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Beat-Frequency Oscillator(BFO) Metal Detectors

In a BFO system, there are two coils of wire, one in the search head, and another smaller one located inside the control box. Each is connected to an oscillator that generates thousands of pulses of current per second, the frequency of which is slightly offset between the two coils.


As the pulses travel through each coil, radio waves are generated and picked up by a tiny receiver within the control box. An audible series of tones based on the frequency difference is produced in the receiver. When the coil in the search head approaches a metal object, a magnetic field around the object is created as a result of interaction with the field created by the coil current. The object field interferes with the radio frequency, and as the frequency deviates from that of the coil in the control box, the audible tone beats the change in duration and tone.

The advantages of BFO-based systems are their simplicity that leads to ease of manufacture and low cost for the consumer. Their applications lie mainly in handheld scanners.

Some non-recreational applications for BFO metal detectors are:

  • Airport and building security – for screening of  boarding passengers and people entering a particular build, such as a school, office or prison
  • Event security - screen people entering a sporting event such as the Olympics, Wimbledon, or the like, concerts or other large gatherings of people such as conferences held at United Nations venues.
  • Item recovery - help someone search for a lost item, such as a piece of jewelry of minute size, such as a dislocated diamond in the depth of a swimming pool.
  • Archaeological exploration – to find metallic items of historical significance

Geological research - detect the metallic composition, such as iron, magnesium, or common metals of soil or rock formations.

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