Metal Detector MkII

This project is an extension of Metal Detector MkI, and shows how metal objects are detected. It is the second in a series of circuits and allows a great deal of experimentation, especially if you have a CRO (Cathode Ray Oscilloscope) and a few items to detect. You can view the waveforms and see exactly how they alter as an object is brought into
Metal Detector MkII - schematic

the field of the coil. Metal detectors will detect ferrous (iron, steel, stainless steel) as well as non-ferrous (copper, tin, gold, lead, silver, aluminium) as well as alloys (brass, cupro-nickel, pewter etc). Depending on the complexity of the circuit, a metal detector will be able to discriminate between a lump of gold and an aluminium ring-pull from a drink-can. The circuit we have presented in this project is very simple and works on the principle of detecting the amplitude of a waveform. This is called AMPLITUDE MODULATION. When a metal object is placed inside the detecting coil, some of the magnetic flux passes into the object and creates a current called an eddy-current. This "uses-up" some of the magnetic flux and thus less flux is available for the receiving coil. This produces a lower output from the coil and causes the second transistor in the circuit to be turned OFF slightly and the voltage on the collector rises. This allows the third and fourth transistors to oscillate and pass a signal to the fifth transistor to drive a mini speaker. As you can see, the circuit consists of a number of BUILDING BLOCKS. All you have to do is understand how each block works and you will understand the whole circuit. The concepts of TALKING ELECTRONICS is to explain how various building blocks operate so you can design your own projects. You can take any of the blocks and add them to your own project, but it will be necessary...

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