esquema detex-3000.doc

Build an induction

balance metal locator

Main Features

• Easy to build & operate.

• Suitable for use over wet

or dry ground including beach sand.

• Adjustment to exclude

ground effects.

Most do-it-yourself metal locators are dif-ficult to build & operate but not this one. This unit is a cinch to put together & is just the shot for finding coins, rings, watches & other valuable metallic items.

By JOHN CLARKE

• Sensitivity control.

• Audible indication via

head¦phone or loudspeaker output when metal detected.

• Sound increases in

frequency as metal moves

under search head.

• Counterbalanced handle

for ease of use.

Of course, as well as finding those more mundane items, a metal locator can also be used to locate the metal of our dreams — GOLD! But let’s be real-istic; not many of us are ever going to strike it rich on the goldfields, although metal locators have detected large nug-gets for a few lucky prospectors.

No, a metal locator is more likely to be used for fun and any profits made from finding coins or jewellery are likely to be quite modest. Then again,

you never know what might be hidden under the next few square metres of beach sand.

The big advantage of a metal locator is that it saves lots of digging. One only has to dig in locations where the metal locator indicates the presence of metal. Of course, not all finds will be of any value except maybe for the recyclers of cans and scrap aluminium.

To overcome this problem, some metal locators incorporate controls

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which discriminate against various types of metals (eg, ferrous metals) which are likely to be of little value. Taken to the extreme, the ultimate metal locator would find only things of value. As expected, metal locators which can discriminate against unwanted metals are usually expensive and can be extremely complicated to use. They are best left for experienced prospectors.

The SILICON CHIP Induction Balance Metal Locator is not a discriminating type and is very easy to use. In fact, there are just three control knobs: Vol-ume, Ground and Sensitivity.

The first control sets the volume of the output from the loudspeaker or headphones. The second control (Ground) is the most frequently used — it adjusts the sound from the loud-speaker so that it produces a low frequency growl when the search head is positioned over the ground. The fre-quency will then increase sharply when metal is detected.

The final control adjusts the sensitiv-ity of the unit and sets the maximum depth at which an object will be detected.

Operating principle

Most simple metal locators operate on the principle of beat frequency oscilla-tion (BFO). In this type of design, the search coil is used as the inductive com-ponent of an oscillator. When a metallic object is brought near the coil, the fre-quency of the oscillator changes slightly due to the resulting change in the coil’s inductance. This frequency change is detected by mixing the oscillator fre-quency  with  a  fixed  frequency  to

produce an audible beat.

It is often claimed that BFO metal locators are able to detect the differ-ence between ferrous and non-ferrous metals. This is because the inductance of the search coil increases with ferrous metals and decreases with nonferrous metals, corresponding to decreasing and increasing beat frequencies respec-tively.

In practice, however, the audible beat can also increase for ferrous metals since eddy current flow in the iron often masks out the effect of increasing inductance. It is therefore impossible to discriminate between the two different

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types of metal.

By far the biggest disadvantage of the BFO technique is that the search coil must be shielded with a metal screen to prevent reaction with the ground. This significantly reduces the sensitivity of the BFO type metal locator, which means that small objects buried in a few centimetres of soil can easily be missed.

To eliminate this problem, the SILI-CON CHIP metal locator uses a completely different operating princi-ple. Unlike the BFO type, it uses two coils in the search head, with one coil driven by an oscillator. The second coil is used to pick up signal from the first.

During construction, the two coils are positioned in an overlapping fashion so that the second coil has minimum pick-up. When metal is introduced, how-ever, the signal level in the second coil increases. This increased level is detected and the resulting signal used to drive circuitry to provide an audible indication that metal is present.

This principle of operation is called “Induction Balance” (also known as “Transmit Receive”) and it provides a far more sensitive metal detector than the BFO type. Its only disadvantage is that the two coils must be carefully aligned during construction.

The depth to which the metal locator can detect metals under given condi-tions is set by the search head coil diameter. The larger the diameter, the deeper it will detect. However, large search coils suffer from lack of pin-point accuracy in finding metals. We opted for a medium-sized search head which provides a good compromise between accuracy and depth.

Of course, there’s nothing to stop you from experimenting with larger search heads if depth is important.

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