Thomas Scarborough has designed a new embodiment of the CCO (Coil Coupled Operation) metal detector, a new genre which he invented in 2004. This is the first IC design to appear on the Internet (there is a transistor design on many websites, and IC designs were published by Elektor and EPE). It took me just eight minutes to design this — which illustrates the simplicity of the principle.
OBSERVATION: On the surface of it, it may look like an IB (Induction Balance) detector — but it is not. It works on the principle of the transformer coupled oscillator (TCO), and requires an AM radio to generate a beat frequency in the speaker. This means that it (also) works on the beat frequency operation (BFO) principle. However, it is not a BFO detector either, for the simple reason that it has two coils instead of one — and it is far more sensitive. With this circuit, I obtained a one-tone shift in an AM radio speaker at 15 cm (6 inches) with a 27mm (1 inch) diameter coin. This is what you should aim for. You can find details of the construction of the (identical) coils L1 and L2 at http://thomasscarborough.blogspot.com/2008/05/bfo-metal-detector-2.html. These should be fixed to a search plate (e.g. hardboard, with cable ties). Note that the orientation of the coils matters: you might need to flip one of them over for best performance. This is an experimental design: experiment with the coils overlap, experiment with the AM radio frequency (you should tune in to a clear whistle, not a hiss, and find the “zero beat” zone), and you might even experiment with C2, or the size and shape of the coils. A screened wire may be taken to the AM radio aerial as shown — if a connecting wire is required at all. Click on the diagram to enlarge. Click on the “Electronics” category top left to follow the electronics trail on this blog.
NOTE: You may re-publish this design, on condition that you acknowledge the designer (Thomas Scarborough) and this blog http://thomasscarborough.blogspot.com/)