This project is a demonstration of a metal detector. When it comes near to any metallic object, the coil of an oscillator changes the frequency of the oscillator to indicate the presence of metal. These types of metal detectors are used by people to locate lost treasures, buried pipes, hidden land mines and much more.
During war time especially these have been used to save many lives by locating mines and booby traps set out by the enemy. This circuit is a low distortion oscillator that draws only one milliamp from the 9V supply. Low power is desirable to allow the nearby metal to have maximum affect on oscillation frequency.
Use a small transistor radio tuned to a weak AM broadcast station as the detector for this oscillator signal. Tune this oscillator until a low-frequency beat-note is heard. This beat-note is the difference between the broadcast station signal and this oscillator signal.
Do not bring the radio closer than necessary. Just enough to obtain about the equal level on the two signals (the radio station and this oscillator). This is the best sensitivity setting. Try using keys, plastic objects, coins, etc. as samples of what to expect when using a metal detector like this.
Of course a real metal detector dose not have a small ferrite coil like this. It is usually an air core coil which is shielded with a aluminum electrostatic shield called a "Faraday electrostatic shield". This project at least gets the point across.