This metal detector project is extremely sensitive and capable of detecting very small objects at a depth of at least 8 inches. It uses beat frequency oscillator (BFO) method. User need to do some adjustment to the relative temperature before use it since it is not extremely temperature stable. The oscillators circuit operate at frequency of 494 KHZ, just below the bottom end of the AM Broadcast Band. Although this is a bit high for a BFO metal detector, it allows for the finding of very small objects such as small rings and coins.
One JFET 2N5951 oscillator working with the search coil operates around 494 KHZ. The other oscillator operates at about 494.1 KHZ. The difference is about 100 HZ which is produced in the mixer stage. When the search coil close to metal, the frequency will shifts a bit and producing an audible frequency shift that can be heard via earphones. A following stage amplifies the audio signal and drives the 8 ohm earphone.
Project Firmware, Schematic and Documentation