Re: How do I make a metal detector, specifically the detector part.

Hi Michael,

Let me say at the outset, I do not design metal detectors. My engineering 
background is in instruments for measuring other 
things, and this is just an overview based of my sketchy knowledge !

The engineering of a real metal detector can be very complex. Metal 
detection is used in many situations, airport gun detection, 
rock crushing plants and textile mills spring to mind. My father was 
designing metal detectors capable of detecting a needle in a 
six foot by six foot bale of cotton 40 years ago ! 

There are several methods of doing metal detection. 

The most basic and easiest to understand  is to wind three wide coils. 
Imagine winding them on a tube. 1,2,3. The centre coil 
(2) is driven with an audible signal, say 5 Khz or something like.

This sets up a magnetic field in the three coils (through the centre), and 
the three coils are said to be coupled. When coils are 
coupled, then what you set up in one coil you can listen to in the others.

Of the other two coils(1,3), both are used to drive an amplifier, but one 
(3) drives it with its connections reversed, we say it is 
out of phase with the first coil. If things are set correctly and tuned 
right, then you can make the first (1) coil's signal be 
cancelled "filled in" if you like by the second (3) coil. If you listen to 
the amplifier with headphone, you would here nothing !

Now bring the coils near a piece of iron, and the presence of the iron 
bends the magnetic field in the coils, and the carefully 
adjusted balance is destroyed. You will now start to here the im-balance in 
your headphones.

There are other methods, another is to create a tuned circuit, like a very 
small radio transmitter, that is used to transmit an audio 
tone on the AM bands. A radio receiver is tuned to the output and listened 
to with headphones again. In the presence of a 
ferro-magnetic metal  (NOT a magnet, a piece of iron or nickel, or steel) , 
then the frequency of the transmitter changes, and 
you have to retune your receiver to make it audible. Non ferro magnetic 
materials are difficult to detect with any of the old style 
systems. It is possible to detect precious metals with modern detectors, 
though I don't know how they work. 

Yet another method involves Nuclear Magnetic Resonance, which works on very 
similar principles to the MRI machines in 
hospitals,but this is really way too complicated to explain here. Suffice 
to say it is the only metal detector that  needs two bottles 
of water to work ! The NMR metal detector is incredibly sensitive to 
changes in field strengths and is the method of choice in 
archeology I am told.

Hope this helps. Many older electronics magazines have carried metal 
detector projects in the past. I suggest you try and visit a 
library and consult them. The British magazine "Wireless World", now 
"Electronics and Wireless World" is a particularly good 
resource, and there are also some extremely good articles on audio 
engineering and stuff in it.

Best wishes.

Steve