This project is perfect for student experiment, not so complicated but very interesting. It is a metal detector that uses utilizes two oscillators that produce radio frequency. One coil is the search loop and the other smaller one is the reference oscillator. The search loop is fixed at 100khz and the reference is adjusted to 100kHz with a tolerance of 250hz. When they are almost the same 100kHz there will be no sound produced. A slight change will produce a note when the search loop is near a metal.

With very minimal components it could be done in a few hours. The sound is audible but a high impedance headphone is better. The search loop is made using one 15 cm diameter and two 16 cm diameter 3mm plywood, put wood glue on both sides of the small one and sandwich it between the two big ones using a clamp. When it is dried up, make ten turns of .25 mm enameled copper wire around it and the ends soldered to marked as points coil 1. If an oscilloscope or digital frequency meter is around, check for its frequency, it is around 104kHz. The second one is smaller and will be placed inside the control box and is adjustable. You will need an inlet pipe and a ball valve with brass nut another is a tank connector using brass nut, both of them are plastic.

The reference coil is made from a 10mm diameter by 50mm long plastic or wood. The number of turns is about 125 from a .25mm magnet wire. The finished coil should fit in the plastic tube. The number of turns will either be added or subtracted to make the frequency almost equal to the first coil. To assemble a detector to be used outdoor will depend on the materials available and your skills to do it.

http://www.easytreasure.co.uk/bfo.htm