AM metal detector

The metal locator uses a one-transistor oscillator and an AM radio to detect metal. Transistor Q1 is a pnp transistor that is connected to an oscillator. Resistor R1 provides the correct base bias and capacitors C3 and 04 and the search coil determine the frequency of oscillation. Capacitors C3 and 04 are fixed in value, but the search coil is an inductor that varies in inductance (and thus varies the oscillator frequency) as metal is brought near it. The oscillator frequency is rich in harmonics and its output falls within the AM broadcast band.
AM metal detector - schematic

The metal detector works by combining its output with the local oscillator of the AM radio. The resulting net output of the radio is a low-frequency audio tone that changes—gets higher or lower—as metal is brought near or taken away from the search coil. Commercial metal detectors use two oscillators, so they don't require an AM radio. This metal locator provides an inexpensive alternative to an expensive commercial metal locator. Parts: ------------- 01, C2 O.O1-?F Capacitor (103) C3, C4 0.001-?F Capacitor Q1 2N3906 Transistor R1 47-k? Resistor R2 100-? Resistor