Metal Detector

[G-R-C]

Hey Gang
I am think of building a metal dectector. Purchased on this summer, but some how
I think its possible to build a better one. After doing a google search and a search on
this forum I'm definitly confused. Google shows very few examples using micro-controllers.
Melanie posted a decent idea for one a while back. However it still seems a bit fuzzy to me.


My question is.......
Can someone point me in the right direction on where to start.?

I Really want to learn how these work so I can make my own from scratch using a PIC.


Please excuse me if this is not the correct place to post this type of question.

Thanks in advance
Gordon

[G-R-C]

After re-reading my first post I should of mentioned I
want to build the metal detector of the style used for searching
the ground for old coins and such. Something like the Whites
style of metal detectors. No this is not a school project, just
have an itch to learn something that interest me.

Thanks all
G

[locko]

Hi G-R-C

I had the same idea, so started with a non-pic project to learn how it works.
The net has several example circuits for BFO and pulse metal detectors.

http://www.rpi.edu/~bradfe/MD/MD.html
http://oldradiobuilder.com/MDET.html
http://www.talkingelectronics.com/projects/MetalDetectorMkII/MetalDetectorMkII.html

I built the circuit from http://www.easytreasure.co.uk/bfo.htm - had good description.
It works but the feedback control is a pain and the depth detection sucks, best I could get was about 2" with a 5" coil in air. Note:You WILL need an Oscope for setting the coil frequency etc.

Due to the terrible depth detection, I have not persued this project any further with using a Pic, but with someone else having the same interest, I may have to start it back up again.

If you have access to PCB Wizard, attached is the circuit I made for the BFO.

Regards,

Paul

Edit: There is a Pic based metal detector called Gold Pic (search the net). You have to pay for the pre-programmed pic chip but I think you can find the circuit for free. I will see if I have anything saved for this and post it if I do.

Edit2: Here is the link http://home.global.co.za/~trh/gp301n.gif

[G-R-C]

Thanks for the links Locko. Some of these I have seen before.
I too was thinking about the BFO detector as well. However
you are first person I have had any conversation with who has some real
feedback on the subject. Seems intersting that your's is only able
to read into the ground just a few inches.

The circuit with PIC is very cool. This looks like a good place to start.
I'd like to write my own code. Maybe it would be possible to block
certain metals like aluminum and iron from being detected.
This way somebody doesn't keep digging up trash.

Any idea why yours only has a few inches of depth detection?

Wonder if the pick-up coil senstivity can be increased/decreased.

Thanks again Locko, this has been a huge step in the right direction.

Gordon

[Archangel]

Any idea why yours only has a few inches of depth detection?

Wonder if the pick-up coil senstivity can be increased/decreased.
Gordon
I have a couple of Ideas. There are basicly 2 types of metal detectors, the TR type( transmit / receive ) and the BFO ( beat frequency Oscillator ) and are used for direrent reasons. The TR type for deep detection uses 2 coils oriented 90 degrees from each other and spaced several feet apart, whereas the bfo usually has a small coil inside and to the front or rear of a larger coil. The larger the coils, the deeper they will search. Metal which has had time to corrode a little will cause the detector to sense it more readily. There are TR type detectors which have coils like the BFO, units and these can be tuned to ignore iron or alum. but if tuned to ignore alum, they will also ignore gold and silver. Many good articles in Rock And Gem magazine about metal detectors, check ebay for back issues.
JS

[locko]

EDIT: Here is all you need right here - a fully explained pic metal detecter with hex file!!
http://geotech.thunting.com/cgi-bin/pages/common/index.pl?page=metdet&file=/projects/stuart2/index.dat



Thanks for the links Locko. Some of these I have seen before.
I too was thinking about the BFO detector as well. However
you are first person I have had any conversation with who has some real
feedback on the subject. Seems intersting that your's is only able
to read into the ground just a few inches.

