How can i make a metal detector

		Quote:			Posted by xavier
	....you were going to build a PI detector. Did you get it started yet ...

Hi Xavier,

Yes, it took a while to figure which PI to build. I had a look at the Hammerhead, Gary's PI and finally settled for the XR-71.  I decided to build a PI first as it is perhaps easier with less fuss involved when it comes to making useable coils, and will probably dig into building a TSG afterwards. Your goodie will be posted early next week.

@Klaipedos - Hi, it seems we've hijacked you're thread - hope you don't mind. But to address your question - it is assumed/necessary to have a basic knowledge of electronics, that you can read a schematic and that you have some tools (soldering iron and digital multimeter) and some practiced skills such as soldering. You can possibly order a kit?, or make your own PCB or send a layout file to a PCB company. I recommend studying about the different types of metal detectors before diving in as each presents different set of challenges. Some projects may be easier while others require some experience and perhaps additional tools such as an oscillosope. Hope this helps.

Best regards
Xero




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		Quote:			Posted by xavier
	Hi Goldigger Thanks for the schematics this will help I would just like to know for the RF in could I use a voltage controlled oscillator and connect that to it?  : Schematic num 200911121400 Regards Xavier Hi Xeronium Thank you very much   now I will be able to experiment with programmable chips on the other hand it means that I will want to learn how to write the program and that dose not sound very easy to me  [teach]is there a tutorial some where that I could have a look at? @Klaipedos Some times jumping in the deep end of the pool is a good idea but it?s all ways nice to know that you have someone close by who can swim my advice to you is start building things like flashing LED?s and then an AM transmitter with a receiver learn your colour code (resistors) and how to test a transistor and so on pretty soon you will have all the basics(you have to burn your self to know the meaning of hot)    . And this is where I do not agree with our friend Xeronium DO NOT BUY KITS you just wont learn much from them and getting it done by your self is so much more satisfying. Once you have acquired more knowledge then you could go for kits. Personally I don?t like kit?s there is no challenge in them for me. Best regards Xavier

Darn! It has been a couple days and I can not remember what I said and the 2 files I was going to upload. 

The VCO, well I think that was my plan, but you need a frequency counter. I have been searching the net for a suitable ICM7217 counter, as it is all in one chip and does not need to be programmed. It has one problem: only 4 1/2 or 3 1/2 digits, so 150 kHz, for example, would read: 150.0 This means only one decimal place.  The 4553 does 3 digits and can be ganged for 6, or 9 digits, that gets complex.

So, then I looked for pre-programmed PIC based counters, and have a mass of stuff to go through, yet, so I can not say if I was successful! 

The PIC counters drive a 1 or 2 X 16 LCD display, which can be cheap, if one shops carefully. Spark-Fun has a kit for about $40, but that is not quite so good for building into something.... I am still looking.

I have 2 counters, neither of which will measure the frequency of square waves, is that not ironic? Sine is OK and triangle is OK.

LOOK MORE now.

goldigger

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		Quote:			Posted by xavier
	Hi Goldigger Well I build the resistor bridge but when I wanted to place the 1uF cap I found that I only have 0.1uF caps DAM I'm mad   and that on a Sunday on top of that the weather is lousy and I cant even go down to the beech  .I made a square wave oscillator that I used on the RF in but I have to change the VR as I only had a 100K and I need a 1M to have a 50/50 wave or dose that not matter?. I would like to know if I want to measure a 0.1uF cap what should the frequency be? Also up to what capacitance would that frequency be good for. And should I not place a rotary SW with pre determined frequency?s? well I hope that you are having a better Sunday than I am. Best regards Xavier

OUCH!

Well, first off, I do not know how well  a square wave will work, because a square wave is a mass of odd harmonics of the fundamental! A simple Wien op-amp oscillator is easy to build and you should use a buffer to avoid interfering with the frequency. 

How do you know what frequency you are using? If you use a 30.72 kHz or 32.78 kHz crystal and divide with a CD4040, you can get a number of frequencies, but they are still square! A simple 2 resistor and 1 cap filter will make a much more acceptable triangle wave. Suppose you use 15.36  (Q1 from 30.72 kHz), and 2 - 4.7k resistors, with .027 ufd cap: one resistor goes to the 4040, the cap goes to ground, with the other resistor across it. Take off the filtered wave at the top of the 4.7k and .027uf..... 

The formula for this is f = parallel R1,R2 X C..... so if R1 and R2 are the same, divide the value by 2 (2.35k in this case.)  F = 1/.0027 X .027 (Use the parallel value of the 2 resistors.)

