RS team

Note: in the Rough Science TV series I used a radio coil in the metal detector circuit board (in addition to the large search coil) but I have suggested that you replaced this in the web site version by a 1mH inductor (coil) (available from Maplins and other electronic supplies).

A metal detector works by making use of the fact that a metal object placed near to a wire coil modifies the electrical properties of the coil. We can devise an electrical circuit incorporating the search coil (the large circular thing at the end of the detector) to form what is known as an oscillator. This is a device that produces an electrical signal that goes on and off very fast (with a frequency of about 100,000 times a second - 100kHz). Now the frequency of the circuit is dependant on a number of things but most importantly it is dependant on the properties of the coil. If metal is brought near to the coil it changes the electrical properties of the coil which changes the frequency that the circuit oscillates at. If we can measure this change in some way we have the basis of a working metal detector.

The shift in the frequency is, however quite small. Aalso the oscillator is outside the range of human hearing so additional circuitry is needed so that we can make a practical detector. Another oscillator needs to built that works at very nearly the same frequency as the first. These two signals are then combined in a circuit called a 'mixer' who's output frequency is simply the difference in the frequency between the two oscillator circuits. Now all you need to do is amplify the signal so that it will run a loudspeaker.

In the Rough Science metal detector I managed to get all the components from the old radios. The components were fitted onto a piece of plastic and soldered together underneath to form a low-tech circuit board. One of the radios was found to have an amplifier that was working and this was used to boost the signal from the mixer electronics to drive the speaker. With all this we had the basis for a working metal detector.

RS team

When all the electronics had been constructed and wired up correctly it was time to test it out. The main search coil had a tuning capacitor wired across it which allowed the frequency of this oscillator to be adjusted a little. The other oscillator was fixed in frequency. The search coil oscillator was adjusted so that the two were very near in frequency and so it produced a low frequency tone in the speaker. When the coil was brought near to metal it changes its oscillator frequency and therefore changed the frequency you hear in the loudspeaker showing that metal is present.

So to detect metal all one has to do is turn on the metal detector and adjust the tuning on the main coil so that a note is heard and the volume adjusted so that it is loud enough to hear. Then by placing the coil near to the ground and slowly moving it across the earth one can detect buried metal because the note of the metal detector will change.

Note: this sort of basic detector is not very sensitive. As a general rule it will detect to a depth that is about the same distance as the size of the diameter of the coil, although this will be dependant of course on the size and shape of the buried metal object.

In order for the metal detector to work the two oscillators need to oscillate at very similiar frequncies. The difference between them would then be very small and we could hear the result as a note in the loudspeaker. In the Rough Science situation, because of the uncertainty and complexity of the circuit it would be very lucky if on turning on for the first time the two oscillators just happened to be set up correctly. So how / why did the Rough Science metal detector work, first time. Was it just luck?
It took me a while to work it out and it's a nice story. The circuits we used are very simple oscillators that unfortunatly tend to produce more than one frequency at a time. These extra frequencies are spurious signals and are often multiples of the main frequency called harmonics. So in reality rather than there being just being a single frequency from each oscillator; there are actually rather a lot of frequencies there all at once. As a result when you tune the capacitor there is actually quite a high chance of one of the harmonics from one of the oscillators being close in frequency to one of the harmonics from the other oscillator. It is these two close harmonics that probably provide the signals in the mixer circuit to get the metal detector to work first time!

Also our ears (and the last stage of the electronics) only 'hear' the products of the frequencies that are close together and we can't hear all the other complex mixture of things goings on. Harmonics are often a very annoying limitation of simple electronic circuits but in this case it was to our advantage as it allowed the circuit to work almost straight away!

Fine gold and nuggets
One last point is worth making. There is a chance of finding a large nugget of gold in a river but to be honest not a very high probability ! Most of the gold as you have seen from the programs is in then form of very fine flakes. The metal detector would have been best used higher up in the mountains where there is greater chance of finding larger lumps of gold.

When I got home and made my self a copy of the metal detector I found out a curious thing. I took two small aluminium box's and ground one up by fileing it to a powder. Then I put the fillings into a plastic bottle. Although the metal detector easily detected the complete box it could not detect the same amount of the powder ! This may be due to the aluminium powder particles being coated in an oxidised layer and therefore they are insulated somewhat from each other or it might be for some other reason. What ever the case this observation might mean that even if you find an area of concentrated gold flakes in the river mud the metal detector might not be able to detect it ! - it needs solid lumps or nuggets.


Dr Jonathan Hare, Physics Dept., The University of Sussex
Brighton, East Sussex. BN1 9QJ

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