Build your own metal detector
Building your own metal detector is an ideal school,
college, or hobby project. Requiring very little skill
Build with confidence this project costs nothing and is guaranteed to work.
BUILD YOUR OWN BFO METAL DETECTOR
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.
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.
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.
This is the only tricky part. The search loop is best wound on to a plywood former. Method 1: Cut three circles from some 3mm plywood, one 15cm diameter and two 16cm diameter. Using wood glue make a sandwich with the 15cm circle in the center. When the glue has set you can wind 10 turns of . 25 mm swg enameled copper wire around the groove in the edge of the former. Connect this coil when finished to the points marked coil 1 on the schematic. Method 2: Cut a 16mm diameter circle from some 10mm plywood. Then with this circle clamped in a vice run a saw around the edge of the circle so as to make a slot about 5mm deep and 2mm wide around the edge to accommodate the windings. If you have access to an oscilloscope or frequency counter make a note of the frequency. Ideally This coil will be oscillating at about 104khz, with an amplitude of about .5v p to p. The second or reference oscillator needs to be made much smaller and if possible attached to the control box so it can be adjusted as the detector is used. To make a really good adjustable reference oscillator you will have to visit a DIY store, what you need are some plastic water fittings, two examples are shown below. The smaller one is the inlet pipe to a plastic ball valve assembly fitted with a brass nut. The larger one is a plastic tank connector fitted with a brass nut from an old tap. Both of these work well and are glued to the control box in a position where they can be adjusted. The reference coil itself is wound on a piece of wood or plastic about 10/12mm diameter and about 50mm long The actual number of turns of this coil depends on the diameter of the former and can only be found by experiment. Start with about 125 turns . 25 enameled copper wire ( this coil when finished has to fit inside the plastic tube ) and remove turns until the two frequencies are close. This coil is attached to the circuit board at points marked coil 2. If all is well the detector should be howling at this point. When the two oscillators are well matched it should be possible by adjusting the brass nut in or out to bring the beat note to a halt or null.
Note. On the working detector shown in these pictures we wound 10 turns on to the searchcoil which then oscillated at 104 khz Then we wound on to a piece of 12mm dia x 50mm long wooden dowel ( taken from a bird cage ) 120 turns of wire. This was pushed inside a threaded plastic tube from a ball valve assembly. This oscillated at 96 khz without the brass nut and increased gradually as the brass nut was screwed on up to 106 khz. This was perfect for tuning the detector.
Searchcoil made from 10mm thick plywood
The reference coil is wound on to a piece of wooden dowel about 12mm diameter x 50mm long. This has to fit inside the Plastic pipe fitting above, and is tuned by moving the Brass nut. Drill a very small hole 1mm through each end of the wooden dowel so that you can pass the beginning and end of the wire through these holes to keep the windings in place.
This large coil is 30cm wide by 60cm long ( 12 inches by 24 inches ) and is made from 10mm plywood. It has 5 turns of wire in a 3mm deep groove cut around the edge with a saw. it oscillates at 104 khz . If you want to make different size coils start with the big one, as with only 5 turns you can only alter it in big jumps eg. 4 turns = 115 khz and 6 turns = 85khz . next make the reference coil to match. next make the next smallest coil and so on. The smaller coils are easier to match up as adding or removing a turn at a time only alters the frequency in small amounts
Building a practical detector.
Building a practical detector for outdoor use will depend on the skills and materials at your disposal. The golden rule is keep it lightweight, avoid using heavy materials such as hardwood or perspex. The round search loop needs to be glued to some sort of handle, with the circuit board inside a small box at the other end for balance. You will need to adjust the reference oscillator from time to time when in use.
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.
Frequently asked questions.
Q. How many meters down can I detect a single Gold coin with this detector.
A. None. For a single coin its about 120 mm if you are lucky. A bit more for larger objects.
Q. I built it but I cant get it to work.
A. Well it does work and usually straight away. The most likely reasons for not working would be. Transistor of the wrong type eg. PNP, or connected wrong. Wrong pinouts. Stripboard can be used but be careful not to get solder bridges across the tracks. If the search coil and reference coil are way off frequency you won't get any sound.
Q. What is the best size for the searchoil.
A. The bigger the coil the deeper it goes. But big coils are no good for small objects like coins and rings. The 16 cm coil is just about perfect for coins and rings.
Q. Can I buy a kit of parts from you for this project.
A. No we don't sell anything to do with this project.
Q. Our son wants to build this detector for a school exam project, could you build it for him and send it to us.
A. No tell him cheats never prosper. ( trouble is cheats do prosper ) .
METAL DETECTORS HOW TO TEST
At first sight of it you would think that testing a metal detector would be quite a simple task. Just dig a few holes bury a few coins and see how deep you can detect them. In practice its not quite so simple or straight forward. Books, Magazines, Field tests, and other publications tend to steer clear of any serious tests, preferring to quote in air tests that are just about useless. Actual in ground performance usually falls far short of advertised depths. So called independent field testers are usually under pressure from editors or on a bung from manufacturers to exaggerate depths.
