This blog provides a brief explanation of how pulse induction circuits detect metal objects.
Pulse induction metal detection relies on the fact that when metal is in a changing magnetic field, eddy currents are induced in the metal. These eddy currents form complete circuits which are equivalent to electro-magnets which generate their own magnetic fields. As these induced magnetic fields fluctuate, they also induce current in the originating inductor (antenna).
If the antenna in this circuit is driven with a current pulse, when the pulse ends, current will continue to flow in the antenna (inductor) and in this case the current will decay because there is a resistive load on the antenna. As the current decays, the magnetic field will also collapse.
The effect of foreign metal in the magnetic field of the antenna and the resulting fields produced by induced eddy currents is to prolong the decay of the current in the antenna. We can detect metal near the antenna by noticing when the decay period starts to extend.
This is an image of the PCB discussed in the videos:
Here is a video showing how this circuit performs this function:
Here is a video showing how the circuit responds to various coins from around the world:
I originally designed this circuit board to experiment with pulse induction metal detection and learn more about it.
The project has been a great way to delve into the subject.
At some point I will convert this circuit to a microcontroller with LCD display (and shrink the board at the same time) and post a blog about it.