Simplest Metal Detector Circuit

Simplest Metal Detector Circuit

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The simplest metal detector circuit consists of 4 components.
The detection-coil consists of 70 turns of 0.3mm wire 120mm diameter.
Place an AM r...

The simplest metal detector circuit consists of 4 components.
The detection-coil consists of 70 turns of 0.3mm wire 120mm diameter.
Place an AM radio near the search-coil and tune it to pick up a squeal.  When a coin is placed near the coil, the tone will alter.
The circuit operates at about 250kHz and the the radio picks up a harmonic. It will detect a bottle-top at about 90mm.

simple bfo metal detector.gif
Another simple Metal Detector Circuit consists of two coils that overlap to provide feedback:

simplest metal detector circuit.gif

Place an AM radio near the coils and a tone will be heard.

Simple BFO Metal Detector

simplest metal detector circuit2.gif

The first two transistors are deigned to produce the exact same frequency. The result is fed to the third transistor to produce a beep-beep-beep in the piezo speaker.
When a metal object is detected by L1, (the search coil) the beeping increases.
The performance of this circuit is no better than the single transistor circuit above and an AM radio. When it is all boiled-down, the first transistor drives the coil and the frequency of the circuit changes when the inductance of the coil is altered by a metal object near it. 
The second and third transistors are equivalent to an AM radio by detecting the frequency of the oscillator and producing a result that is the difference between the frequency produced by the first transistor and the frequency produced by the second transistor.

The next circuit is even more complex.
It uses integrated circuits (IC's) to detect the difference between the two frequencies.
The circuit is very old. All the IC's can now be replaced by an 8-pin microcontroller and the circuit will be much simpler.
Don't forget, it is no better than a single transistor and an AM radio and is not worth constructing, however the text is interesting . . .

 

 

 

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