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HOW TO MAKE A METAL DETECTOR
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HOW TO MAKE A METAL DETECTOR
See it working on Youtube here
There are three or four Youtube videos of the project if you can find them.
Building your own metal detector is an ideal school,college, or hobby
project. Requiring very little skillor equipment.
Build with confidence this project is completely free, costs nothing, is
guaranteed to work, and has been built by thousands worldwide.
Use this Google custom search to find information and suppliers for
Transistors,resistors,capacitors and other electronic components.
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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
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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.
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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, Radio Shack, 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 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
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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
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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
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