A metal detector is a pretty awesome tool. They can detect metal objects, or metal inside of other objects. Metal detectors have many uses ranging from high profile security checkpoints at airports or government facilities, to simple hobbyist toys for those who want to go looking for hidden treasure.
So exactly, how does a metal detector work? In this article, I’m going to be discussing the metal detectors used by hobbyists. You know, the ones you see the guys on the beach walking all over the place with, with headphones on? Those kind. First, let’s talk about the parts of a standard metal detector.
The Parts of a Metal Detector
The search coil is at the bottom, with a stem reaching up to the controls, handle, and arm cuff. Now, let’s discuss what each part of the detector actually does.
How Metal is Detected
We’ll start with the most important part, the search coil. The search coil is what actually senses the metal. An oscillator(a component used for producing alternating current) transmits an electromagnetic field into the ground through a coil, which will energize any metallic object in that field. The metallic object will then transmit a signal that can be detected with the device.
With most detectors, the objects must be within a foot of the surface of the ground to be detected. This depends on the size of the coil used. Smaller coils may only reach a depth of 6 inches.
How it Lets Us Know it Found Something
Once a metallic object is detected by the coil, the signal is sent up to the control housing where it will give an indication of how strong a signal it has detected. An audible signal is usually sent to the user who can plug in a set of headphones to the device. The louder the sound, the stronger the signal received.
There will also be some sort of indicator on the device such as a simple needle display, to full-blown LCD displays. Some devices can even tell you what type of object it is detecting.
Of course, this is a simplified version of how a metal detector works, but to get a little closer look, let’s take a look at the different types of detectors and the technology that they use.
The 3 Types of Metal Detectors
There are 3 types of technologies used in metal detectors. Each one is explained below.
Very Low Frequency (VLF)
This technology is also known as “induction balance,” and is probably the most common type of technology used in detectors today. They are versatile, meaning they have the ability to detect a wide range of material.
As described above, VLF detectors use 2 coils to transmit and receive electromagnetic signals.
- Transmitter coil – the outer coil used to send the electromagnetic signal into the ground.
- Receiver coil – the inner coil used to receive electromagnetic signals.
The process used to distinguish between the 2 types of signals is known as “phase shifting.” In layman’s terms, it’s the difference in the 2 wavelengths that the transmitter coil is sending, and the receiver coil is receiving.
If an object in the ground is present, something called a “phase demodulator” detects the difference in wavelength, and sends an audible tone to the user.
“Discrimination” is a technique available in many detectors which allow you to control the sensitivity of the detector. The higher the discrimination setting, the more likely you are to miss certain objects. The best way to learn the discrimination characteristics of your detector is to set out various types of objects you would be searching for and test them out.
The frequency range used in “VLF” is 3-30 KHz, while the wave length is 10-100 km. As an interesting side note, the VLF radio bands used by these detectors are also used by the military for communications with submarines.
Pulse Induction (PI)
Another kind of technology is Pulse Induction. These are different from VLF detectors in that they use a single coil, rather than two. Repeated pulses are sent to the coil, which produce a magnetic field aimed toward the ground.
If a metallic object is detected, an opposite magnetic field will be generated, and is picked up by a “sampling circuit.” The sampling circuit then measures the time it takes for the magnetic field to decay. If it takes longer than normal (as if no metal objects were present) for the field to decay, an audible tone is sent to the control housing via an “integrator.”
The user is then able to determine when the metal detector is picking up on something.
Pulse Induction detectors are not as good as VLF detectors at discrimination, but have the ability to pick up on objects a little deeper in the ground. They are also great for using in areas with minimal trash, such as the beach. If you’re heading to the beach, Pulse Induction is the technology you want.
Beat-frequency Oscillation (BFO)
These are the simplest form of metal detector, and use the oldest form of technology among the three discussed here.
Two coils are used in this device. However, one of the coils is located at the bottom, and the other is located up higher in the control housing. Both coils transmit electrical current at frequencies just slightly different from each other. If the search coil a at the bottom passes over a metallic object, the object then alters the frequency at which the search coil is transmitting.
This widens the difference in which the two search coils are transmitting, which produces an audible tone. The tone is then amplified and sent to the headphones, allowing the user to know when an object is detected.
Hopefully this overview of how metal detectors work has helped to give you a better understanding of these interesting and cool devices. If you need help finding the right detector for you, please check out the rest of this site to help you in your decision.