Home Made BFO metal detector


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 Comment/How 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 Comment/How-to so be nice :)

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:


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

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 ===== 2.2 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
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.

Step 2:

Make the pick up coil

This was the most confusing part for me. This is why I made this Comment/How-to. all the other website give you vague descriptions of a coil and pretty much I wanted something right so here's my attempt at explaining everything right.

The first coil we are gona make is the receiver coil. To match the inductances of both coils I used this calculator to figure it out:


So I figured out to use these items to match my inductors:

A medicine bottle from CVS pharmacy. this bottle is about 31cm diameter in the bottom and about 32cm at the top, without lid. this bottle is a dark orange color. I left it sink in water for about 10 mins to remove the paper outside, and the remaining glue acted very well with the wires. I drill a two small holes at the top of the cannister, just below where the thick part that locks the lid is, and passed the wire from the outside, to the inside and back out. This locks the wire there and prevents it getting loose. the holes are within 3 mm of each other. Then I wound the 26 AWG wire 90 times around the bottle all the way to the bottom of the bottle. this must be done perfectly and without overlap. once the 90 turns are completed you can then make the spaces between the last few turns really big to make it go to the bottom. I made a small notch on the bottom of the cannister to prevent it from unwinding, then two small holes in the bottom ,to pass the wire through. this locks the wire and you have a perfectly nice coil. you may leave about 3 inches of wire left to work with. Then wrap around with black tape and superglue the last cut of the tape to itself to prevent it from peeling.

After this I frilled a small hole in the top of the cannister, you may use a small bolt to pass trough the hole to screw on through the middle of the cannister. this will adjust the inductance. No matter how close both inductors they will always need adjustment. Don't use a screw that is too big, the adjustment will be too coarse and you wont get it right. you may also get some small washers or nuts of steel (no pun intended). I will explain later.

Step 3:

Search Coil

this is simpler than the pick up coil but requires more patience. The website says to join 3 slices of wood together bla bla bla... I don't time for that or so much wood, or time to clean up all the wood shavings. I tried using plexiglass but its too hard to work with. SO I used a regular 1/2 inch thick plywood. I used a paint bucket to make the circle. so thats how big you need it to be.... or 23.87CM in diameter. Then I cut 4 small triangles to reduce weight. Now that I'm done I should mention: Go WILD making holes and whatever you want to the inside to remove weight. Plywood its not very heavy, but when moving at the end of a stick it picks up inertia.

I made tow small notches at the top and bottom sides of where I wanted to start. Remember to start and end in the same place. Then I TRIED to wind the 26 AWG wire. I say tried because it was horrible. What I did to fix this was to wind about 3 turns of wire, then tape it in 3 different places around the circle. Then wind the rest. this holds the first 3 turns from unwinding and if you use soft black electrical tape the wire will squeeze into the tape and griping on. its frustrating but it can be done. you wana do exactly 11 turns of wire. Then pass the wire trough the bottom of the detector, and back to the middle, don't cross over the windings, this will mess it up in some way I don't know exactly. This is why there are notches in top and bottom. After I got all the turns I superglued them at the tape spots, then superglued them at the notches, then taped all around and across the area. The most critical points for the tape I superglued the tape too.

This completes the search coil, for now.

Step 4:

Make the handle and body

Once you got both coils together, plug it all in and test it, if it works somewhat right, then its good, you can't tune it on your hands because when you install it it will change a lot.

My first idea was to use a long stick with a handle, and to use a T and two elbows in PVC to atach the search coil and give it mobility. Well scratch that, after I saw some desings I said the simpler the better. So I decided to reconfigure the layout to what it is now. starting from the top:

the T is conected to go under MY elbow and provide support, followed by small tube into a 90 degree elbow. This follows trough my hand for grip for about 4 inches. Then conects to a T. In one side I have a tiny piece of tube and a end cap fitting, in the other side the long tube down to the detector.

The electronics where fitted inside a soap box, for 99 cents at walmart. Or you buy the 3 or 4$ project box at radio shack.

In order to make some brackets to hold the equipment I used PVC pipes. I heated them until they are soft then step on them to lay them flat, sometimes I hamer them so they stay flat. I used a Heatgun, but you can use a toaster oven, regular oven, a BBQ propane torch or whatever. A ligther wont work, is too small ( or is it?)

The first small piece was heated and made flat. Then heated in the midle and once soft, pushed on top of a pipe in order to achieve the desired shape. Mine looked like a U with two flat sides. I then drilled holes for the small screws in the pipe and box. Then a small hole in the midle to screw ti to the main chassis. Remember to cover the screws inside the box with tape or somethign they can cause shorts.

The pick up coil was left outside because it wouldn't fit. I atached it the same way the box was, but wraped arround. Two small holes in top and bottom of the bracket where drilled for the screws. Notice the bracket only holds the coil it is not physiscally attached to it, its sort of "clamped" down. Avoid using any metal near the coils.

Before you glue any PVC together make sure you have everything the way you want it forever. PVC cement glue wil no let go ever. I used spray on adhesive 3M it worked allright.

I decided it would be easier to pass the wire to the search coil inside the pvc pipe.

To atach the search coil I heated the pvc and when soft I bent it to the right angle I wanted it to. Then used 2 screws to hold it after it cooled down.

I wraped everythign in tape and superglued the tape endings to itself.

Step 5:

Tune it up!!!!

Now that everything had been installed onto the body and the detector now looks more like somethign is time for the finishing touches. turn it on and listen for noise. Then screw the screw into the canister slowly and listen for change. If the tone goes higger, you are good. If you put it all in and nothing changes, you need more metal. One way to test this is to shove a big piece of metal inside the pick up coil, if you see a change and in some point beeping then you need more metal. IF it gets worse, then you have too much inductance, and the search coil is under inductance. try adding some screws to the detector plywood boddy. you have to get both inductors at same frequecy. with mine i needed more metal inside my pick up, and what i discovered is that metal close to the coils will change it more than in the midle of the canister ( like the screw). So now get those washers and nuts and put them inside the canister. Push them in until you get the sweet spot. then get some superglue and glue them there. this will add a lot of induction to your coil. then use the small screw to adjust it tight.

There's a lot of playing arround in this area, the better you make thigns the better they will work. IF the screw is too loose then it will go in and out of frequency all the time, this hapened to me and i'm in the process of re-engineering it. hoep it helps sned any Questions I'll try to help.


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