Use a 5 k pot across the cap and you have a way  to vary the filtered signal to an op amp.

How big a pot are you using in the bridge? I would say it is safe to use 100 ohms from either end, and maybe 10 ohms. So, for a 5k pot; 100 to 4900 ohms or 10-4990

If you use a FIXED 100 ohms and adjust the FREQUENCY, until it nulls,  use: L = 100ohms / F in mHz X 6.283

By doing a bit of calculator work, this works out to:

  100 uHy = 159.159 kHz
  500 uHy = 31.832 kHz
  1000 uHy = 15.915 kHz
  5000 uHy = 3.183 kHz
  10,000 uHy (10 mHy) = 1.591 kHz
  50 mHy = 318.3 Hz

  formula to find likely frequency using 100 ohm resistor, in the bridge, instead of a pot:

    100 ohms / L X 6.283

Am I answering your questions OK?

OK, with the square wave osc, it sounds like you are using an LM/NE555?? forget the duty cycle, a bit, anyway, if you use a D type flip-flop and divide the oscillator signal just once, it is now 50% duty cy, but half the frequency. If you want to use both FF's, you will have to make the oscillator work at its upper range.... a 7555 would be a bit better, or a LM567.

The  problem with the 2 resistor + 1 cap, is it is frequency sensitive, and would work best with a crystal derived frequency.... but you have to have a pot. in the bridge, not a fixed resistor. b This is why I suggest a Wien osc., because it is sine, and variable. You can have several ranges by switching or plugging in the capacitors. I have been  collecting Wien circuits and will upload one, eventually.

I have a circuit designed for measuring home made detector coils and will  eventually upload that, too.

I have to run.... will continue this next posting. 

goldigger



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		Quote:			Posted by xavier
	Hi Goldigger Thanks for the info much appreciated I did place a cap and resistor as a buffer but not the same values and I was getting a sinusoidal wave on the other end so that should be OK but will have a good look at all that. Regards Xavier

Yeahbut! By BUFFER  I  actually meant a buffer amp.

OK I am uploading 2 versions of an as simple as you can get Wien oscillator. If you can not get an LM353, 3 TL081s or TL071s can be used but they MUST have a split supply.... they go weird with a single supply. The use of LM353 allows a single supply and the are good to at least 1 mHz, not so the LM741.

I think the circuits are straight forward and do not need much explanation.  The Wien osc. is good to feed an LCR bridge

goldigger

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		Quote:			Posted by xavier
	Hi Goldigger Thanks for the schematic's as for the supply I always use split voltage on my op-amps +6 -6 virtual ground. This one that I build is not working it?s 60% out probably the frequency that?s no good (not sinusoidal) and yet when I looked at it, it was sinusoidal and it is drifting but then I am using a sound card scope (not very good) I am going to the workshop to take care of this Wien Bridge just got to get it working right. Once again thanks for the schem . Best regards Xavier

Sounds good, you will get it licked, eventually.... The one resistor (where it goes, I am not sure, this minute,) Is supposed to be a positive temperature coefficient, meaning the resistance rises with heating. 

I think the bulb, in the inverting input circuit of the op amp, is the resistance in question. It is a low current higher voltage and I think a 12v, 50 ma grain-of-wheat bulb will work. The article mentions 24 v, 60 ma. The resistor that goes from the inverting input, to the output (the feed back resistor) may need to be adjusted a bit, for a 12v, 50 ma bulb.

I do have a circuit that uses an FET, instead of the bulb, but it is complex. 

Drift might be fixed with zero coefficient caps. Try to use metal film resistors, too. Mylar caps are supposed to be good. 

The circuit I uploaded does not use nor need a split supply, if you use a split supply, it will screw up as it was designed for a single supply.

What I said was: IF you use anything but an LM741, which was designed for single or split supply, for the buffer, such as a TL081, the TL081 HAS to have a split supply, it will not work with a single supply, like the 741 will. I have tried it, long before I discovered this... I almost swore TL081's were no good for anything!   

I will eventually try this circuit (I copied it and embellished it a bit,) when I can get a programmer and type in a listing for a PIC16F84, to programme it for a freq. counter. I have a clutch of 16F84s, a clutch of 16F628s and some 16C54 OP's. To used the OP's, (one-time programmable) I need to have a 16F84 circuit to emulate a 16C54, and be sure the programme works, first!! 

goldiigger

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