do my field tests to see what is actually happening on the receiver side of things then use that info for further development like for frequency analyzer then using a programmable chip PIC
And a display to indicate whether it?s gold or silver .later on I want to translate that data to graphics. I will need to use a set of receiver coils we did have a chat about that some time ago where I was thinking of using three coils and you said that you were also working on a design but with 16 coils (if my memory serves me right) this brings me to something else that I am looking for , a list of the different frequencies that different metals would best respond to Do you know where can I find one?

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Xavier


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Offline GoldDigger1950
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« Reply #8 on: February 17, 2010, 05:40:53 pm »
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Don't go getting into the heeby jeeby stuff now but do a Google search for metallic resonant frequencies to figure out your own chart of metallic response frequencies. Most resonate in the Mhz range so that's a useless number unless you use a harmonic. When you do find a nearby harmonic that fits into the audio range in which you are working, find the next nearest harmonic and use a point in between them as your target frequency. Don't for a second believe that the resonant frequency or a harmonic is what you want. You want the midpoint between two harmonics. The resonant frequency, when transmitted into a metal, is completely absorbed with no residual field strength reflected. The next harmonic will also absorb but not 100%. Then on down to perhaps the 32nd order harmonic and you might find yourself in the audio range. Here you might find a 70% absorption but that 30% leftover most often isn't enough. You need to get to an area between the harmonics where absorption does NOT happen and that is in an area between the harmonics. That way the signal is not absorbed but is aligned along the surface making your metal target into a miniature transmitter with maximum return signal.

Using a variable oscillator in the audio range and a wideband receiver antenna will allow you to crank through the audio range and use empirical rather than calculated measurements. Watching the output of your receiver and marking the best reflection frequency will zero you in on your target metal. Audio signal generators are useful for this and can be found in second hand shops and pawn shops dirt cheap. You might even befriend a local technician and get him or her interested in helping you.

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« Reply #9 on: February 18, 2010, 05:49:16 am »
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Ok I'm with you there GD and that?s what I'm looking for thanks for your help
Can you then tell me what is the formulea to find the next harmonic? I read that if you have a 20Khz transmiter a 60Khz reciever will have a better reception so from that I can take it that 60Khz will be the third harmonic true or false? 

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