NE5520 / NE5521 based discriminating circuit

[Lake]

Anybody tested this NE5520-based idea for metal detector?

http://www.tinaja.com/glib/hackar2.pdf
Pages 27.3 - 27.4.

Is the third coil necessary? I didn't get any sense to output when just two coils were used. Are some current driving transistor necessary to get more depth?
For this circuit I had to add also some resistance to sender coil to get the signal stable.

[Carl-NC]

You can omit the 3rd coil, just make sure the other two (TX and RX) are inductively balanced, e.g., DD style.

I had completely forgotten about this article... thanks for reminding me!

For those who might not know, tinaja.com is Don Lancaster's site, with all the articles he's written for various electronics magazines over the years. It is, IMHO, the single best electronics web site on Earth.

- Carl