Making a metal detector?

gareth evans snipped-for-privacy@yahoo.com wrote

Not necessarily any heterodyne.

They vary the effect on the field.

Ditto.

You didn?t look in the right place.

Well, the basic principal is what you say. I remember one Tandy used to sell had more than one coil for help in location, but apart from ferrous against non ferrous metals its hard to know what precision of type of metal it may have. Modern ones could be made to have profiles of common items I guess. Brian

Not very helpful unless you don't know what the right place is!

Brian

That is one of the more common old designs. Although pulse induction is also used today which comes with a different set of problems. These days I expect there are some based on convolutional codes too.

They often have two coils in the head. One the sending coil and the other the receiving coil and any metal in the sweet spot changes the coupling between them. The change is made audible in a traditional manner of a changing frequency and sometimes a meter indication.

Engineering it to have a narrow sweet spot for detection and tolerance of salt laden waterlogged ground are the main practical difficulties.

Most these days can distinguish ferromagnetic materials like iron and steel from non-ferrous metals. A few high end ones can recognise high conductivity in a ring - but none can avoid finding coke can pull rings :(

At a handwaving level this isn't a bad introduction to the various types of metal detector around and their method of operation:

I used one 50 years ago where the coils were enclosed in a vertical tube approx 3 inch diameter and 2/3 long.

It was used on tidal mud and could detect large ferrous targets buried 6 foot down and even tiny ferrous targets buried a few feet, or more.

gareth evans wrote on 31/08/2021 :

Basic Grade 1 electronics.

Two frequencies are mixed together. One is from a crystal,controlled oscillator the other is controlled by a large coil which forms the head of the device. A variable capacitor sets that one's frequency to be somewhat above the crystal frequency. As metal arrives in the vicinity of the larger coil its inductance increases and the frequency drops towards that on the Xtal oscillator. When the beat frequency drops into the audio range it appears as a whistle. The mechanical construction is more challenging than the electronics

If you are only interested in ferrous objects (buried gas pipes, unexploded bombs, etc. :-)), a proton magnetometer is the way to go.