BFO Metal Detector
by Rachel and Steve Hageman
This article describes a very basic BFO detector which uses only two transistors and an LM389 audio amplifier. The LM389, in fact, contains the two transistors that form the search and reference oscillators so the whole circuit is comprised of a single IC and a handful of discrete linear components. Accurately matched transistors are not really necessary in the oscillators so you can use discrete transistors and an LM386 audio amp instead of the LM389.

This article was published in the "Design Ideas" section of the December 18, 1997 issue of EDN, a popular electronics trade magazine that offers free subscriptions to qualifying individuals.

BFO Metal Detector (PDF-72K)