# 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
39
1
Hi guys, I would like to try to build this 555 metal detector circuit for fun.

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?

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
5,450
1,066
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.

Apr 7, 2015
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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

circuitfreak2000 likes this.
7. ### Alec_t AAC Fanatic!

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

circuitfreak2000 likes this.
8. ### circuitfreak2000 Thread Starter Member

Apr 7, 2015
39
1
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
39
1
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
1,235
384
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
5,450
1,066
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
1,190
156
I think in one of my books has plans for a metal detector .. Let me check..