Tuesday, 23 April 2013

Simple metal detector.

Hi everyone

This time I m gonna show you how a simple metal detector works.

I will be using the oscillator circuit and the coil transformer in the previous post, which I have modified it into a simple metal detector. The circuit is shown below:

The LED is made an indicator for the detector. The coil transformer is the detection coil. Object is put into the centre of the coil for detection.

I have tested this detector with different materials and got several results:

When I inserted the metallic object such as screwdriver or scissors, the LED dimmed.

However when I inserted a ferrite rod (which is not a metal), the LED brighten up.

Here is the video:

Note that the ferrite rod increases the LED brightness just a little, but if I connect the secondary winding of the coil transformer to the voltmeter instead of connecting it to the LED, the secondary voltage indeed increases but it is not sufficient to make increasing brightness of the LED apparent to the eyes.

The detector can be improved by using circular (disc) coils as the primary and secondary winding of the coil transformer instead of using the cylindrical coil transformer as I am using, and inserting the object to be scanned in between the discs.

The detector works by the principle of different core material for the transformer.

Any object inserted into the coil transformer will be automatically assigned as the core of the transformer.

If you insert a ferrite rod, it will act as the core. Since the ferrite rod has high magnetic permeability, the magnetic flux around the coil transformer finds a less reluctance (magnetic resistance) path, which is the ferrite rod, to flow through. Hence, the field lines reinforce together, reducing the lost magnetic flux and hence, making the LED shines brighter.

However if you insert a metallic object, it will create eddy current within it as a reaction to the changing magnetic field it resides within. This eddy current will in turn create other changing magnetic fields that oppose the magnetic field the metal resides within (which is the magnetic fields of the coil transformer), cancelling the magnetic fluxes together and hence generate less voltage across the LED, dimming it as a result.

This detector, although works, is not suitable for practical metal detection since you have to put the tested material/metal into the coil, which is not practical if you want to detect any hidden metal across a flat surface. Therefore, for that purpose, the coil needs to be flat as well to facilitate smooth scanning of the flat surface. Hence, flat coil transformer is frequently used in the actual metal detector design.

Flat coil is like a mosquito’s coil, and it is very hard to make by hand, and requires special tools and materials. That is why I resorted to using cylindrical design like the coil transformer.

I think that’s all for today. Thanks for reading.

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