Making My Own Metal Detector

Mention the words metal detector and you'll get completely different reactions from different people. For instance, some people think of combing a beach in search of coins or buried treasure. Other people think of airport security, or the handheld scanners at a concert or sporting event.

A basic metal detector consists of an electronic box and battery case on one end, with a brace or handle for the operator's arm. An insulated wire wraps around a telescoping shaft and into a round plastic disk called the coil. This disk comes off the shaft at an angle which allows it to be held parallel to the ground. The operator straps on or grips the electronic box and turns on the power. The idea is to slowly sweep the coil end over the ground until an electronic signal is heard. This lets the user know that some metallic element is buried directly beneath the area swept by the coil.

Metal detectors work on the principal of electromagnetics and their effects on conductive metals. There are actually two separate elements in the coil of a typical unit. One is a high-powered coil of metal which uses the battery power to generate a penetrating magnetic field. This coil is called the transmitter. As the elecromagnetic field enters the ground, anything metallic will become charged with magnetism, similar to a paper clip become magnetized after contact with a standard bar magnet.

Michael FARADAY observed (1831) that when a magnet is moved through a closed coil of wire, a current is induced in the wire. The direction of the current flow is such as to create a magnetic field opposite in direction to that of the change in the field produced by the magnet. Faraday then replaced the magnet with an electromagnet.

Two coils were wound close together, the first being connected to a battery and the second to a galvanometer, which measures small currents.

Metal detectors must also be adjusted to eliminate false positives generated by natural deposits of metal in the soil or sand itself. Most units allow users to change the sensitivity of the coil in order to cancel out the background clutter. Some other uses of metal detecting technology include security inspections at airports, government buildings and other public places. Construction crews and woodworkers also use hand-held metal detectors to find dangerous nails or other metallic debris in reclaimed building materials and trees.