See below



The circuit with PIC is very cool. This looks like a good place to start.
I'd like to write my own code. Maybe it would be possible to block
certain metals like aluminum and iron from being detected.
This way somebody doesn't keep digging up trash.

Yes you can, it has to do with the return signal. A metal near the magnetic field will affect the frequency of the search coil (Phase shift). On the BFO I made, a copper coin would increase the tone but my gold ring would decrease the tone.
Some good info here: http://www.gi.alaska.edu/~jesse/treasure/misc/howdetector.html



Any idea why yours only has a few inches of depth detection?

There are lots of things that affect the depth, voltage in the coil, size of coil, the sensitivity of the balance circuit. If I set the zero beat to just triggering with no metal, it can detect a single penny at 5-6 inches in air. My test circuit was using 5 volts and when I went to a real circuit with a 9 volt bat, the depth detection increased. Some metal detectors use 18 volts in their circuits - could be for more coil volts?
With the size, Joe is right, larger = deeper, but you will sacrifice the size of object you can detect.



Wonder if the pick-up coil senstivity can be increased/decreased.

I’m sure it can but I think switching to a good PI circuit would improve things greatly.



Thanks again Locko, this has been a huge step in the right direction.

Gordon

From what I have read, a pulse induction system is the way to go. Better depth but more complicated to build. Here is another site that has lots of good links:
http://geotech.thunting.com/cgi-bin/pages/common/index.pl?page=metdet&file=projects.dat

I think it is best to start with deciding what you want to detect and go from there.
This has got my interest again, may have to start loosing more sleep to yet another project…. :)
Sorry none of this really covers a pic chip, but I never got that far last time. I think the pic is probably the easier part od a good detector, the circuit and coil design is the complicated part.
Regards,

Paul

[G-R-C]

Joe and Paul
Thanks for the help again fellas. Really like the PIC based link.
I will to modify this idea as my starting ponit and go from there.
Any idea if a microcontoller is used in the expensive profressional
detectors? Seems to me there have to be some form a programmable
device to block the iron and trash metals.

I think I have enough information now to be dangerous. This is going
to a fun project. Can't wait to let the smoke out of some electrical
components.


Thanks agian guys
Gordon

If at first you don't succeed, let the smoke out of another PIC.

[locko]

Gordon,

Just an FYI, I took a closer look at the last pic based circuit diagram from the geotech link today and there are a couple of small mistakes. The coil, R27 and IC3 should all be connected to the 9 volt rail not the 16 volt rail.

I'm thinking you could go for menu driven LCD, programmable presets and a selectable 'smaller; coil for pin pointing at the push of a button......oh and of coarse the obligatory flashing lights and bar graphs :) Hope you didnt want any sleep, hehehe

Regards,

Paul

[ScaleRobotics]

I noticed that some of these links here were dead. Here is one link with a description of each of the most popular detector types, with schematics.

http://www.hobby-hour.com/electronics/metal_detectors.php


5000

5001

5002

5000
And one more link http://www3.telus.net/chemelec/Projects/Projects.htm

[BoyceNoun]

Thanks for the links Locko. Some of these I have seen before.
I too was thinking about the BFO detector as well. However
you are first person I have had any conversation with who has some real
feedback on the subject. Seems intersting that your's is only able
to read into the ground just a few inches.

The circuit with PIC is very cool. This looks like a good place to start.
I'd like to write my own code. Maybe it would be possible to block
certain metals like aluminum and iron from being detected.
This way somebody doesn't keep digging up trash.

Any idea why yours only has a few inches of depth detection?

Wonder if the pick-up coil senstivity can be increased/decreased.

Thanks again Locko, this has been a huge step in the right direction.

Gordon
I know nothing about the BFO detector. I just heard about metal detector and gold detector.

[kellyseye]

The single most important part of any metal detector is the coil assembly. A PIC is usally only used to make 'fancy' displays of the resultant signal but you can't make a silk purse from a sows ear - in other words, without a decent coil/detection system using a PIC won't add any additional advantages.

