Most pulse induction detectors use a single monoloop coil. With this technology, short pulses of high current are sent through this coil. Each current pulse generates a magnetic field that can penetrate the ground. When the current pulse is switched off, the magnetic field will not stop immediately. The remaining magnetic field is detected by the coil and the strength of this magnetic field is measured. After this measurement a new pulse is generated and the process repeats itself. Most pulse induction detectores have a pulse frequency of 100 Hz - 1 kHz.
Emission of a magnetic pulse
When there is a metal object under the coil, the metal will generate an opposite magnetic field. Because of this relatively small opposite magnetic field, it will take longer before the current in the coil will decay completely. So, the metal causes a kind of echo in the coil.
Opposite magnetic field in the metal object
Echo generation in the coil
The electronic circuit inside the detector measures the pulse width of the current/voltage decay in the coil. When the pulse takes longer than normal to return to zero, there is a metal object under the coil. The extra time determines the tone that is sent to the headphones.
Voltage flyback pulse over the coil with and without metal target
This type of detector has the disadvantage that it can discriminate very badly between different types of metal.
The biggest advantage is that it is highly independent of the type of ground. That is what makes it the detector of choice on beaches where the salty seawater provides very good conduction.
Another advantage is that this detector goes deeper than other types of detectors. Using a bigger coil in combination with higher currents can result in major depth.
