metal detector using colpitts oscillator

Discussion in 'The Projects Forum' started by lephuduc, Oct 11, 2012.

  1. lephuduc

    Thread Starter New Member

    Oct 11, 2012
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    I'm studying about metal detector using colpitts oscillator, can anyone help me to explain how the attached circuit work? can it detect metal easily? and the function of capacitors?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    How do you know it works at all?

    It may detect, but I see no way for it to indicate anything. No sound or light output.

    Metal near the coil changes the impedance of the coil, thereby changing behavior of the circuit. Probably changes frequency of oscillation.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    You can search on google : metal detector, metal detector using ne555, then you will get more and more circuits.
     
  4. radiohead

    Active Member

    May 28, 2009
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    I agree. It may detect metal, but there are no specs on how to wind the coil. Here are a few schematics to get you started with a quality machine.
     
  5. radiohead

    Active Member

    May 28, 2009
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    Here's a schematic of another good machine.
     
  6. lephuduc

    Thread Starter New Member

    Oct 11, 2012
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    Thanks for help. actually I just want to design a simple circuit, the circuit I attached is just my beginning. If you understand how it work, please explain it?
     
  7. radiohead

    Active Member

    May 28, 2009
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    Basically, the circuit will generate a signal through the coil. The signal will penetrate the ground. The coil will have to be tuned to balance with the ground so no sound will be heard in the earpiece (for a beat frequency oscillator type). When the coil passes over a ferrous metal, the electrical balance of the coil will change and the frequency difference will be heard as a tone in the earpiece (or deflection in a meter). There are hundreds of simple metal detector schematics on the web. Some are better than others, but most simple ones won't detect more than a few inches. Below are some links for history and theory of operation for VLF, pulse induction and BFO types.
    http://electronics.howstuffworks.com/gadgets/other-gadgets/metal-detector.htm
    http://inventors.about.com/od/pstartinventions/a/Metal_Detector.htm
    http://www.hobby-hour.com/electronics/metal_detectors.php
    http://www.discovercircuits.com/S/s-metal.htm
     
  8. vk6zgo

    Active Member

    Jul 21, 2012
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    It doesn't look much like a Colpitts oscillator to me!
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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  10. vk6zgo

    Active Member

    Jul 21, 2012
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    Yes,on looking at it,& Googling,I see the resemblance.

    The strange thing is,that most of the Colpitts circuits I have seen over many years,have been common anode (yes I date back that far) or common collector,with the cathode or emitter returned to the capacitive tap.

    These are fairly rare on the 'Net,& I don't instantly recognise the other,in real life,(in my experience) less common forms.
     
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