Discriminative Metal Detector 1978

[Esteban]

I have schematic BFO 5th harmonic like the posted, that use NE561 IC and 741. I have the re-published article in Spanish from English language magazine.

[mikebg]

Thanks Luciano.
The circuit diagram is similar to items BM8041 and NM8041 of Russian company Master Kit.
www.masterkit.ru
The principle of operation is known as a method for rapid measurement of very low frequency - countdown. In this method measures the period of the signal, and frequency is calculated as the reciprocal value.
This is the block diagram: