A metal detector is a hobby and/or professional device that consists of a carrying handle (usually with a support device for your forearm), a control box at the top, an extendable shaft, and a search coil. So, how do metal detectors work (simple explanation)?
A metal detector works by transmitting a signal into the ground. When the detector coil passes over a metallic target, it receives a return signal that the detector’s electronics can interpret as a target. This interpreted signal is presented to the user via the control box as a visual display, an audible tone, or both. The coil is responsible for the detection of metal, the control box contains the interpretive circuitry and headphone jack, and the shaft adjusts for proper distance between the operator and the ground.
Underground targets made of copper, gold, silver, nickel, zinc, or lead are more conductive than items made of foil, iron, or tin. The metal detector’s circuitry can use this information to display pertinent and useful target information to the operator. For example, this helps discriminate between junk targets and good targets.
There are different kinds of coils used on metal detectors. The coil contains both a transmit and a receive loop of wire. The coil is what your detector uses to sense metal targets in the ground and allows the control box electronics to distinguish between different types of metals in the two main groups: ferrous (iron) and non-ferrous (aluminum, gold, silver, etc.)
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A Bit More technical
Metal detectors make use of electromagnetic induction science to sense metallic elements in the ground. The science says that changing magnetic fields cause changing electric fields, and vice versa. If you’re interested, this is covered in more detail in another post (see the “Bonus Material” here.)
Two Primary Technologies
There are two primary technologies in play for hobby/professional metal detectors: Very Low Frequency (VLF) and Pulse Induction (PI).
VLF uses the transmitter loop in the coil to send signals into the ground and then compares them to the return signals from detected objects to produce an alert to the operator. A PI metal detector sends very rapid signal pulses into the ground and measures the time difference between an expected return pulse and the actual return pulse. The two different technologies serve different detecting goals.
You may hear someone speak of a VLF metal detector’s “operating frequency.” Your detector’s transmit coil sends alternating electric current into the ground, so the operating frequency is the number of times the direction of the current is changed per unit of time. Depending on the design, modern metal detectors can send out between 5,000 and 25,000 waves per second. So you’ll see that operating frequency referred to as 5 – 25 kiloHertz (kHz) per second.
Metal Detector Frequency Ranges and what they’re best for:
- Less than 8 kHz – Large items, buried deep
- Around 12 kHz – General coin detection but also many types of deeper objects
- Around 18 kHz – Small coins and jewelery (even in trashy areas)
Note of caution: If you’re near detectors on the same frequency, you are likely to get electromagnetic interference (EMI) from them. Distances to avoid are about 20 feet or less.
TIP: If you have a newer detector with a “frequency shift” feature, you can set this up or down a notch to avoid the interference
Old School – Beat-Frequency Oscillation (BFO) Technology
This early metal detector technology hasn’t been produced much in decades. In its day, it was simple to make and produce. They are sometimes referred to as “universal detectors” because of the range of targets you could detect with one machine. Modern technology made them obsolete.
If you feel like a real challenge (or at least an education), here’s a video I found on how to build one today.
How deep can a metal detector go? It varies, but a good rule of thumb is approximately as deep as the coil diameter. Be aware that there are a number of factors that influence the final answer. These include target size, shape, orientation, conductivity/material, soil type, and ground slope.
- Depth: Generally 4 to maybe 18 inches, though there are some high-cost deep metal detector models that can hit up to 65″
- These shallower depths are usually the limit of where objects lost within the past several hundred years will be found.