Rough Science has a gold theme running through it, and one of the challenges for the first programme 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 are also magnetic. A good test for a metal is to see if it conducts electricity but this does not help us much when it is buried under ground! If we were looking for iron we could modify a compass in some way but unfortunately as gold is not magnetic we need to find some other way to detect it.
What is a metal detector?
A metal detector is an electronic device that consists of a large coil of wire, called a search coil (a search coil is the round end of the metal detector) and some special electronics to allow us to 'hear' the presence of buried metal by a change in the note from the metal detector's speaker.
How does it work?
The metal detector's electronics supplies 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 it.
An electronic device that produces electromagnetic fields is called an oscillator (an oscillator is an electric circuit that is capable of switching on and off very quickly). This is invisible, rather like the magnetic field lines around a magnet but constantly changing.
If the search coil is resting on the ground the field extends 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, cause the field to be enhanced or reduced.
The electronics can also detect if the field produced by the coil is altered in any way. If the search coil encounters metal, the electronics detect the alterations in the field, producing a corresponding change in the note from the speaker. This change in note tells us that we have detected buried metal.
The electromagnetic field is rapidly changing perhaps at 100,000 times a second - far above what the ear can hear. You may know that two musical instruments can be tuned together by playing the same note when the instruments are close together. We can do the same thing electronically with the oscillators used in the metal detector.
We can mix the search coil oscillation with another built-in oscillator near 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 with metal present, it will shift the frequency of the search coil producing a change in the pitch of the note.
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 the amplifier to power the speaker for the metal detector.
Electronics and circuits
A radio is an electronic device comprising electronic components, including resistors, capacitors, transistors, diodes etc. The components are connected together to form a circuit, using wires and soldering. At the heart of the electronics are the transistors. The 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, space travel - they have revolutionised our world.
How the metal detector was made
First the search coil was constructed. A wooden former was cut out with a 30cm diameter. Upon the former was wound about 20 turns of insulated wire and then fixed to a wooden handle.
We decided to build the electronics in small plastic boxes - the sort that are used for food storage - and made as watertight as possible. The battery was placed in one box, while the electronics was built into another. The spare radio was used as a speaker amplifier and was fixed near to the top of the handle.
The metal detector electronics were connected together to form the correct circuit using wire and metal solder. We used a plastic sheet as a circuit board upon which to fix the components. Holes were drilled where the component wires needed to go and then the component ends were soldered and connected using wire underneath the homemade board and then placed in the waterproof box.
The search coil electronics - oscillator - was built using a transistor and a few resistors, capacitors and connected up to the coil. A tuning capacitor (a radio tuning knob) was used from the scrapped radio and wired across the coil so its frequency could be adjusted. Then the second oscillator was made. This was identical to the first but had a smaller coil instead of the large search coil. As the coil is smaller and not near to the ground it is unaffected by any metal buried in the ground. A coil from the scrapped radio was used for this.
Next a 'mixer circuit' was made, this electonically combines the two oscillator signals so that we can detect the difference between them. This 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 stages i.e. circuit blocks were correctly wired to each other. All this was put together and 'fired up' live on TV - and by a miracle it worked first time!
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 indicating buried metal.
If you would like to have a go at making your own metal detector, have a look at the step-by-step instructions in the metal detector interactive.