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How does the BFO metal detector works?

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  • How does the BFO metal detector works?

    Hello everyone!
    I am skylly, I am new here, and want to learn more technical knowledge about metal detector from here.
    I attempt to make a simple BFO metal detector first.
    I know that the search frequency and reference frequency of BFO type is slightly different.
    But when I look at the "pocket metal detector" by Frank Hughes, url: http://www.sm0vpo.com:800/_visitors/...aldetector.htm
    The reference oscillator is 400KHz and the search frequency is 100KHz,
    why they are such a big difference and whether it or not to still work properly?

  • #2
    On this site you can find many pdf's of homebuild projects, including BFO's, which will include explanations of operation. There are also other technical papers, and even patents available.

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    • #3
      Originally posted by Skippy View Post
      On this site you can find many pdf's of homebuild projects, including BFO's, which will include explanations of operation. There are also other technical papers, and even patents available.
      yes I know, but my question is I think that one doesn't work...

      Comment


      • #4
        Not sure what you mean by 'I think it doesn't work', you mean you've built it and can't get it to work? You don't understand the explanation of it's operation? It looks vaguely like the multiplying mixer in another BFO project, the ETI561. Take a look at that project, it includes an explanation of the mixer operation.
        Edit: OK, I see your problem now, he's messed up the circuit diagram. That's not a D-type ff, the pin 13 should be connected to pin 2 of the NAND gate circuit, not the power supply.
        Edit again: I can see another reason it might not work. I've just seen the headphone socket wiring, he actually used the earpieces as the on/off switch. The full circuit power flows through the earphones. Hence why low impedance 'phones are suggested, high resistance ones will drop too much voltage. I've actually used a very similar circuit myself, to switch on a circuit using headphone insertion, though on my circuit I only used the phones to turn on a transistor, which then turned on the circuit. I thus only had micro-amps going through the 'phones. You can get headphone sockets with an electrically isolated switch built into it, usually activated by the tip of the plug pushing on it. These are a alternative way of achieving the power-switching.

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        • #5
          Originally posted by skylly View Post
          Hello everyone!
          I am skylly, I am new here, and want to learn more technical knowledge about metal detector from here.
          I attempt to make a simple BFO metal detector first.
          I know that the search frequency and reference frequency of BFO type is slightly different.
          But when I look at the "pocket metal detector" by Frank Hughes, url: http://www.sm0vpo.com:800/_visitors/...aldetector.htm
          The reference oscillator is 400KHz and the search frequency is 100KHz,
          why they are such a big difference and whether it or not to still work properly?


          Circuit will work, difference in ref. and search freq. is OK, harmonics of search freq. is used, eliminating problem whit two oscillators on same freq. locking together, typical issue whit BFO, this will actually work better increasing frequency shift 4 times. Properly connected, as on schematic, power supply and headphone will work (headphone is connected to mono, remaining contact is used for power). Don’t expect much from this ckt. performance.

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          • #6
            Actually, both headphones are used for power, connected in series, and they should both make a sound, too. The cheap earphones used in the photo will have a resistance of 20 to 30 Ohms, I recommend using similar resistance earphones.
            tepco is correct about the mode-locking of the oscillators, the fact that both oscillators share the same piece of silicon and the same power-supply makes coupling between them likely. The other reason these circuits are difficult to get working is the oscillators rarely work at the frequency they should, especially the R-C one.

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            • #7
              Look at the schematic once more, both earpieces are connected in parallel (mono), remaining pin is used to apply power to the device via –V.

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              • #8
                Yes, you're right, I didn't see that on the circuit (though he does mention it in the text). I don't think it would be easy to add the link in practice, these 'phones always have a moulded plug. I don't think those 'phones in the picture have been modified. Another practical problem is the fact that walkman-type 'phones are usually only 60cm long. This restricts the way you use the detector somewhat. I don't mind getting on my knees to dig, but I don't fancy searching that way!
                To improve the chances of this working, some power supply decoupling would help, 100nF and 10 microfarad straight across would be a guide. Interesting that his 12V battery has 7 cells in it. The more I look at the project, the more I think it will be a pain to get working.

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                • #9
                  Thank you for your reply, I already know about the working principle.

                  Few days ago, I was completely in accordance with the author's schematic to do a version, but I use 6v battery pack to do the power with a buzzer instead of headphones. I use 40106 for ic1 and 4011 for ic2.
                  But it does not work, no sound the buzzer. Perhaps I soldered a problem, when I have more time to carefully check the circuit board.

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                  • #10
                    Perhaps if you had told us all this in the first place we could have saved a lot of wasted time...... the 40106 is a Schmitt inverter, and won't work in this circuit, especially the L-C oscillator. And you might have trouble with the 6V power supply, 4000 series CMOS IC's don't work fast at lower voltages. And why did you change the earphones to a buzzer??? If you haven't got any earphones, use a loudspeaker, from 4 ohms to 60 ohms, such as the kind you get in PC's etc. It will be very quiet, but it will at least work.

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                    • #11
                      I have checked my circuit boards once, and found no problem.
                      and I use the sound card oscilloscopes check out the search circuit is oscillation.
                      So I suspect that is the principle of the circuit,
                      but you all say there is no problem on the circuit.


                      Sorry for my poor English.

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                      • #12
                        Please re-read all the replies VERY carefully. The answers are all there. Sound cards are completely unsuitable for diagnosing this kind of circuit.

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                        • #13
                          Hi, leave this schematics and
                          maybe this is an better start-up circuit for you:

                          http://www.extremecircuits.net/2010/...ocator_13.html

                          Google is your friend: before changing an IC to other type, please always check the IC-datasheet.

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                          • #14
                            I have built that circuit, it's the EPE mini-pinpointer one, by Scarborough. It looks simple, but again, it's difficult to get going. The LC and RC oscillators run at frequencies that are hard to determine, the power-supply needs good decoupling and regulation to avoid frequency locking. And the RC oscillator is sensitive to capacitive effects, so when you put your hand near the pot, the frequency changes a lot. It has poor temperature stability too, though that can be tolerated.

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                            • #15
                              This project might also be worth a look. It does go into some reasonable construction detail, so the chance of success is higher.-
                              http://www.easytreasure.co.uk/bfo.htm

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