Metal detector Sensor working Principle and Applications

 Metal detector


Metal detector is a device that can detect metal, the basics can make a sound when it is near some metal.

Metal detectors work on the principle of transmitting a magnetic field and analysing a return signal from the target and environment.

when some metals are coming close to the coil the amplitude of the reflective pulse is getting little lower and a duration of the pulse a little longer.

The need for detection is very clear to protect our self from any kind of danger. 

Principle of operation

The operation of a metal detector is based on the principle of electromagnetic induction.

Metal detectors contain one or more inductor coils. When metal is placed in a close proximity to a varying magnetic field (generated by the coil or coils), currents are induced in the metallic part.

These current are called eddy Currents. The eddy Currents, in turn, induce their own magnetic field ( called eddy field).

These fields act in such a direction as to oppose that generated by the coils. The resultant field and using a specially designed electronic circuit can indicate the type of material being magnetized.

Here in our metal detector circuit transistor is used as a colpitts oscillator.

If we increase L1’s inductance it will cause the decrease in frequency and if we decrease this L1’s inductance it will cause the increase in frequency. 

Types of Metal detectors

  • Beat-frequency oscillator (BFO)
  • Pulse induction detector (PI),
  • Very Low Frequency (VLF). 

How Do Metal Detectors Work?

Metal detectors work by transmitting an electromagnetic field from the search coil into the ground. Any metal objects (targets) within the electromagnetic field will become energised and retransmit an electromagnetic field of their own. The detector’s search coil receives the retransmitted field and alerts the user by producing a target response. Minelab metal detectors are capable of discriminating between different target types and can be set to ignore unwanted targets.

1. Control Box

The control box contains the detector’s electronics. This is where the transmit signal is generated and the receive signal is processed and converted into a target response.

2. Search Coil

The detector’s search coil transmits the electromagnetic field into the ground and receives the return electromagnetic field from a target.

3. Transmit Electromagnetic Field (visual representation only - blue)

The transmit electromagnetic field energises targets to enable them to be detected.

4. Target

A target is any metal object that can be detected by a metal detector. In this example, the detected target is treasure, which is a good (accepted) target.

5. Unwanted Target

Unwanted targets are generally ferrous (attracted to a magnet), such as nails, but can also be non-ferrous, such as bottle tops. If the metal detector is set to reject unwanted targets then a target response will not be produced for those targets.

6. Receive Electromagnetic Field (visual representation only - yellow)

The receive electromagnetic field is generated from energised targets and is received by the search coil.

7. Target Response (visual representation only - green)

When a good (accepted) target is detected the metal detector will produce an audible response, such as a beep or change in tone. Many Minelab detectors also provide a visual display of target information such as an ID number or 2 dimensional display.

Key detecting concepts


The frequency of a metal detector is one of the main characteristics that determines how well targets can be detected. Generally, a single frequency detector that transmits at a high frequency will be more sensitive to small targets and a single frequency detector that transmits at low frequencies will give more depth on large targets. Minelab’s single frequency technologies are VLF and VFLEX.

Minelab’s world-leading BBS, FBS, MPS and new revolutionary Multi-IQ technologies transmit multiple frequencies simultaneously and are therefore sensitive to small and deep large targets at the same time. 

Ground Balance

Ground Balance is a variable setting that increases detection depth in mineralised ground. This ground may contain salts, such as in wet beach sand or fine iron particles, such as in red earth. These minerals respond to a detector’s transmit field in a similar way that a target does. Due to the much larger mass of the ground compared to a buried target, the effect of mineralisation can easily mask small targets. To correct this the Ground Balance setting removes the responding ground signals, so you clearly hear target signals and are not distracted by ground noise.

There are three main types of Ground Balance:

Manual Ground Balance

 Manually adjust the Ground Balance setting, so the minimum amount of ground signal is heard.

Automatic Ground Balance

The detector automatically determines the best Ground Balance setting. This is quick, simple and more accurate than a manually set Ground Balance.

Tracking Ground Balance

The detector continuously adjusts the Ground Balance setting while detecting. This ensures that the Ground Balance setting is always correct.

Minelab detectors use exclusive advanced technologies for superior ground balancing capabilities that cannot be matched by any other detectors.


Discrimination is a metal detector’s ability to identify buried targets based on their conductive and/or ferrous properties. By accurately identifying a buried target you can decide to dig it up or consider it as junk and continue searching. Minelab detectors produce target identification (Target ID) numbers and/or Target Tones to indicate the type of target that has been detected.

There are four main types of discrimination in Minelab detectors:

Variable discrimination

 The simplest type of discrimination which uses a control knob to adjust the level of discrimination.

Iron Mask/Iron Reject

Used mostly with gold prospecting detectors to ignore iron junk.

Notch discrimination

Allows specific target types to be accepted or rejected.


The most advanced form of discrimination. Target IDs are plotted based on both ferrous and conductive properties on a two dimensional (2D) display. Individual segments or larger areas of the display can be shaded to reject unwanted targets. 

Detection depth factors

The most common question about metal detectors is ‘How deep do they go?’

The simple answer is "as deep as the diameter of the coil". So detectors with larger coils will detect deeper.

However, detection depth also depends upon detector technology and many environmental factors. A more complex answer usually starts with ‘It depends...’ The depth that a metal detector can detect a target depends on a number of factors:

Target Size

Large targets can be detected deeper than small targets.

Target Shape

Circular shapes like coins and rings can be detected deeper than long thin shapes like nails.

Target Orientation

A horizontal coin (e.g. lying flat) can be detected deeper than a vertical coin (e.g. on edge).

Target Material

High conductive metals (e.g. silver) can be detected deeper than low conductive metals (e.g. lead or gold).


  • Airport and Building Security 
Metal detectors are used for airport and building security to determine whether guns, knives, or other weapons are being transported onto aircraft or into public buildings.
  • Construction Industry 
Metal detectors are also used in the construction industry to locate steel reinforcement bars embedded in concrete, and to pinpoint metal pipes and wires in floors and walls. This is useful in avoiding unnecessary damage when replacing plumbing or wiring in a building or house.
  • Civil Engineering 
In Civil Engineering, metal detector are used to locate rebar (strengthen steel used as rod in concrete) .Rebar detectors are less sophisticated. Detectors can only locate metallic objects below the surface.
  • Land Mine Detection 
Land mine detector can sense as little as 0.5 grams of metal. So, now-a-days militaries are using land mine detector to identify the land mine. Military has used metal detector to pinpoint buried land mines since world war1.


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