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?

Thanks for your help.

 

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

 

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)

 

Is your plan to build a 555-based metal detector or just a metal detector?

If it is the latter, then the TDA0161 (http://datasheet.eeworld.com.cn/pdf/STMICROELECTRONICS/177624_TDA0161DP.pdf), which is obsolete, but still widely available works well. The CS209A works similarly and is a current chip, I think.

John

 

Can someone please show me what values in this air coil calculator result in 10mH (millihenry). http://www.66pacific.com/calculators/coil_calc.aspx

 

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

 

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

 

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

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.

 

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

 

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...

 

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.

 

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