Instructables
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.
 
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Step 1: Parts

Parts list

Power source:
Any 9v battery PP3 is ideal.

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

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

Transistors:
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.
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paulinex1272 years ago
hi! i would like to know where the 220 uf electrolictics connect, because the two
have the B-, and if possible where the two electrolictics are going to connect.
Best regards,
Paulino Alves paulino_al@iol.pt

p.s. i´m trying to build your project, but i don´t understand what i mencioned!
I have just bought a GC-1005 metal detector from ebay,
is this good to get started with?

please replie
baker amin3 years ago
Hi,
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.

jack0023 years ago
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.
toben2224 years ago
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.
yahklfe Azayles3 years ago
xwtdqdsrhj
bobberts3 years ago
can anyone tell me how to figure s voltage drop across any of the resistors??? cause there's no ground in this circuit :s
jaschhina4 years ago
Hi ,
I am working on project of metal detector .
Could we find out that how far it can detect.

if we want more distance metal to detect what I have to do for it.

Thanks
anasko4 years ago
vere gooooooooooooooood
You can find the original of this project on my site http://www.easytreasure.co.uk
this is where it was stolen from. There is also a video of it working here
http://www.youtube.com/watch?v=aSqmFWtoXYw 
If you check it out on my site there is nothing to pay its free.
anasko4 years ago
yese vere good

rickove5 years ago
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.
rickove5 years ago
Pinout for common BC548 NPN appear same as BC184 NPN transistors.all OK.
4star5 years ago
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
rickove 4star5 years ago
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?
rickove rickove5 years ago
See in the photo MKT capacitors used for most all, except the Electro caps.
Panther_6456 years ago
Which transistor can be used in place of BC 183B transistor.
I used a C1815 transistor and it's works well.
rabiif5 years ago
very good
kingwaffles6 years ago
What gauge of wire did you use?
JoelDude6 years ago
my BFo metal detector

OK I'm finished here it is!!
JoelDude6 years ago
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:
http://www.coilgun.info/mark2/inductorsim.htm
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 " general" use npn. I've spent3 days and nothing.
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
Hen8 years ago
Does Anybody Know How To Make A Homemade Metal Detector? If So Tell Me How This Is VERY IMPORTANT!!!!
JoelDude Hen6 years ago
SOMFG!!!!!111eleven one!! ROFL STFU LOL!!!!!!
really...
duh!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Mouserz Hen6 years ago
hahaha
um. thats what this instructable is about......
amlan_20088 years ago
PLS SEND ME THE DETAILS OF TRANSISTORS USED IN BFO METAL DETECTOR
Coffee bean8 years ago
Part of this site is relisting projects so they can become more widely used!!!!
jurgis (author) 8 years ago
vbrtrmn jurgis8 years ago
Nice work, really cool to read. Thanks!
Silas8 years ago
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..
Silas8 years ago
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: http://home.clara.net/saxons/bfo.htm as i said before.. Also This instructable should NOT be able to participate in the contest! Honestly, HE STOLE IT!
ll.138 years ago
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).
Silas8 years ago
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!
ll.138 years ago
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)
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