Help with building an air coil

Discussion in 'General Electronics Chat' started by circuitfreak2000, Apr 21, 2015.

  1. circuitfreak2000

    Thread Starter Member

    Apr 7, 2015
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    Hi guys, I would like to try to build this 555 metal detector circuit for fun.

    [​IMG]

    Would it be possible to replace the 10mH choke with a DIY air core?

    I used an air coil calculator to figure out how to get 10mH and it seems
    this might work according to the calculator:

    Diameter: 5cm
    Turns: 10
    Length: 0.2cm

    this results in a perfect 10.0 microhenries, would this air coil work?

    Thanks for your help.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    10 mH is 10 mili Henry and not 10 micro Henry(wich would be written as 10 μH) , there is a difference of a factor 1000 X.

    Bertus
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    A 10mH air coil will be huge...

    Most metal detectors use coils that resonates at ~1MegaHz (MHz); the 555 circuit pictured above resonates at ~1kHz (also a 1000:1 difference)
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    cmartinez likes this.
  5. circuitfreak2000

    Thread Starter Member

    Apr 7, 2015
    39
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  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    For 10mH the air coil need to be large. For example air coil wound on a 10mm tube (45mm length ) will have 10mH only if you wound 1830 turns (11 layers) using 0.22mm (31 AWG) coper wire. And the resistance will be equal to 32Ω.
    Try this java applet
    http://www.coilgun.info/mark2/inductorsim.htm
     
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  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    One example: 142 turns, 25cm diam, 1cm length. (N.b. 1mH=1000uH).
     
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  8. circuitfreak2000

    Thread Starter Member

    Apr 7, 2015
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    Thank you guys. Got it now! :) The higher the frequency, the smaller the coil.

    Indeed, it would be impossible to make a little air coil of 10mH by hand.
     
    Last edited: Apr 21, 2015
  9. circuitfreak2000

    Thread Starter Member

    Apr 7, 2015
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    I did find another circuit which works at 104khz and uses two 34uH air coils. This circuit doesn't use a 555, instead only transistors. It looks like these coils will be much easier to make ;)
     
  10. RichardO

    Well-Known Member

    May 4, 2013
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    I played with trying to make a metal detector once. I have to admit, up font, that I never got it working to my satisfaction.

    The pick-up coil was wound on a reel that had held SMD resistors. The circuit used 2 CD4046 PLL chips -- one for the oscillator going to the coil and the other as a FM detector. The coil needs to use large wire to get the resistance low otherwise the Q will be too low. Other details escape me right now...
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Build an oscillator that runs about +-1Khz from a strong local AM broadcast station. Wind the pick-up coil to make the oscillator run at that frequency. Get a portable AM transistor radio that tunes aforementioned AM station. Tune in the station. Tune the oscillator to get a nice beat note in the radio (the AM station frequency +-1KHz).

    Whenever the pick-up coil gets near anything that changes its inductance, it pulls the oscillator, which changes the beat note in the speaker. Very sensitive way of building a metal detector.

    e.g. say the radio station is on 1160kHz (1.160MHz). Build the oscillator so that it runs at 1159kHz or 1161kHz. That makes the beat a nice audible 1oooHz.
     
  12. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    I think in one of my books has plans for a metal detector .. Let me check..
     
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