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Offline SimpsonTopic starter
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« on: January 24, 2009, 08:37:16 am »
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BFO Metal Detector

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by Rachel and Steve Hageman

This article describes a very basic BFO detector which uses only two transistors and an LM389 audio amplifier. The LM389, in fact, contains the two transistors that form the search and reference oscillators so the whole circuit is comprised of a single IC and a handful of discrete linear components. Accurately matched transistors are not really necessary in the oscillators so you can use discrete transistors and an LM386 audio amp instead of the LM389.

This article was published in the "Design Ideas" section of the December 18, 1997 issue of EDN, a popular electronics trade magazine that offers free subscriptions to qualifying individuals.

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« Last Edit: May 21, 2009, 05:07:11 pm by Christian »
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Offline Eugene52
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« Reply #1 on: January 24, 2009, 02:32:50 pm »
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Hello Simpson and Everyone , the BFO Metal Detectors are Great for Electronics building and experimenting . The BFO type of Metal Detectors have long been replaced by the much better I.B. [INDUCTIVE BALANCE] VLF [VERY Low Frequecy] Metal Detectors. Also Very popular are the P.I. [PULSE INDUCTION] Metal Detectors . BFO Detectors are the easiest to build , but you will not detect coins very deep . SO , Next I would suggest building a P.I. Detector , it will detect coins deeper and Bigger objects Very Deep ,  and the P.I.'s search coil , is easy to make . LAST : The best detector to build ,the most Challenging and "REWARDING" is the 'I.B.-VLF" .  PLEASE REMEMBER: When you build a "I.B.-VLF" , the search coil used with your project must be "COMPATIBLE" ! Meaning , the search coil is at the correct frequency of your projects tx-oscillator circuit , and be in 100%  working order , or you will never align and set the electronics properly on your circuit board that you are building !!............ Eventually , on this Great thunting.com site will be many schematics/constuction projects to choose from ..........SOMETHING FOR EVERYONE !  Clapp Clapp Clapp.........................Regards................. Wise Wise Wise.........................Eugene52

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« Reply #2 on: February 06, 2009, 03:23:17 pm »
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Eugene . There are a lot of schematics on the net ( take Carl Moreland's forum ) We all have schematics to choose from . What I like to find out is who has the drawings of the coils . These coils are expensive and kept close to the heart of the detector manufacturers  . A schematic is not the most important part of the detector . It is the coil . If you don't have a notion of how to build the coil you are doing a lot building for nothing . So Eugene if you have any knowledge of coil building let us know .  Cornelius

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« Reply #3 on: February 06, 2009, 04:04:05 pm »
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Quote:Posted by Cornelius
Eugene . There are a lot of schematics on the net ( take Carl Moreland's forum ) We all have schematics to choose from . What I like to find out is who has the drawings of the coils . These coils are expensive and kept close to the heart of the detector manufacturers  . A schematic is not the most important part of the detector . It is the coil . If you don't have a notion of how to build the coil you are doing a lot building for nothing . So Eugene if you have any knowledge of coil building let us know .  Cornelius


Hello Cornelius , You are 1000 % Correct !! The search coil is the Heart of any metal detector . You can build a hundred VLF / IB circuits .. BUT , if the search-coil is not made correctly , the Metal Detector will "NEVER" function correctly . Yes , I have made some various vlf search coils and rememember that  they must match the manufacturers specs "100%" , no exceptions , do not listen to what other Technician's tell you . There is no "CLOSE ENOUGH" when it comes to search-coils . Many things are required , that must be perfect to achieve proper discrimination / phase / operating frequency / sensitivity ....Things like  1. Resonance and "q" of the coil  2. Inductive Reactance of the search coil at operating frequecy  3. Dc resisistance /number of winding /diameter of coil/direction of windings / shielding or no shielding ,and shapes of the coils. There are many formulas to calculate , if not you will have many Headaches getting your Metal Detector to work perfectly !!!. ....................My favorite Metal Detectors are Tesoro , and I have a whole ton of Tesoro coil information I can post here if you need it , let me know !! .........But it will be later on , several days before I can get to it !! Search-Coils are not hard to wind , I just do not like spending $20.00 for a cheap piece of plastic made in arizona !!.............Someday , maybe I will be able to form/make my own coil housings.........Regards...............Eugene52

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Offline Cornelius
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« Reply #4 on: February 06, 2009, 04:06:41 pm »
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I would like it if you did mail the particulars of the Tesoro coil to me  . Use my E-Mail number if you have to . Cornelius

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« Reply #5 on: February 06, 2009, 04:25:54 pm »
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Quote:Posted by Cornelius
I would like it if you did mail the particulars of the Tesoro coil to me  . Use my E-Mail number if you have to . Cornelius


Great Idea Cornelius , as soon as I get all the files together , I will e-mail it to you . Sometime's it is very hard to post documents and files here because of the File-Size limitations . The Thunting.com is a great forum  , but if we were able to post everything we needed , the administrator would need to purchase a million tera-giga-byte server-storage for Thunting.com  Grin Grin Grin..........Regards...........Eugene52

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« Reply #6 on: May 21, 2009, 05:03:29 pm »
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Cornelius and Eugene52 I need your help
I was thinking to build the Shadow detector but I couldent find any explenation or drawing of the coil perhaps on of U can help me
best regards fsss

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« Last Edit: May 21, 2009, 05:07:46 pm by Christian »
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« Reply #7 on: May 21, 2009, 05:43:36 pm »
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Hello fsss , If I remember correctly the search coil for the project had to be purchased and was sold by ambit international . This search-coil  has been discontinued and has not not been available for many years. The good news , this project was copied and modified from an old radio shack metal detector design but you can try an older bounty hunter search coil .........  I think this Shadow is around 15 or 16 khz ? You should be able to find or even make a new search coil . I will do some more checking and see what I can find out.................Regards................Eugene

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« Reply #8 on: May 21, 2009, 06:00:12 pm »
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great please do
This project was the most informative that I found
the only part missing was the search coil
if you can direct me to sites that provide direction or drawings to bilding, winding a search coil it will be great this is my first project
I don't know if my coice is right but I hope to get as much material about it before buying or getting the omponents
thanks
fsss


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« Reply #9 on: July 19, 2009, 11:28:39 pm »
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Late,late post...

This is not a good circuit but can be improved with two diodes, two resistors and another capacitor.

If you want to know, just ask me... I cut my teeth, on BFO.

Brian AKA goldigger (thats why I got  no teeth now.)

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