Very low frequency metal detectors are often referred to as VLF detectors, and are made up of a transmitter coil and a receiver. The transmitter coil contains a wire that conducts electricity, which changes direction several times in a second. The number of changes in energy direction per second equals the frequency of the transmitter. The receiver picks up frequencies of the target objects. For a science fair project, a student can select a number of objects made of metal, plastic, glass, and wood, and test the reaction of the metal detector in contact with the different objects. The student can also conduct a test on metals that vary in inductance and resistance to observe and record how the detector's sound or visual indicator changes with each. The variations are due to the phenomenon known as phase shifting, which is the difference in timing between the transmitter coil's frequency and the frequency of the metals.
Pulse induction metal detectors, commonly called PI detectors, don't differentiate between types of metals, but they are helpful in detecting metals that are deep in the ground. The PI machine sends pulses that create magnetic fields. The student can view the magnetic field that is created using a Gauss meter or magnetic field viewer film, and should observe and record how the magnetic field changes as the metal detector encounters plastic and metal objects.
A beat-frequency oscillation metal detector contains two coils of wire, one in the control box and the other in the search head. The wires are connected to an oscillator, which generates pulses. The pulses go through the wires and generate radio waves. The student can hear the waves through the receiver, and should observe and record how the tones and number of beats change as the detector intercepts the magnetic field of a metal object. They can note the number of beats per minute when the machine is near different types of metals or other objects made of other materials.
Building a simple metal detector is a way for students to find out how metal detectors work. This project is suitable for a team of two or even more students. Students at fifth grade level or higher should be capable of building a metal detector by following the guidelines of a teacher or instructions in an electronics manual. The students can build any type of metal detector. For instance, for a beat-frequency oscillation metal detector students need a 9-volt battery, capacitors, resistors, transistors, and an audio output device such as headphones.