Metal detectors are devices used to find hidden metallic items. These items range from old coins to gold nuggets and metal trinkets, and they are commonly found at beaches and historic sites. All metal detectors operate by sensing changes in magnetic waves caused by metal. However, some types are more sensitive and expensive than others.
Very Low Frequency
A very low frequency, or VLF, metal detector is the most commonly used type due to its wide range of uses. With a VLF detector, a variety of different metals can be detected. This type of detector operates using two round coils of wire. The coil on the outside is the transmitter. This part emits a low-frequency magnetic field onto the ground. A metal object under the dirt causes this field be reflected. A separate inside coil is the receiver. This second coil amplifies any magnetic reflections causes by a hidden object. If these signal reflections are present, an audio tone is played.
In addition to these two coils, a VLF metal detector includes an electronic circuit used to filter the signal from the receiver. This circuit is essentially a "tuner" that allows the user to ignore certain levels of signal and focus only on the desired range. Because different types of metal reflect different signals, a treasure hunter can "tune out" signals from aluminum and steel and focus on valuables such as gold and silver.
The pulse induction, or PI, type of metal detector is a more specialized type of unit most commonly used by treasure hunters searching for items deep under the surface. Large versions of PI detectors are also commonly used at security checkpoints to detect weapons.
A PI detector usually uses only one coil. As with VLF types, a coil sends out a magnetic field into the ground. Any metal underground reflects this signal. Instead of using a separate coil for detection, however, a PI unit quickly switches modes to "listen" for the reflected signal. Using this timed pulse method, the device can discriminate between signals it has sent and stray signals from the surrounding area. This makes PI detectors more accurate but also more expensive.
Beat Frequency Oscillation
A beat frequency oscillation, or BFO, type of metal detector is the most simple and inexpensive design. For this reason, BFO metal detectors are popular with beginners. Like a VLF detector, this type uses two separate wire coils for detection. An oscillator creates a constant signal at a set frequency, which is emitted by one of the coils. The second coil detects the interference of this frequency caused by metal objects, which results in a changing audio tone.
The BFO detector operates similarly to the VLF model but lacks the filtering and fine-tuning ability of the latter design. This makes the BFO detector more prone to mistakes and interference and less able to differentiate between trash and treasure.