Using an oscillator running at 455 kHz, the metal-detector circuit produces an indication on the meter Ml. When the oscillat
or frequency changes because of metal in the field of Ll, the change will show as an increase or decrease in frequency, which produces a charge in the meter reading. The ceramic filter FILT1 produces a selective bandpass that yields this effect. Ll can be a 4" diameter coil wound on a suitable plastic form. About 10 turns of #26 wire are required. Use a frequency counter to adjust Ll and verify that Ql is operating on or near 455 kHz.