Instructables
Picture of Home Made BFO metal detector
I read a couple of do it yourself metal detector write ups in the web as well as the one in the Instructables page that looks exactly like the one on a page. So I decided to make my own too. However I made up most of it as I went around because the information on the web was incomplete or not very precise. So I decided to make one with exact measures and post it here so that everyone who wants to try it can be confident that they are making something that will work. Here's my first Instructable so be nice :)
 
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Step 1: Build the circuit board

When i was looking in the internet for a metal detector circuit, I saw a couple of desing but they where not specific enough to go into detail and i figured a average person with no knowledge of electronics cant figure it out, so after much joining the dots i finally made one and decided to share it here. all the circuit design is from the link provided "easytreasure". The pictures of my device are mine.

Creative Commons License
This work is in the Public Domain.

The circuit board is the heart of the detector. There are 2 different styles out there for the home made version. I based my design on this circuit shown here. I build it to the same layout of the circuit, but not necessarily of the print out supplied. in other words, is the same circuit but looks different from the paper. The circuit I copied from this page. I did not design this circuit:

http://www.easytreasure.co.uk/bfo.htm

And it really easy. the parts can all be found on Radioshack for about 10$ total or ordered on the web for about 1.50$ total+shipping. Beware that I emptied the Radioshack out of capacitors and actually had to go to another Radioshack to find the missing one's.
here's a part list:

Power source:
Any 9v battery PP3 is ideal. Remember to also buy a 9V connector for your battery

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 ===== 2.2 Mega ohm
2 off 39k

The resistors can be 1/8 of a watt if you use 9V because according to my calculations the highest wattage going through the smallest resistor in the circuit is about .080 watts, and the 1/8 handle up to .125 watt. Radio Shack did had most of them but there was some that where hard to find. so you can either buy some and add the values up to the desired value, or buy bigger and use the Resistance division formula (Total R= 1/r1 +1/r2 + etc). I bought a small bag of 1/8 watt resistors that comes with like 120 resistors from all ranges possible to mankind (maybe not that much) but it had all necessary and now I got a big inventory of surplus resistors. it was the most expensive at 12$.

The transistors are npn. Radio Shack had this small package that comes with 15 NPN resistors for multipurpose general use, and they work fine. You can make 2 circuits and still have some to spare to in case you mess up and need more. Its about 2.50$ for the package.

You can also buy a PCB board to make the circuit and soldering iron (10-15$) 25 watt will do and soldering thin. You can use the cheap stuff for this, don't buy the silver one, too expensive, no difference in this project.

When you're there, get the cable for the coils. You will need about 4 FT of small 22 gauge DUAL wire (that is two wires joined, else you need 8 ft) and some 26 gauge for the coils or whatever you wana use. I bought a small bag that had 3 rolls of insulated enamel wire, 30, 26 and 22 AWG.

The following information, references and quotes are from the Designer website
http://www.easytreasure.co.uk/bfo.htm
"
Coil A = Search coil: Coil B = Reference coil: NC = No connection: B+ = Battery + 9V PP3 or similar : B- = battery -

Notes for the electronics beginner.

2 off 220uf / 16v Electrolytic : These are 220 microfarad / 16v working voltage. You can use a higher working voltage but not less. Higher working voltage capacitors work just the same but they get physically bigger. They have a negative lead that must be connected to the battery - track. These components must go in the correct way round.

5 off .1 and .01 polyester : These also have a working voltage. 63 volt in quite common and will be ideal. If you want to use the pcb layout above you will need capacitors with 5mm lead spacing. .1 can be marked as .1 or 100n or sometimes 104 : .01 can be marked as .01 or 10n or sometimes 103. These components can go in any way round.

All resistors 1/4 watt 5%: These are general purpose carbon film resistors with a 5% tolerance and rated at 1/4 watt. You could use resistors of a higher wattage as this does not affect the working they just get bigger. 1 watt or bigger will not fit on the board. These components can go in any way round.

