How can i make a metal detector

Hi Mike

Thanks for all the info I really do appreciate it let me tell you what I did .I found a lot of schematics on the net but they were either to complex or to simple like BFO and so I took ideas from a few of them then put it all together and it worked it did however take me the better part of eight months to gather all the info then draw up the schematics get all the parts ho by the way where I live it?s hard to get parts so most of them I get out of old VCR?s radios and so on even an old telephone any how I use a sound card scope not the best thing but it gives a very good idea of the frequency and what the heck if it?s out by 10% or more as I will have the same error on all my measurements  I tuned the transmitter at 8Khz I was thinking to tune it up to 14 or 16 KHz but now all I want to do is to get it going the right way again LOL As you say it?s more than probable that it?s the most ridiculous problem and that I am over looking it all the time the thing that gets me is that it works fine just that it works back wards I just don?t get it .

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Xavier

Hmmm I will have a look but I think that I can find all that I need right here thanks any how

Happy hunting
Xavier

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Xavier

I'd like to build a detector that has deep penetration and excellent discriminating capabilities when needed. And it would be neat to build one that has vlf,tr, and bfo or pulse in sanme unit--kinda hitting all possibilities with one detector!

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		Quote:			Posted by xavier
	Hmmm I will have a look but I think that I can find all that I need right here thanks any how Happy hunting Xavier

I was going to ask, when you wound the new coils did  you use the same size wire and same number of turns?

One turn out (so simple) can make a big difference in resonance.... its not hard to do.

Are the TX and Rx circuits both resonant circuits? I ask because I have a design (circuit) for a balanced detector that has no designed resonance, anywhere. But it also has no rejection nor ground balance. (does not need ground balance.)

I might make a suggestion, make about a 15cm loop, connect a diode to it,  and a volt-ohm meter (DVM) to it. Lay this coil right  on top of the Tx coil and take a meter reading. Write this down, put the old coil back on and do the same for it... if  the new coil is substantially lower in output, this may be the problem, or part of it.

One sneaky way to measure frequency is with a radio capable or receiving CW or sideband transmissions, a lot of  radios, made in the last 30  years, will do this. While it is not as accurate as a frequency counter, it has been in use for a long time.

Tune to around 100 kHz, turn on the detector, and find its harmonic signal, mark down the exact frequency and then continue  to tune upward until you find another harmonic. Subtract the 1st frequency from the second and this is usually double the fundamental frequency. Unless this is a non-digital, analogue machine, it will be all odd harmonics, while an analogue oscillator will produce both. (an analogue, 8 kHz will have a tone every 8 kHz.)

For 8 kHz, odd harmonics, you should find one at 104 kHz and the next at 120 kHz... next 136 kHz .... 152 kHz and so on.

18 kHz fundamental will be 126 kHz, 162 kHz, 198 kHz, etc. They are all odd harmonics but double the fundamental, apart.

Of course, the tuning is *zero-beat,* and you tune for zero between the tones that indicate a signal presence.

This can be done at a higher frequency but the harmonics get weaker.

Just a couple suggestions.

goldigger

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		Quote:			Posted by xavier
	Hi Goldigger THANKS a lot for that valuable information I thought of using a radio for the tuning but I did not know how to go about it one's again thank you very much I will be doing tests in just a few moments. I did find some problem with the bucking coil and got that sorted out now the coil works but another phenomena has produced it self I detect non ferrous metals such as gold, silver, copper, bronze and alloys with out any problems (I get an audio signal) but I have no audio signal on ferrous type of metals such as nails I have to look at the UA meter or I miss them come to think of it I involuntarily made a discriminating device . Hi Mike Yes you are quite right I did have the same problem when I was still on the bread board and it did get me going for quite some time but I did find the problem as you know I took the working coil of to try the new coil well all that it was was an adjustment of the loose wire that came from the bucking coil it just needed to be moved a few cm away from the TX coil Regadrs Xavier

Very interesting problems!

There  used to be an LC bridge, for sale, years ago, in the surplus stores, that was made in the 30s, and it used the zero-beat method, but not as I described. I said odd harmonics, because that is what you get with square waves and many non-digital oscillators are over-driven and produce square waves through clipping. I found out about the odd harmonic bit through some article I read, 2 or 3 years ago..... so I am not so smart! 

I was wondering what country you were in, if you would like to say. Myself, I am in Canada, and all parts, mostly, have to come from the US. One of my favourite sources is BG Micro (bgmicro.com,) I also like the Electronic Gold mine and Jameco. It is easy to get the addresses by Google search and most of them have a download-able pdf catalogue. And they all will do mail delivery, as long as they get enough for shipping.

I was thinking of trying to build a simple induction balance detector, I would also have to go through the careful coil placement so it is nice to see what others are doing. 

My radio (Ham and all-band) will receive right down to zero Hz, so, as a zero-beat measuring device, I have it made! It is easy to work on frequency checking a VLF detector or the radio Lowfer band. (115 to 175 kHz, or there about.) It detects upper or lower sideband and code, but not FM. I want to design a phase detector for the Lowfer band, to operate VMSK, as they are calling it. This sends and receives serial data at 90 degrees phase shift.

To work the Lowfer band I am going to have to wind some BIG coils, 3 meters across and about 12 to 20 turns. I need a pair of these, one 90 degrees to the other, and fixed so I can turn both as a single assembly. Just a bit harder than making a 2 box detector. 

goldigger

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