Metal Detector
by Jonathan Hare

Iron, gold and other metals
What is a metal detector?
How does it work?
How we made the detector
Electronics and circuits
How the metal detector was made
Using the metal detector

The challenge
Rough Science has a gold theme running through it, and one of the challenges for the first episode was to make a metal detector to help locate this precious metal.

Iron, gold and other metals
Metals are usually shiny, hard and conduct electricity. Some metals are also magnetic. A good test for a metal is to see if it conducts electricity, but this doesn't help us much when it's buried underground! If we were looking for iron we could modify a compass in some way, but unfortunately gold is not magnetic, so we need to find some other way to detect it.

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What is a metal detector?
A metal detector is an electronic device made of a large coil of wire, called a search coil — the round end of the metal detector — and some special electronics that allow us to 'hear' the presence of buried metal through the metal detector's speaker.

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How does it work?
The metal detector's electronics supply the search coil with a signal that makes it produce an electromagnetic field (an electromagnetic field is any moving or changing electric or magnetic field) around itself. The electronics also detect if the field produced by the coil is altered in any way.

An electronic device that produces electromagnetic fields is called an oscillator (an electric circuit capable of switching on and off very quickly). This is invisible, like the magnetic field lines around a magnet, but constantly changing.

If the search coil is resting on the ground, the field it generates will extend outwards and downwards to a depth roughly equal to the diameter of the coil. The field is affected and modified by nearby objects. Insulators such as dry rock don't change the field very much, but metals, which conduct electricity, will noticeably enhance or reduce the field.

If the search coil encounters metal, which affects the field, the electronics detect that alteration and produce a corresponding change in the note from the speaker. This change in sound tells us that we have detected buried metal.

The electromagnetic field is rapidly changing, perhaps at 100,000 times a second - much faster than our ears can hear. You may know that two musical instruments can be tuned to each other by bringing them close together and playing the same note. We can do the same thing electronically with the oscillators used in the metal detector. When we mix the search coil oscillation with another built-in oscillator at close to the same frequency, the result is a note that we can hear. In normal use, one oscillator is adjusted so that there is a constant note produced by the metal detector. If the search coil passes over ground where metal is present, it will shift the frequency of the search coil, producing a change in the note's pitch.

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How we made the detector
The building of a metal detector can be seen in three sections:

1. Two electronic devices or oscillators (one using the search coil)
2. A mixer - to combine the two oscillators
3. A power amp to drive the speaker.

We used three scrap radios to provide the electronic components for the metal detector. One of the radios was working, so I used its amplifier to power the speaker for the metal detector.

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Electronics and circuits
A radio is a device made up of electronic components such as resistors, capacitors, transistors, and diodes. The components are connected using wires and soldered to form a circuit. At the heart of the electronics are the transistors. A transistor can be used to amplify small signals into bigger copies of the original, and switch voltages on and off very quickly. The versatile properties of the transistor has led to them being used in almost every electronic invention, including radios, computers, mobile phones, and space travel - they have revolutionized our world.

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How the metal detector was made
First we constructed the search coil. A wooden form was cut out with a 12-inch diameter. Insulated wire was wound about 20 times around the form and then fixed to a wooden handle.

We decided to build the electronics in small plastic boxes, the kind used for food storage, to be as watertight as possible. The battery was placed in one box, while the electronics were built into another. The spare radio was used as a speaker amplifier and was attached near the top of the handle.

The metal detector electronics were connected to make the correct circuit using wire and metal solder. We used a plastic sheet to map out our circuit board. Holes were drilled where the component wires needed to go, and then the component ends were soldered, connected with wire underneath the homemade board, and placed in the waterproof box.

The search coil electronics (the oscillator) were built with a transistor and a few resistors and capacitors, and connected to the coil. A station tuning knob taken off the scrapped radio became our tuning capacitor, and was wired across the coil so we could adjust its frequency. Then we made the second oscillator, identical to the first one but with a smaller search coil. Because that coil was smaller and further from the ground, it would not be affected by any buried metals. A coil from the scrapped radio was used for this.

Next a 'mixer circuit' was made. This circuit electronically combines the signals from the large and small coils so we can detect the difference between them. That generates the signal that we will eventually hear (after it has been boosted in the amplifier stage) in the speaker. This note will change when we encounter buried metal and will be our indicator from the metal detector.

Finally, we wired up the battery to the radio and all the other circuits and made sure all the various pieces, i.e. the circuit blocks, were correctly wired to each other. The whole thing was assembled and 'fired up' live on TV - and by a miracle, it worked the first time!

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Using the Metal Detector
To use the metal detector you connect the battery and adjust the volume control on the speaker amp. The knob on the search coil tuning capacitor is adjusted until a note is heard from the speaker. If the search coil encounters metal, the note from the speaker will change to indicate the buried metal.

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If you would like to try making your own metal detector, take a look at the step-by-step instructions in the metal detector interactive.

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