Simple BFO metal detector

BFO ( beat frequency oscillator ) metal detectors use two oscillators, each of which produces a radio frequency. One of these oscillators uses a coil of wire that we call the search loop. The second oscillator uses a much smaller coil of wire, and is usually inside the control box and is called the reference oscillator. By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. It has been found in practice best to make the search oscillator fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. This gives a beat note of 250hz to 0 to 250hz. The beat note disappears or nulls when the two oscillators are about equal. This type of detector is most sensitive when the beat note is close to zero, about 5hz ( motor boating ) any slight change being noticeable.

Step 1: Parts

Parts list

Power source:
Any 9v battery PP3 is ideal.

2 off 220uF 16v electrolytic.
5 off .01uF polyester.
5 off .1uF polyester.

All resistors 1/4 watt 5%
6 off 10k
1 off 1K
1 off 2.2m
2 off 39k

All BC 183B. Just about any small signal npn with a gain of 250+ will do. There are hundreds to choose from.

Audio output:
A 2.5 inch 8 ohm speaker will work but headphones or earpiece are preferable the higher the impedance the better.

Many of the above parts could be salvaged from a broken transistor radio.

Once the components have been obtained the circuit can be built in a few hours using copper clad stripboard, or if you the facilities make a printed circuit board using the layout below. The original layout as below should print out at about 50mm x 100mm.
<p>im trying to build this as a design project for my electrical classes. i would like to see a full shot of the metal detector to get a full perspective of what went where and what all was going on.</p>
<p>what is the range of detection for this circuit ? if i need 30 cm range , is it good for that ?</p>
when can the zero rate and does not accuse the presence of metals which can be cause?<br><br>I did the site http://detectordemetais.r30.com.br/detector-de-metais/circuito-detector-de-metais-caseiro/<br><br>&nbsp;&nbsp;Only fit as I approach the soldering iron connected to the network 110v .<br><br>
<p>could I use a ferrite instead of a search head for using it as a pin pointer?</p>
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hi! i would like to know where the 220 uf electrolictics connect, because the two<br>have the B-, and if possible where the two electrolictics are going to connect.<br>Best regards,<br>Paulino Alves paulino_al@iol.pt<br><br>p.s. i&acute;m trying to build your project, but i don&acute;t understand what i mencioned!
I have just bought a GC-1005 metal detector from ebay,<br>is this good to get started with?<br><br>please replie
Hi,<br>thanks for this instructable. Can I use this method to make a security system like those we see in airports, those which look like doors that passengers pass to check them. <br> <br>
No connection means the wires at the junction there at the bottom where the one goes up and down and the other goes left and right do not connect. The one passes over the other. The bottom of the 10K resistor will go to the base of Q3 and will NOT connect to B- but pass over it.
hey, sorry, but really: what does NO CONNECTION mean...? where should i connect the 10K ???
It means the two wires crossing over each other are two separate wires, and not connected at the node.
can anyone tell me how to figure s voltage drop across any of the resistors??? cause there's no ground in this circuit :s
Hi , <br>I am working on project of metal detector . <br>Could we find out that how far it can detect. <br> <br>if we want more distance metal to detect what I have to do for it. <br> <br>Thanks
vere gooooooooooooooood<br />
You can find the original of this project on my site http://www.easytreasure.co.uk<br /> this is where it was stolen from. There is also a video of it working here<br /> http://www.youtube.com/watch?v=aSqmFWtoXYw&nbsp; <br /> If you check it out on my site there is nothing to pay its free.<br />
yese vere good <br /> <br />
In search coil build as photo,use nylon bolt an nut an plastic ,polycarb L shape for brackets,to reduce Metals in search head an connector cable from search head short as possible an screen cable to control unit.for best signal to noise performance for this basic unit. metals in centre of search head make balancing the coil for this simple circuit harder to achieve an less sensitive overall once built.
Pinout for common BC548 NPN appear same as BC184 NPN transistors.all OK.
can any one explain this circuit for me.....and is that no connection..does that mean we dont have to connet the 10k resistors there.then where do we connect it..i've tried to make this project but..can we use bc548 instead of bc183
a very simplistic schematic,as the transistors all appear to be NPN types from Q1 thru to Q6, BC548 transistors should suit.just remember correct pinout for this transistor for collector,base, emitter pins. most capacitors by their values could probably be Green caps! poly/mylar type. cheap, an a couple of cheap 220uf Electro caps 16v types or better. B- an B+ to connect battery 9V. Getting the coil winding right ,an matched is the tricky part?
See in the photo MKT capacitors used for most all, except the Electro caps.
Which transistor can be used in place of BC 183B transistor.
I used a C1815 transistor and it's works well.
very good
What gauge of wire did you use?
<a rel="nofollow" href="https://www.instructables.com/id/Home-Made-BFO-metal-detector/">my BFo metal detector</a><br/><br/>OK I'm finished here it is!!<br/>
this thing is harder to build than it looks. I built it to spec and nothing. I've used this calculator to find the inductance:<br/><a rel="nofollow" href="http://www.coilgun.info/mark2/inductorsim.htm">http://www.coilgun.info/mark2/inductorsim.htm</a><br/>and I have both inductors within .007mh difference, using 9v battery and tiny headphone. I hear nothing n i got voltage on both coils. i can't find what's wrong. I used radioshacks transistors &quot; general&quot; use npn. I've spent3 days and nothing.<br/>
Ok i figured it out: the diagrams in the transistor package are very confusing. all transistors were backwards. it works but it still not finished i'm working on a more detailed instructable of this same one so everyone is more confortable at atempting it
Does Anybody Know How To Make A Homemade Metal Detector? If So Tell Me How This Is VERY IMPORTANT!!!!
SOMFG!!!!!111eleven one!! ROFL STFU LOL!!!!!!
duh!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
um. thats what this instructable is about......
Part of this site is relisting projects so they can become more widely used!!!!
<a rel="nofollow" href="http://home.clara.net/saxons/bfo.htm">http://home.clara.net/saxons/bfo.htm</a><br/>is my home page<br/>
Nice work, really cool to read. Thanks!
P.S hehe, no i also don't want an argument, were all makers, we should stick togheter.. I'm merely pointing out that this is not a "valid" entry..
I'm not saying it's mine. What i'm saying is that, ok he put it in, but he should have added credits, and asked the owner if he could do it.. Am i sure?? YES! Check out this link, this is where he stole it from: <a rel="nofollow" href="http://home.clara.net/saxons/bfo.htm">http://home.clara.net/saxons/bfo.htm</a> as i said before.. Also This instructable should NOT be able to participate in the contest! Honestly, HE STOLE IT!<br/>
Ok fair enough if you originally put this idea on (with ALL nessecary copyrights) but are you sure this is exactly the same as yours??? What was that about donkeys???? did you possibly mean llama??? P.S. i do not want this to turn into a huge debate(argument).
How would you feel if someone stole a project of yours, posted it as there own, and then win a maker contest?! I bet you would feel like Big Bubba aka. hung like a donky, paid you a visit!
so what who is so perfect that they keep every single copyright law in the world?? how many pirate MS XP copies are there????(not exactly related i know)
Sure enough #1 on google, search bfo+metal+detector.. I'm sure your well aware that if this isn't yours, then what you did, is illegal, right!?
I have seen this project before, and honestly, i don't think it's yours. If it is, then i'm sorry for the incinvinience. Just a heads up!!!
Well documented, good instructable.

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