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  1. #1
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    Sensitive metal detector using microcontroller ADC channel.

    need help with metal detector circuit if i use arduino/PIC micro controller ADC channel.

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    if i want to switch the coil inductor at high current using mosfet for greater depth. because direct connection of
    switching signal from arduino pin is not the correct way for switching at high input current. and to use ADC channel of arduino.
    i will measure the change in impudence of the LC circuit at the IN4148 & non polar capacitor junction point.

    can anyone have any idea?

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    Metal detector with Arduino ADC channel.

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    i need help with sensitive metal detector design. i'm using swg20 x 15 wound on an

    iron ring with 6 inch diameter. can this design be used for a sensitive metal detector by using

    one of the ADC channel of arduino?



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    Re: Metal detector with Arduino ADC channel.

    It might work as a strong magnet detector but never as a sensitive metal detector!

    Metal detectors work by measuring the effect the metal has on a magnetic field, either by absorbing or de-tuning it. Your design does not create any magnetic field and even if it did, winding the pick-up coil around an iron ring would concentrate the field inside the ring when you really want it to be outside it. Additionally, the signal picked up at the ADC input will only be a few mV, far too low to be useful.

    Consider using flat coils (pancake coils) instead of the iron ring and maybe making a single transistor oscillator with frequency set by the resonance of the coil and a small capacitor. You can then use the Arduino to measure the frequency and see it change when a nearby metallic object alters its inductance.

    Brian.
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    Re: Metal detector with Arduino ADC channel.

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    this type coil?

    how much inductance is necessary for a good metal detector. can we use bifilar winding?



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    Re: Metal detector with Arduino ADC channel.

    That is the correct type of coil but the inductance depends upon how you use it. In its simplest form, you use the inductance and a small capacitance across it to tune an oscillator then measure its frequency with the Arduino. What that frequency actually is, and hence the inductance is your choice. You will probably be looking at > 30 turns in a practical system so use thin insulated wire. I can't think of any point in using bifilar windings in this application.

    Choose a frequency that is relatively high so small inductance changes create a large shift but not so high that the Arduino has difficulty measuring it. Try also to use a relatively small tuning capacitance across the coil so the frequency is more dependent on the inductance than the capacitance. The formula for resonance is:
    1/(2*pi*sqrt(L * C)) where L is in Henries, C is in Farads and the resulting frequency is in Hertz.

    There are other methods of detecting metals in the vicinity but the frequency shift method is easiest to implement. You could also consider using two coils and looking for differences in conduction between them as metalic objects perturb the magnetic field or a different method of using two or more coils and looking for imbalances in inductance or conduction. Be aware that different metals will change the inductance in different ways, for example ferrous metals near the coil will increase inductance but brass will reduce it and some alloys or bonded metals will make almost no difference at all.

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    Re: Metal detector with Arduino ADC channel.

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    i think that it will may be better to use colpitt oscillator interfaced with arduino for metal detection.



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    Re: Sensitive metal detector using microcontroller ADC channel.

    As is mentioned also on the source webpage of the schematic, to make this circuit working is a bit problematic.
    I think any of the many classic schematics available on the net would work better than this Arduino project.



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    Re: Sensitive metal detector using microcontroller ADC channel.

    the schematic along with arduino source is available on github. but the output of the colpitt oscillator is not

    square wave. can i use 555 ic as schmitt trigger to converter the oscillator input to square wave and then compare

    the change in frequency?



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    Re: Sensitive metal detector using microcontroller ADC channel.

    Not a 555, that would be a bad choice. A comparator would be best with a schmitt trigger logic gate next best. If the Arduino has a schmitt trigger on one of its port pins that would work too. I use Arduino IDE extensively for programming ESP8266, ESP8285 and ESP32 but have little experience of the Arduino controllers themselves.

    It probably will work with that oscillator as long as you keep the inductance of the loop as high as practical and the capacitors as small as possible. The frequency is decided by both but the capacitors will not be affected by the nearby metal.

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    Re: Sensitive metal detector using microcontroller ADC channel.

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    i 'm going to use the above circuit of metal detector but here in the schematic they use 4.5vdc

    both for the colpitts oscillator & arduino. how the circuit can changed so that oscillator section operate with 12vdc or 9vdc & 2 or 3 amp

    input and to take 5v output signal from the oscillator that must be the i/p to the interrupt signal at T1 because every arduino

    pin has max i/p voltage of 5.5v & 40mA i/p current?
    Last edited by codemaster11; 16th December 2019 at 04:28.



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    Re: Sensitive metal detector using microcontroller ADC channel.

    I wouldn't do that because there is a possibility the output of the oscillator will always be above the logic low level of the Arduino and the signal would not be detected at all. A better solution would be to use a small 5V regulator to drop the 9V/12V down to a stable 5V to power the whole circuit. An inexpensive 7805 regulator should work perfectly. The other point is that if the voltage supplying the oscillator changes, such as if a 12V battery was discharging, the frequency might also shift and that might be recognized as a metal object nearby.

    The "2 or 3 amp" comment is not relevant, the circuit will draw as much current as it needs and that will probably be far less than 1 A.

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    Re: Sensitive metal detector using microcontroller ADC channel.

    its right to use 7805 regulator. but if to increase the range/depth of the detector, the problem will

    be solved only by using greater numbers of turns of the coil?



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    Re: Sensitive metal detector using microcontroller ADC channel.

    You will have to find the best number of turns by experimentation.

    The construction of the coil will make most difference to sensitivity, you want it to be fairly wide diameter, I would guess around 15cm would be a good starting point. If you make it smaller it will be more sensitive but it will only cover a small area. Making it bigger will increase coverage but make it less sensitive.

    I would start with 50 turns of coated wire (as used in transformers) and mount it so it is protected from being touched but not close to any other metal items. For example, fixed to a plastic sheet. Again there is a compromise, fewer turns will make it oscillate at higher frequency which might make the frequency shift harder to detect. Adding more turns will make it more dependent on the coil itself than the metal you are trying to find.

    Personally, I would change the two capacitors across the coil to 1nF and experiment with the coil to aim for a frequency of around 20KHz to 30KHz.

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    Re: Sensitive metal detector using microcontroller ADC channel.

    i make the search coil with inner diameter of about 1.3 inch & outer diameter of about 8 inch with wire having length of 25 meter

    as shown in post # 4 & design the circuit layout as according the schematic in post # 10 but i didn't works.

    i don't know the reason why it is not working?
    Last edited by codemaster11; 14th January 2020 at 08:39.



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