How Metal Detectors Work

Anatomy of a Metal Detector

A typical metal detector is light-weight and consists of just a few parts:

  1. Stabilizer (optional) – used to keep the unit steady as you sweep it back and forth
  2. Control box – contains the circuitry, controls, speaker, batteries and the microprocessor
  3. Shaft – connects the control box and the coil; often adjustable so you can set it at a comfortable level for your height
  4. Search coil – the part that actually senses the metal; also known as the “search head,” “loop” or “antenna”

Most systems also have a jack for connecting headphones, and some have the control box below the shaft and a small display unit above.
Metal Detectors
Operating a metal detector is simple. Once you turn the unit on, you move slowly over the area you wish to search. In most cases, you sweep the coil (search head) back and forth over the ground in front of you. When you pass it over a target object, an audible signal occurs. More advanced metal detectors provide displays that pinpoint the type of metal it has detected and how deep in the ground the target object is located.
Metal detectors use one of three technologies:

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

In the following sections, we will look at each of these technologies in detail to see how they work.

VLF Technology

Very low frequency (VLF), also known as induction balance, is probably the most popular detector technology in use today. In a VLF metal detector, there are two distinct coils:

  • Transmitter coil – This is the outer coil loop. Within it is a coil of wire. Electricity is sent along this wire, first in one direction and then in the other, thousands of times each second. The number of times that the current’s direction switches each second establishes the frequency of the unit.
  • Receiver coil – This inner coil loop contains another coil of wire. This wire acts as an antenna to pick up and amplify frequencies coming from target objects in the ground.

The current moving through the transmitter coil creates an electromagnetic field, which is like what happens in an electric motor. The polarity of the magnetic field is perpendicular to the coil of wire. Each time the current changes direction, the polarity of the magnetic field changes. This means that if the coil of wire is parallel to the ground, the magnetic field is constantly pushing down into the ground and then pulling back out of it.
As the magnetic field pulses back and forth into the ground, it interacts with any conductive objects it encounters, causing them to generate weak magnetic fields of their own. The polarity of the object’s magnetic field is directly opposite the transmitter coil’s magnetic field. If the transmitter coil’s field is pulsing downward, the object’s field is pulsing upward.

he animation above demonstrates VLF technology.

The receiver coil is completely shielded from the magnetic field generated by the transmitter coil. However, it is not shielded from magnetic fields coming from objects in the ground. Therefore, when the receiver coil passes over an object giving off a magnetic field, a small electric current travels through the coil. This current oscillates at the same frequency as the object’s magnetic field. The coil amplifies the frequency and sends it to the control box of the metal detector, where sensors analyze the signal.
The metal detector can determine approximately how deep the object is buried based on the strength of the magnetic field it generates. The closer to the surface an object is, the stronger the magnetic field picked up by the receiver coil and the stronger the electric current generated. The farther below the surface, the weaker the field. Beyond a certain depth, the object’s field is so weak at the surface that it is undetectable by the receiver coil.

For more Detail: How Metal Detectors Work


About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

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