The working principle of this cheap and easy to build metal detector circuit consists in mixing two equal frequencies which causes a low-frequency interference. When one of the oscillators become unstable then the frenquency of the interference will be modified. The metal detector circuit is built with CD4011.
The oscillator is built with NAND N1 and a ceramic filter of intermediary freq. (470kHz). The second oscillator is with N3 and a LC combination. The freq. of this osc. is adjusted in such way that will produce an audible oscillation of both freqs.
Thru N4, the signal from the variable osc. is amplified. If the sensor coil L1 is closer to a metal object then it will modify the auto-induction of the coil, the osc. is unbalanced and the sound will modify.
The metal detectors coil is made of: 70 turns of enamelled copper with diam. of 0.3-0.6 mm on a 5 cm diam.