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20 June 2003
A Simple BFO metal detector
Source: Essex Metal Detectors
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 (motorboating) any slight change being noticeable.
Parts list
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
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.
Other Transistors will work. For example ---- 2N2222A,2N3904,2SC2240,MPS6520,MPS6512
Construction
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 10 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 100khz but remember this
detector will work at any frequency from 50khz to 150khz. 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 130 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
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 Qtone detectors manufactured at Tunbridge Wells Kent 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.
Improvements
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 around 100khz has been found to be best. Keep
it neat tidy and simple
