Using electronic diagram below can be built a simple metal detector. This detector will help you find objects composed of materials with relatively high magnetic permeability and also indicate if the object inside the detector coil is good or bad conductor properties
Metal detector is powered symmetrical from two 9V batteries, each loaded with about 15 mA. Detector coil L1 is part of sinusoidal oscillator built around transistor T1.
Needle deviation is a measure of the change in frequency, since the direction of deviation depends on the type of material detected by the coil. Instruments used here is zero central ± 50 uA.
Coil has 40 turns of enamelled copper wire, wound on a plastic template with a diameter of about 10 cm