How a Metal Detector Works

Oct 17th, 2009 | By | Category: Detecting Tips

A typical metal detector used for detecting buried coins, gold, or landmines consists of a circular horizontal coil assembly held just above the ground.

A pulsed or alternating current generates a time-varying magnetic field around the coil, as shown in the diagram below. This field induces currents in a nearby metal object which, in turn, generate a time-varying magnetic field of their own. These fields induce a voltage in the receive coil which, when amplified, reveal the presence of the metal object or target.

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There are two broad types of metal detector, classified by the type of magnetic field generated by the transmit coil.

  1. Pulse induction (PI) detectors typically generate a transmitter current which is turned on for a time, and is then suddenly turned off. The collapsing field generates pulsed eddy currents in the target, which are then detected by analysing the decay of the pulse induced in the receiver coil.
  2. Continuous wave (CW) detectors generate a transmitter coil current which alternates at a fixed frequency and amplitude. Small changes in the phase and amplitude of the receiver voltage reveal the presence of metal targets.

Most metal detectors amplify the differences in the receiver coil voltage caused by nearby metal targets and generate a sound signal audible to the operator (aural display) when a target is detected.

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