Transistors: The bc 184b transistor is described has Audio, low current, general purpose NPN . These are quite easy to get in the UK but may be difficult to get in other countries. There are hundreds of types of small plastic NPN transistors available around the world and just about all will work in this circuit. You will have to be sure of the pinouts though. You can get the pinouts for most transistors from manufacturers websites. This will be the most likely problem area when building this project. These components must be connected correctly. PNP types won't work.
"


I used a Car hands free headset to canibalize the speaker to use for this circuit.
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magnific, please contac me to jonath55@hotmail.com

cmalandi4 months ago

Can someone please tell me if this detector can be used in gold prospecting....plz

jhunix036 months ago

plz help me idid but it seems not working!ive checked the schematic but it was right i test the transistor it was ok and the capacitor.i use a 2n2222a/ksp2222a pls help me why i did not work! pls answer me fast

nikkoleo27137 months ago

Can it detect GOLD ?

PLease reply ~

pablomar337 months ago

one question....detect gold?

TommyVarro1 year ago
Wow, brilliant job! you just made it. How many bulks you paid for your home made metal detector?
jswavy1 year ago
How thin or thick was your coils? And how many rings did you make with it as well ?
kameel2 years ago
its work but when i use large coil (60x30 cm) its detect just 20 cm or less for large object what i make wrong ???? cau help me plz
JoelDude (author)  kameel2 years ago
I have no idea what you may be doing wrong, too high frequency, or very low inductance may be your problem, it decreases the magnetic field growth. decrease the turns in both coils, or increase them? I'm no expert, good luck.
kameel2 years ago
i made every thing but resistor 0.5 watt and 2.2mega 1 watt and when i make test the sound i have just esh sound and there is no change so what wrong i made ???
JoelDude (author)  kameel2 years ago
the wattage doesnt matter. if you hear static sound then its working. try to insert many metal objects into the small inductor, see if it makes it better, if not, remove everything of metal from the litle inductor then add metal to the big coil on the ground. find a way to make them the same. good luck
kameel JoelDude2 years ago
its work but when i use large coil (60x30 cm) its detect just 20 cm or less for large object what i make wrong ???? can u help me plz
jrryrggd12 years ago
I have tried mutiple times to build this with no luck, have used both circuit designs. All I get is a clicking sound when I first turn it on, then nothing else.
Any ideas what I might be doing wrong?
JoelDude (author)  jrryrggd12 years ago
There is no way to tell, if you are using ONE, just ONE item that is different, that could make all the difference. Do you hear static? it could be that the frequencies are way off. maybe you are wiring it wrong. without me being able to look at the circuit in my hand, there is little to nothing I can do.
14jpm3 years ago
hello ... I really liked your article and I would like to know how I can modify it to find metal but 10 or more meters away, thanks
Slangman 14jpm2 years ago
Your not going to build that your self, that would be impossible
JoelDude (author)  14jpm3 years ago
maybe increase the voltage, thus increasing the magnetic field. but beware resistors may need to be bigger, and so on.
john henry3 years ago
im going to make one! i should have most of the components in my "parts box" lol
thanks!! for clarify some things the other site was weird....
i was wondering why you use "off" in the parts list?
why not use x instead lol
(2 off 220uF 16v electrolytic) (2x 220uF 16v electrolytic)
i took an electronics class for 2 years (2hours 30min a day) when i was in high school and never seen a parts list with "off" describing how many you need...
people dont just go around saying "i need 3 off loaves of bread" or
" 2 off cans of beans" lol...
off is describing if something is on or not lol.....
people also dont say i want 5x oreos 1x cup of milk lol
off just didn't make sense when used in the parts list
even if you got the list from the other site i would of fixed it. lol
(sorry im not trying to be rude or anything)
(lol electronics class was the highlight of my day any other class and im bored to death)....................
off is how engineers describe parts lists.
I know, i am an engineer
What type of engineer are you the word off means not on, like light is on or off, if you want to say you have resistors you use of not with a double f
JoelDude (author)  john henry3 years ago
Thank you Captain Proper Gramar!
Its Grammar Nazi btw
lol i was just saying.......

so what have you found so far with your metal detector?
im sure everyone else is curious if you found anything with this home made metal detector.
JoelDude (author)  john henry3 years ago
I've barely used it, it was fort a college presentation on oscillating circuits.

