How do metal detectors work?

How do metal detectors work? Generally, metal detectors work using electromagnetic currents or radio waves, or a combination of the two. Whether being used to find buried treasure, hidden land mines, or for airport security, all metal detectors pretty much operate on the same basic principles.

There are three main technologies that answer the question ‘how do metal detectors work’.

• Very low frequency (VLF)
• Pulse induction (PI)
• Beat-frequency oscillator (BFO)

VLF metal detectors are also referred to as ‘induction balance’ because they use two coils of opposing electromagnetic current, known as ‘transmitter’ and ‘receiver’ coils. The two coils create the electromagnetic field – any conductive materials the field passes over notifies the receiver coil, and in turn, the operator of the metal detector.

VLF detectors are the most commonly used metal detectors because of their ability to ‘discriminate’ between types of metal.

A PI metal detector might use one coil as both transmitter and receiver or have 2 or 3 coils working in concert. PI-style detectors send strong short pulses of current – sometimes as many as 1,000-per-second – to create the opposite magnetic field. These bursts generate a reflected pulse, which can be likened to an echo. A sampling circuit then monitors the length of the reflected pulse to identify conductive objects.

PI metal detectors aren’t as good at discriminating different metals like the VLFs, but they have other specialized uses, such as superior depth detection. And they are very good in marine situations because the pulsing electromagnetic currents are not affected by salinity.

In BFO technology, the coils are connected to an oscillator. An oscillator can be described as a processing library that generates a (sometimes complex) series of waves. In this case, radio waves.

So the coils of a BFO metal detector are connected to an oscillator that sends thousands of offset pulses of current per second. It uses the radio waves to create audible tones based on the discrepancies between frequencies.

BFO technology sounds like the most complicated but is actually simplest. So simple, that with access to the right components you could build a BFO metal detector at home. They are simple and inexpensive but do not offer the accuracy and control of VLF or PI systems.

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