Metal Detecting Basics
I have been metal detecting for decades. When I was a teenager, my dad, an electronics engineer who worked for NASA, built a metal detector in our garage from kit parts. With my background in Electronics and experience as an Electronics Instructor, my goal here is to provide some basics for our members who may be interested in this hobby.
Metal detectors work by taking advantage of certain electrical principles.
Ampere's law states that a changing electric field produces a magnetic field. This is how electromagnets work and is the underlying principle for making electric motors turn.
Faraday's law states that a changing magnetic field produces an electric field. This is how electric generators work. A magnet is turned inside a coil of wire to produce electricity.
A generator uses Faraday's Law to generate electricity. That electricity travels through a wire, and that electricity generates a magnetic field around the wire. This happens with any conductor and is not limited to wires. Conductors are not just metals. Metals are the best conductors, but other minerals, like salt, also conduct electricity. The difference is that non-metals are either poor conductors or are insulators that have near-zero conductivity.
The metal detector transmitter coil sends a magnetic field into the soil. That magnetic field produces a very small electrical current and thus a tiny magnetic field in any conductors in the soil. The metal detector's receive coil detects these tiny magnetic fields and transmits the data to the control box for analysis. The signals are so weak that they must first be amplified before they can be analyzed.
In the earliest models of metal detectors, the signal was converted to an audio signal. It was up to the operator to know what these sounds indicated and how to interpret them. Modern-day detectors still produce an audio signal, so I will cover how that works.
The sound produced has several distinct qualities that vary according to the target detected.
Targets are first separated into ferrous (iron) and non-ferrous. This is an easy task because iron is a great conductor, therefore producing a stronger magnetic field, and iron can be magnetized, so it holds the magnetic field longer. Non-ferrous targets can still be great conductors, like aluminum, gold, and silver, but they are not magnetic and do not hold the magnetic field.
The soil itself produces a signal as well, due to soil conditions such as moisture content, salinity (salt content), and composition (iron particles). Since the soil covers a larger area, the audio signal produces a slight buzz or hissing sound in the background. As the detector moves over the soil, a target produces an anomaly with a sound that is louder than the background noise.
The target sounds produced may vary by detector manufacturer, but generally sound like a bloop, blip, bleep, or buzz sound. The volume of the sound indicates the size of the target. Larger targets sound louder, while tiny targets may be barely audible above the background noise. The pitch of the sound (bloop, blip, or bleep) indicates the type of metal detected. The clarity of the sound indicates the clarity of the signal received. A coin has well-defined edges and a distinct shape, so it will have a clear, crisp sound. A wad of aluminum foil will have a more buzzing sound.
The best way to learn what these sounds mean is to practice with a detector. Many dealerships that sell metal detectors will allow you to rent one. That allows you to test drive various brands and models to find what you like before making a purchase. Many detector manufacturers set up test grounds with various soil types (sand, clay), density (gravel, rocks), and composition (high mineral content, iron, saltwater).
I set up a "coin garden" in my backyard that I would use for practice. Set up an area for use and use the metal detector to find, then remove, all metallic objects in that area. At certain locations, I would place various coins (penny, nickel, dime, quarter, half-dollar) as well as some junk items like a nail or screw, pull-tab, aluminum foil, bottle cap, key, etc. Allow several inches between targets so your coil does not detect more than one at a time.
As I would pass the coil over the target, I would try to remember what that target sounded like. After a long winter, it was a great way to remind myself what the detector was telling me. I would then test my knowledge by detecting another part of the yard and attempting to identify the target before digging it up. The key to remember is that your detector cannot lie to you; it is a machine. It is up to you to understand what it is saying.
With the advances of technology, modern detectors can analyze the data in much more detail and present the information as audio signals and visual displays. The visual displays will tell you approximately how deep your target is, as well as the most likely metal type. They provide a Target ID number to help narrow down what your specific target might be. (More on Target IDs later) They provide circuitry for ground-balancing, which helps remove the background noise produced by the soil, resulting in a more accurate signal for processing. They have a Discrimination circuit to block out iron targets, like nails. Some even have a "Notch" function so you can teach the detector what targets you wish to ignore, like bottle caps and pull tabs.
Detectors have become even more specialized, with circuits designed to specifically detect certain types of targets. My first two metal detectors were Bounty Hunter brand. The Bounty Hunter Pioneer 505 was an excellent machine, and I used it for almost twenty years. They work very well for coin-shooting and relic hunting. Every trip was successful in bringing home at least ten coins and occasionally a piece of jewelry or a metal toy.
With my interest being primarily gold prospecting, I invested in more specialized machines. I have a Garrett AT Gold Pro, which is designed to detect small gold nuggets, and I also have a Falcon MD-20 handheld detector for locating gold veins in rock or gold content in rock samples. I will explain more about those machines later.
Feel free to ask questions about this section. I will add more content to this thread later.