I used it to find a spoon the professor buried to see if it worked. I used to find come screws i lost while working on a motorcycle in the grass. I may upgrade it since I'm now working making mine detectors for the military and they use metal detectors as well as Ground Radar.
nice good luck
abhargav2 years ago
hello if we have to increase the range say fing any metal upto 3ft then wat hv to do?
JoelDude (author)  abhargav2 years ago
higuer voltage would do this, but you need to watch for the transistor's power dispation, and the resistors too. I dont know the answer to that Q.
perhans2 years ago
I would like to make a metal detector for finding nails etc. in wood, so I want to be able to find small objects but also within a short distance. Can you say something about how to modify your project in order to accomplish this?
JoelDude (author)  perhans2 years ago
then you want a very small inductor at the bottom, whatever diameter you see fit. Maybe Two medicine bottles, with same windings, should match themselves easily. then move the pill container around the wood. if I'm not mistaken, higher frequency is less current on the coil which means better battery life, less flux around the core, so its more directional and precise. Thats is all a guess. good luck.
syiqin2 years ago
hi,, its it we have to match both of the inductance value ,, then only it will works? if thats so,,

can i have your inductance value of both coil,, coz i dont have anameled with 26 AWG of diameter ,, i just have 0.2mm.. i will try to match your value with this diameter wire of mine,, pleeeeeeeeeeeeeeez reply me,, TQ =)
JoelDude (author)  syiqin2 years ago
You can use whatever wire you want. Just got to calculate the inductance of both and they have to match. Use online calculators, look for " air core inductor calculator" and you use the size of wire, diameter and turns, and you can get in the ball park, use the screw to fine tune them.
RaniaMomo2 years ago
thank you sooooooooooooooooooo much
Adnan munir2 years ago
SIR,
can i use any other ic instead of cs209a n making single chip metal detecter.
Hi Joel I am a amateur at this but I Have built this circuit 3 times now and cant git the speaker to make a sound I used all new parts when I built it each time.
Just don't know what I'm doing wrong. Can You explain whats happening in this circuit so that I can trouble shoot it better. All so I used the NPN 4011 and was wondering if there are different values of the same transistor or some thing.
Any help will be greatly appreciated.
JoelDude (author)  bcrabtree19792 years ago
Every single transistor has different values, and so minor tweaks may be need. It seems the Transistor you are using is WAY too old, and it was difficult to even find the Datasheet for it. Its also very high frequency and low gain. to drive the speakers you need high gain.

Try a 2n2222 or a 2n3904 transistors, easy to find anywhere.

I already explained what goes on in this circuit. The top small Capacitor-Inductor (LC) oscillates at the same frequency as the bottom LC circuit. They cancel each other out, when metal aproaches the bottom inductor, it changes the freq, and that difference peak pulse is aplified from left to right, trough each transistor, then to the speaker.
Can you replace the audio output with a visual signal? (small bulb / led / etc.)
JoelDude (author)  FirstAttempt2 years ago
You could but i dont know hwere exactly, i would suggest on the Collector of Q4 and ground
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 have been checked.
JoelDude (author)  baker amin3 years ago
yeah you would have to make people pass trough the inductor at the bottom basically. match the inductance of each, make the bottom one a circle 6 ft in diameter. you can bend the circle into a square. you can also use the inductor on the side panels of the "door way" where people walk trough and make it oval so it covers the whole door area, doesnt have to be perfectly cicular.
thank you very much
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