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Simple pulse induction metal detector

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LAN_403

Reaching

Thu Jan 10 2013, 07:10PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Hey Guys.
it must have been years since i started my last project here.. god damn i missed it, but i had much to do for work and so on .. but now im back, hehe.

I want to start with a simple diy metal detector. i have worked on the shematic for a few days now and the breadboard version is working very good. with a simple selfwound coil with a diameter of around 16cm and 50turns of 0,3mm wire i can detect iron within a distace of about 30 to 40cm. big pieces are even better to detect. but even small pieces of coin size are easy to find.

Within the next few days i want to build it on pcb with a nice case and so on to try it outside and possibly find something^^

attached is the shematic and my board layout.. Just ask me if you want the eagle files to build it on your own.

when i had the idea of building a metal detector, i searched the web for some circuits but couldnt find much without microcontrollers. nothing simple around except those bfo dudes, but thats not performant enough though^^
so is started to work on a circuit on my own. i came up with a fairly simple design with only a few ics.

A IRF 630 is powered by a ne555 at around 440Hz. this mosfet generates an ac in the search coil. a few simple diodes, resistors and a capacitor are used to get the back em pulse from the coil which changes in voltage and time when a piece of metal comes closer to the coil due to eddy currents generated in the piece of metal. a tl072 op amp is used to get a halfwave sinusodial looking pulse, which changes in voltage when a piece of metal comes closer to the search coil. for simplicity this pulses are fed through a diode and a capacitor forming a simple rectifier. the resulting dc voltage is then fed to a vco 4046 to generate a audible sound variing in frequency when metal comes closer to the search coil...

thats it. Really easy

Reaching

Thu Jan 10 2013, 07:11PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

forgot the attachement^^

1357845103 76 FT148849 Metalldetektor Board

1357845104 76 FT148849 Metalldetektor Shematik

Shrad

Fri Jan 11 2013, 10:17AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

thanks a lot for this design!

I suppose by 30 to 40cm you are talking free air detection?

I'm curious to build such a thing and try it out in my yard (I'm next to a 17th century battleground)

I'm also curious about separating search coil in two, with a greater turn count for the listening coil as to gain sensitivity (maybe a television deflection coil? already has a gorgeous collar ^^)

Reaching

Fri Jan 11 2013, 01:47PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Yes, the 30 to 40cm are through air. i havent tested the thing outside yet, but now its weekend and ive finished the pcb and the searchcoil, so its just a matter of a day or so to try it. just got to find some sort of rod or something to mount this all together.. the battery will be aproblem too cause the detector is drawing around 0,7Amps,, really wasteful :P but you need some current in the coil to detect, no way around that..

How i made the search coil..

i had some flexible plastic around here which i slotted in the middle with a dremel, then bend it and wound 50 turns of 0,3mm on it. some isotape to prevent the thing from going apart. i searched the web for a good coil design and almost every coil had this aluminium shield around it, so i made that too. got some alu tape and wound it around the entire coil, just leaving a 1cm gap or so and connected the shield on one end of the coil. Works really good and its stable enough and really lightweight

1357912024 76 FT148849 Metalldetektor Platine

Ash Small

Fri Jan 11 2013, 03:27PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Reaching wrote ...

the battery will be aproblem too cause the detector is drawing around 0,7Amps,, really wasteful :P but you need some current in the coil to detect, no way around that..

What voltage do you need? have you considered a small motorcycle battery? the come in 12V and 6V. otherwise LiPo's, I suppose.

Reaching

Fri Jan 11 2013, 03:55PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

i need 12v.. i have some battery packs here for those nimh batteries. but i doubt that they will last long enough.. ill just give it a try. i have some of those big lead acid batteries here too but those are heavy compared to the small nimh packs..

Ash Small

Fri Jan 11 2013, 07:15PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Something like a YB-4L

weighs 1.5kg, and could be strapped to your belt. It would give several hours of use at 0.7A, and can be charged using any 12V battery charger. Cost ~15 Euro.

radhoo

Fri Jan 11 2013, 08:17PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

In RO, it's illegal to build, hold or use a metal detector

, unless one gets a permit, but it's impossible to get a permit for a home-built device.

At what distance does it detect a 1 EURO coin?

teravolt

Sat Jan 12 2013, 05:22AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I've seen pictures of metal detectors wiht big loops over a meter for deeper stuff like meteorites. Do you think your circuit could do that

Reaching

Sat Jan 12 2013, 02:20PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

this circuit just detects the back em pulse generated in the coil. so i think, with a bigger coil, youll get better results. maybe with more current you get better results, so building a coil with less resistance would be even better. i just dont know, cause ive build this one coil so far.

you can add a pot instead of r11 to vary the frequency. 440hz suited best for my coil, for other coils it may be different.

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