Simple project arduino uno12/8/2023 ![]() The presence of a metallic object near a coil will change its inductance. The inductance of a coil with N windings and a radius R is approximately 5µH x N^2 x R, with R in meters. This effect is called self-inductance, and the unit of inductance is Henry, where a coil of 1 Henry develops a potential difference of 1V when the current changes by 1 Ampere per second. Thus, a voltage will develop across the coil that opposes the increase in current. According to Faraday’s law of induction, a changing magnetic field will result in an electric field that opposes the change in magnetic field. When electricity starts flowing through a coil, it builds up a magnetic field. A thin piece of aluminium foil oriented in the plane of a coil will give a much stronger response than a heavy metal bolt. ![]() The weight of the object does not matter much. Thus a metal disc in the plane of the coil will give a much stronger response than the same metal disc perpendicular to the coil. It is most sensitive to objects in which a current can flow in the plane of the coil, and the response will correspond to the area of the current loop in that object. As a rule of thumb, the detector is sensitive to objects at a distance or depth up to the radius of the coil. To interpret the signals and to understand what materials and shapes the detector is sensitive to, it really helps to understand the physics. An additional advantage is that all can be powered from a single 5V power, for which a common 2000mAh USB power is sufficient and will last many hours. LED’s, a speaker and/or headphone are then added for signalling the presence of metal near the search coil. Instructions for DIY metal detectors are plenty, but this recipe is particular in the sense that it requires very few components in addition to an Arduino microcontroller: a common capacitor, resistor and diode form the core, together with a search coil that consist of circa 20 windings of electrically conducting cable. Check your local regulations on how to act in case of an eventual find, in particular in case of hazardous objects, archeological relics or objects of significant economic or emotional value. Metal detection is a great past-time that gets you outdoors, discover new places and maybe find something interesting. *** A new version has been posted that is even simpler: ***
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