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  • big coils

    im working on a big coil for looking for meteorites. it osc is 125k . checking (air test) i notice a frequency change at about 5ft with a plate. now the loop is about 4 1/2 feet across. im going to make a second osc at 125khz and combine the to freqencies. anyone got any suggestions for improving it. i here the higher frequencies get better penetration into the ground. true/false?

  • #2
    Take a look at the metal detector projects section on this site. There's one with a frequency multiplying mixer, I think it's the ETI561 project. That might give you some ideas, as might some of the other BFO projects.

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    • #3
      i have gotten the second ocs to 123k the first one with the big loop is running at 128k. i am wondering if i took the difference of the to(5k) squared it off and used a pic chip to measure the pulse width , would it be sensative enough?

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      • #4
        Don't let me stop you putting a micro in it, could be fun. But you will find your ear is just as capable. Set the difference freq to 200Hz, and you will find a frequency change to 220Hz is audible.

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        • #5
          i moved a small plate towards small coil and it drove the freq up to 132k . i will place the small coil in a box in such a way to enable the frequency adjustment by adjusting the plate distance from the coil.

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          • #6
            What you are describing is a BFO design. Besides being relatively less sensitive, BFO's also have a stronger response to minerals, making them less stable. Most meteorite detectors use PI designs.

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            • #7
              so does higher frequencies get better pentration? this is my first attempt at making a metal detector. basically its two ocs tuned to the same frequency.

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              • #8
                Generally the opposite... lower f's go deeper.

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