HOW TO MAKE A METAL DETECTOR

Click here to see it working on Youtube

http://www.youtube.com/watch?v=kEGPv0pu9cg&feature=related

http://www.youtube.com/watch?v=Y8he8JkC39c&feature=related

http://www.youtube.com/watch?v=es_h70Ky-tM&feature=channel&list=UL

You have to cut and paste these other links to see these Youtube videos

Building your own metal detector is an ideal school, college, or hobby project. Requiring very little skill or equipment. 

Build with confidence this project  is completely free, costs nothing, is guaranteed to work, and has been built by thousands worldwide.

This must be a really good project because people keep stealing it and calling it their own.

THE LIST OF SHAME

like here            Steve Robday/make projects

and here     Simon Fraser University Canada

and here                                              Matni

and here                             Scott W Hotaling

and here      Radioshack The Great Create. They never created it. They stole it from here.

Make projects and Radio shack recomend that you buy a pack of 500 resistors for this project.

as well as a lot of other expensive stuff

But like all losers they all made mistakes copying so you are better off here with the original that works

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.

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 ===== 2.2 Mega ohm
2 off  39k

Transistors:
All BC 184B, or 2N3904, or 2N2222A. 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, or purchased from companies like RS Components, Maplin Electronics, Farnell, or Digikey who's adds often appear at the top of this page..

Once the components have been obtained the circuit can be built in a few hours using copper clad stripboard, or if you have the facilities make a printed circuit board using the layout below. The original layout as below should print out at about 50mm x 100mm.

Coils
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  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 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 (tuning 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 dia through each end of the dowel so you can pass the ends 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. I have designed this circuit not to be to fussy about wire thickness. .25 .31 .4 will work. Must be insulated wire. copper enameled. I have made coils that work well using plastic coated wire  7/0.2mm. Plastic coated is OK for the large coils with just a few turns, but gets a bit bulky for the tuning coil and small coils.

This 600x600 coil made from plastic overflow pipe and bends from B&Q has 4 turns of 7/0.2 plastic coated wire and gives a really good signal on something like a manhole cover. It needs a very small amount of thin paint run around the inside to stop the wire from vibrating