You may be surprised to learn that many of the modern coins being minted today are actually made of steel. A steel blank is struck and then coated with a very thin layer of copper. Probably three quarters of the UK 1p and 2p copper coins have now been replaced with these steel clones. If selecting coins for metal detector tests it is easy to spot the steel coins by using a magnet. The pure copper 1p and 2p are ideal for detector testing, and will give about the best depth you will get on coins. The older English coins such as the 1 penny and 1/2 penny are also suitable for testing if you have them.
Using a magnet to separate out steel coins
Whether you are testing your detectors depth on a single coin, or a hoard ( see jar below ) it is most important to dig the smallest diameter hole possible. To avoid the detector detecting the hole. If you dig a large hole to bury a jar of coins and do some tests you may find that after you have dug up the jar and filled in the hole. The detector still detects the hole where the ground matrix has been disturbed. If you are fortunate enough to have a nice piece of lawn that is uncontaminated with junk. It can be useful to bury a row of coins. Say a line that is parallel to a fence, with the coins about 60cm apart and gradually getting deeper. You need to remember where the thirst coin starts, perhaps in line with a post or some other object, and about a metre from the fence. You need to check the ground with your detector before you start to make sure it is not full of junk. If the grass is damp you can carefully cut out with a sharp knife a small plug of grass about 50mm diameter. then with a spoon or similar tool dig your hole to whatever depth you like. Put in the coin, making sure it is laying flat and not on edge then fill in using all the soil you have dug out and finally replace the plug of grass. in a couple of years this type of test patch will give very accurate results. But make a note of how deep you have buried the coins, it is easy to forget. Put the first coin at 50mm then going down 25mm each coin.
An alternative method for testing on single coins is to make a set of testing plugs as described below. The basic idea is that a bullet shaped plug that contains a coin, made from hard wood, Nylon, or other engineering plastic is banged in to the ground with a hammer and driving rod to a particular depth. The ground needs to be not to hard for the plugs to survive. After the plug can be retrieved by being pulled out of the ground by a strong nylon cord, to be used again. The plugs themselves can be roughly made from hardwood, or machined from some sort of engineering grade of plastic or Nylon.
Plug components machined from Delrin ( an engineering plastic )
A couple of finished plugs with coins securely glued in place with two part epoxy adhesive
Driving rod is made from 25 mm diameter ally bar marked with 25mm graduations
Close up of a plug made from Paxolene with old English penny glued inside
When testing your detectors ability to find a hoard it will be necessary to prepare a sample hoard to be contained in a jar or cloth bag. Using a magnet make sure there are no iron clad coins. it takes about 500 coins to fill a coffee jar. It is most important that you dig the smallest diameter hole possible, so as to disturb the soil as least as possible. There are very few detectors that will detect this type of hoard more than about 45cm. Try it and see for yourself. Using this method it is difficult to know if the detector is detecting the hole you have dug or not. Sometimes you can remove the hoard to find the detector still detects the hole. As with burying single coins if you can leave your fake hoard buried for a few years it will then behave more like a genuine hoard. When doing these tests experiment with your detectors settings. All metal mode, various disrim levels/programs, ground exclude. If you have the time and can be bothered the only true way to test a detectors is to bury your hoard without actually digging a hole. You need to find a steep bank or ditch, or even dig one. Then with a small hand trowel dig some small tunnels at different depths horizontal so that you can push your Jar/bag of coins sideways as far as possible. this method will give results close to a genuine hoard.
Coffee jar containing about 500 copper coins
PINPOINT PROBE FOR METAL DETECTING
BUILD YOUR OWN
Apart from a pair of headphones and something decent to dig with, the most useful aid to metal detecting is an electronic pinpointing probe as described here.
The time spent locating tiny objects that have been detected, especially in muddy and sometimes bad light conditions can be considerable, and is time that is better spent looking for the next target.
Pinpointing probes are not a new idea. They have been on sale for at least twenty years, although very little development or improvement have been made to the original idea. The 1st probes were home made devices, cobbled together by enthusiasts, usually made from parts taken from hand held metal detectors of the type that are sold in DIY stores. The type of device that is used for detecting nails, pipes, or wires in floorboards.
One thing that is certain though, once you go detecting with a pinpointing probe you will soon find it an essential tool that is a must have aid for your detecting adventures.
This probe is a DIY project, we do not sell ready made probes. You do not need ant electronics knowledge to build this probe although if you do have some it may help. To build this probe you will need the following.
Ready assembled and tested circuit board ( only available from us ). all parts are soldered to the board, including ready programmed pic 12F675 microcomputer.
Piece of ferrite rod: 10mm diameter 50mm long **** see note 1
Piece of plastic overflow pipe 22mm dia 150mm long
22mm plastic pipe clip
Small Plastic/Aluminum box about 120mm long x 65mm wide x 40mm deep
Small speaker 40-50 mm dia 8 ohms
Some enameled copper wire .25 or .31 or .4 Enough to wind about 100 turns on ferrite above **** see note 2. Enameled copper wire is now included with the board
Some fine solder
A small soldering iron
1 PP3 Alkaline battery
1 battery holder
1 battery connector
A small amount of hook up wire
About two hours of your time
From left to right: Small plastic box, battery holder with leads, pipe clip, pushbutton, on/off switch, and two small 8 ohm speakers at the front.
