Metal detector
.In old TV drama " Combat", German soldier used metal detector, " metal finder" , to find the land mine under the ground. They swung the ring with bar over the ground. I made it. See fig1! With coil and variable capacitor, tunable oscillator is constructed. In the figure it is indicated as VXO. But it is VO. " Not whisky!" On the other side, Ceramic oscillator is constructed with Murata Ceramic filter "SFP450H". It oscillates 450kHz. These double HF signals are detected with diode detector. And we can get the audio signal as the beet of these two oscillator's frequency. The coil of the first oscillator "VXO" is constructed very big coil around the cover on the rim of the plastic dust bucket. Using the frequency counter or the test scope, you must tune the frequency of the first oscillator to be about 450kHz, with changing the capacitor of the oscillator. If some metal be near from the coil, the oscillating frequency of the first oscillator changes. Therefore you can know the metal under the ground with changing the beet sound. This metal detector can detect the tuna can or Aluminum case from the distance of 10cm. But it can not detect the coin.


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http://www.intio.or.jp/jf10zl/metald.htm