SAXONS@clara.net 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 70khz and to arrange for the reference oscillator to be adjustable 70khz 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 ( motorboating ) any slight change being noticeable.
Power source:
Any 9v battery PP3 is ideal.
Capacitors:
C11= 220uF 16v electrolytic.
C1,C2,C3,C4,C10= .01uF polyester or polycarbonate.
C5,C6,C7,C8,C9= .1uF polyester or polycarbonate.
Resistors:
All resistors 1/4 watt 5%
R1,R2,R3,R4,R5,R7= 10k.
R8= 1K.
R9= 2.2m.....( not 22m as on circuit diagram )
R6,R10= 39k.
Transistors:
All BC 183B. Just about any small signal npn with a gain of 250+ will do
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.
Once the components have been obtained the circuit can be built in about two hours
using copper clad stripboard, or if you have the facilities make a printed circuit board
using the layout below.
Coils
This is the only tricky part. The search loop is best wound on to a plywood former. to
make this former cut three circles from some 1/8 inch plywood, one 6 inch diameter and two
6.5 inch diameter. Using wood glue make a sandwich with the 6 inch circle in the centre.
When the glue has set you can wind 15 turns of .4mm or 28 swg enamelled 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 and connect the battery. If you have access to an ocsilloscope or
frequency counter make a note of the frequency. Ideally This coil will be oscillating at
about 70khz but remember this detector will work at any frequency from 50khz to 100khz.
The second or reference oscillator needs to be made much smaller and if possible attached
to the circuit board. A small plastic coil former with screw in ferrite or brass slug is
perfect for this. The actual number of turns of this coil depends on the diameter of the
former and the type of slug used and can only be determined by experiment. Start with
about 150 turns ( you may have to use thinner wire ) 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 screwing the slug in or out to bring the beat note
to a halt or null.
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. There are two ways to do this. By putting a variable capacitor across coil 2. The larger the capacity the better. Or by changing the inductance . The very best commercially made BFO detectors varied the inductance of the reference coil in some way. On Q tone detectors manufactured in Tunbridge Wells some years ago the reference coil was placed inside a plastic tube about 1" diameter which was then filled with epoxy resin for stability. Then the frequency could be adjusted carefully over a wide range by a brass ring that was a snug fit on the outside of the tube, this was an excellent method. Yet Another detector by the same company used a reference coil wound on a solid plastic former ( it could be made of wood ) about 3/4 inch diameter. This former is drilled and tapped about m5 or 1/4 inch through the centre, so that a Steel or Brass bolt can be screwed in or out to adjust the frequency. This is also an excellent method.
The BFO detector suffers from drifting due to falling battery voltage and the effects
of temperature on the coils. You can run this detector from a 12 volt battery pack via low
dropout 8 volt regulator LM 2930T 8.0 is ideal. If possible use Polycarbonate capacitors
for C6,C7,C8,C9. When the coils are working properly coat the windings in paint, varnish,
or resin for stability. If experimenting with different frequencies below 100khz have been
found to be best. An improvement in sensitivity can be obtained by making the reference
frequency an exact multiple of the search frequency, although this also increases the
Drift. Keep it neat tidy and simple
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