Elektor magazine, November 1981, has a good (very, very good) metal detector circuit (with pcb layout etc) if you can get a back issue - my pdf of the article runs to around 8.5Mb in size.

[bethr]

Another good source is the BS2 Parallax forum, where is a small design called Sensitive Metal Detectors, wiring and source code are available from there, i think is easy to experiment with, porting the code as well !

http://forums.parallax.com/showthread.php/62768-Sensitive-STAMP-based-Metal-Detector-enjoy!!?p=467144#post467144

not tried yet but soon.

[andywpg]

Elektor magazine, November 1981, has a good (very, very good) metal detector circuit (with pcb layout etc) if you can get a back issue - my pdf of the article runs to around 8.5Mb in size.

Went hunting for this when I saw your post and found THIS (http://www.ebay.co.uk/itm/Elektor-Electronics-digital-collection-1-Issues-1-to-152-1974-1988-/281142542425?pt=UK_Collections_Lots_Books_EH&hash=item417566c059). I ordered one immediately, what a great resource.

[BoyceNoun]

I noticed that some of these links here were dead. Here is one link with a description of each of the most popular detector types, with schematics.

http://www.hobby-hour.com/electronics/metal_detectors.php


5000

5001

5002

5000
And one more link http://www3.telus.net/chemelec/Projects/Projects.htm

Can this circuit diagram go with any kind of metal detector (http://www.detectorall.com/categories/Metal-Detectors/)?

[sayzer]

Hi Andy,

Here is a link (http://www.digiwood.ee/8-electronic-projects/2-metal-detector-circuit) for you. It has a Metal Detector circuit with 12F1840.


Also, below is the circuit schematic.

http://www.digiwood.ee/images/metaldetectorimages/schematic_700.png (http://www.digiwood.ee/images/metaldetectorimages/schematic_700.png)

[andywpg]

Hi Andy,

Here is a link (http://www.digiwood.ee/8-electronic-projects/2-metal-detector-circuit) for you. It has a Metal Detector circuit with 12F1840.


Also, below is the circuit schematic.

http://www.digiwood.ee/images/metaldetectorimages/schematic_700.png (http://www.digiwood.ee/images/metaldetectorimages/schematic_700.png)

Thanks much!

[Andybrown]

Hello

First of all Thank you again for sharing this information with me.


Hi Andy,
Here is a link (http://www.digiwood.ee/8-electronic-projects/2-metal-detector-circuit) for you. It has a Metal Detector circuit with 12F1840.


I have taken a look shortly at the details of this design.
On the hand I have checked local markets for this PIC12F1840 ; it is not available at the moment at local shops, and on the other hand this project only has HEX & ASM files as sources for this mentioned micro controller.

Also, I am very interested in to use the DT-Ints (Darrel Taylor Instant Interrupt routines) and use LCD, of course to add a keyboard for the purpose of frequency adjustments, battery level monitoring and other facilities and etc. So I decide to switch to a better PIC chip such as PIC16F877A or some other equal types as well.

For now, it is difficult for me to adapt the ASM file to a program with PicBasicPro language.

Would you please do a great favor to me and just give me some clues that HOW I should use DT-Ints for handling this issue? for the reference of this below schematic.

Many many thanks again for your help and always kindly support forum members.

Regards,

Andy Brown

[Ioannis]

Hi Andy and welcome back. Sorry for the kick!

About the firmware, I'd recommend to start without DT-INTS, as it is not necessary, at least for the first experiments.

Look at this phrase: "In my design pulses occur at 2 millisecond intervals. Pulse duration is 140 microseconds." This is easy for any PIC to produce.

Start by this and then try to check with analog input for the decay time it takes. Maybe comparator is better choice than ADC as it is much faster.

Ioannis

[Andybrown]

Hello Ioannis,

Thank you for ...


Hi Andy and welcome back. Sorry for the kick!

About the firmware, ...

I would start to challenge and back to expert guys, such as you, with further questions.

Regards,

Andy