Note 1: Ferrite rod is a black material that can usually be purchased from electronic component/amateur radio type shops. It usually comes in lengths about 100 to 150mm long. You only need 50mm so you will have to cut it and it is about as hard as glass. To cut ferrite rod you will have to score it in some way with the edge of a file or a hacksaw blade. Then press it over a matchstick or similar until it snaps. It is a bit like cutting a tile. If you have an old broken radio with an AM band then there will be a piece or ferrite rod inside that can be salvaged. Sometimes ferrite rod is square section say 6 x 12 mm, this will be OK.
Above is a selection of old radios purchased from a car boot sale one Sunday morning for less than a pound each. The two small radios at the front also contained very useful tiny speakers.
A selection of ferrite rods and speakers easily salvaged from the above radios.
Note 2: Enameled copper wire can also be purchased from electronic component suppliers, but you could make a thousand probes with the wire from a small reel. So again look in old broken radios etc. You need about 2-3 metres of wire. Plastic covered wire will do but it has to be very thin. The type of wire that is wound on ferrite rod aerials is too thin for this project. If you don’t mind buying a small reel of wire then .25 copper-enameled wire will be perfect. We have made probes using .31 and .4 wire
For the cost of the above items you could probably buy a ready made probe, but it won’t be as good as this one. The commercially made probes I have seen do not work as good as this one. They use crap components that soon break. They always have some sort of tuner that gets jammed with mud. And you can’t say “ I made it myself ”.
Four modes of operation . The probe gives an audible bleep when the tip comes close to metal. It also gives a visual indication with a bright LED that can be situated to shine on the target area. Modes are selected by pressing the pushbutton in a cyclic manner. E.G.. Normal audio: Normal audio plus LED: Normal LED only ( for silent searching ): Geiger ( ticks like a Geiger counter and speeds up when near metal. Then back to normal audio again. Each time the probe is switched on it starts in the same mode as when it was last used. Tuning is automatic. Pressing and holding the pushbutton for about 3 seconds will turn on the bright light to be used as a small torch. Uses Pic 12F675 microcomputer.
Battery low indicator.
Each time the probe is switched on it does a battery self test. If the battery is OK the probe bleeps once. If the battery is getting a bit low the probe bleeps twice
To begin with you must construct the coil. You need a piece of ferrite about 50mm long 10mm diameter or square section about 6 x 12mm. On the end of this rod that has not been cut ( the smooth square end ) you must glue a plastic or cardboard disc. This disc needs to be a nice fit into the plastic overflow pipe above ( about 19mm diameter ). Then wind the copper wire ( 100 turns . 4mm ) starting close to the disc at one end slowly moving along the rod to about 10mm and then back again until you have wound on 100 turns ( any direction ). You need two flying ends coming from this coil that will later be connected to the circuit board. Secure these ends to the ferrite rod with tape but do not cover the wound part.
From left to right: Ferrite rod, 3 pieces of ferrite taken from radios and wound with 100 turns of wire, 125mm long x 22mm dia plastic overflow tube, wound ferrite rod glued to plastic disk.
The Probe itself:
This tiny coil needs to be fixed inside the plastic overflow tube. How long you cut this tube is up to you. On most probes the probe tube is about 100mm long with about another 25mm going into the box. So you cut the tube 125mm long, or it can be longer its your choice. The coil is fixed as close as possible to one end of the tube with the flying leads coming out of the other. You need to block the coil end of the tube some way with a rubber/plastic cap or tape. You need to pour something down the tube to secure the coil in place. Make sure the far end of the tube is well sealed with masking tape or a glued in plastic disc or cap, then pour in for example. Two part Epoxy glue, Epoxy Resin, or silicon sealant. Paint or varnish may take a long time to harden so avoid this. Two part car body filler may do if you can get it down the tube.
Placing wound coil into plastic tube.
Wound coil inside plastic tube well sealed
Pour in a small amount of wax, epoxy resin, or two part glue.
Examine the LED ( Light emitting diode ) supplied with the board. You will notice that on the LED one lead is a bit longer than the other. Carefully solder a piece of Red wire to the longer lead and a piece of black wire to the other lead. Try not to get the LED to hot. If you do not have red or black wire use other colors as long as you remember what color is connected to the positive ( longer ) lead. LED’s will not work if connected the wrong way round. Slide a piece of rubber sleeve down one of the wires to stop shorting.
Ready assembled and tested circuit board Chip in the center is Pic 12F675 Microcomputer
You have to buy the circuit board from us. We do not sell to dealers agents or middlemen. The price for the board and the LED is £25.00 inc postage to anywhere. We accept payment by Cheque, Money Order, or Cash. Make payment to: DAVID SMITH. ( must be in Pounds Sterling ). As long as we get the money some way you will get the board. If sending cash don’t send coins. So many people had trouble getting the enameled copper wire so now it is included with the board.
Customers from outside EU should make sure such items are allowed in their country. Any customs label will be marked "sample of merchandize".