In this circuit, oscillator Ql runs at about 15 kHz and feeds mixer Ul. Ul has an internal oscillator that runs a
t around 15 kHz. CI5 is used to zero-beat both oscillators. When a magnet is brought near Ll or L2, the magnetic field shifts the permeability of their respective cases. This changes the oscillator frequency, and the audio note is passed through filter L3, C16, and C10/R7 to amplifier Q2, There the audio is amplified and drives meter Ml via rectifiers Dl and D2.