Single Transistor Metal Locator

This is a metal locator circuit. This circuit is useful to detect metal. This circuit uses a AM radio and one-transistor oscillator. This circuit is inexpensive. The correct bias is provided by R1 and the frequency of oscillation is determined by search coil, C4 and C3. The search coil is formed by inductor which has varied inductance depend on the metal around it. Capacitor C4 and C3 is fixed in value, and the transistor that is used in this circuit is PNP transistor. Here is the schematic diagram of the  circuit:

The output of the oscillator falls within the AM broadcast band and the frequency of the oscillator is rich in harmonics. The output of the oscillator is combined with AM radio to detect the metal. If the metal is taken away from the search coil, the a low-frequency audio tone will decreases and increases when the metal is near. The AM radio can be removed, if this circuit uses two oscillator like Commercial metal detectors.