I made a Metal detector circuit

[neptune]

Here i am posting the block Diagram of metal detector circuit.
the primary coil will generate an alternating magnetic field which will strike metal and produce eddy currents in metal, these eddy current will generate its own magnetic field in opposition to main magnetic field and will be picked up by secondary coil.
Secondary signal will be amplified and rectified to be compared from main signal.

comparator will be normal op amp which will amplify the diffence in both the signal.
this signal will then switch ON the transistor which is connected to Buzzer. if signal is strong enough then only will transistor switch ON, this will be determined by base reistance which is variable resistance.

Do you think this circuit will work ?

 

[alec_t]

The main problem is going to be eliminating cross-coupling of energy directly from the Tx coil to the Rx coil. Any such cross-coupled energy is likely to be vastly greater than the eddy current energy you are trying to detect. So your signal will be swamped by noise.

 

[sram]

Do you want to come up with your own circuit, or do you need to make a metal detector circuit somehow? If the case is the later, read about BFO (Beat frequency oscillation). It is the easiest method to build a metal detector. You will be feeding two frequencies(from oscillator circuits) into a frequency mixer chip. You can set the mixer to give you the difference in frequency as an output. This difference will then be amplified and fed into a speaker. You will have two coils of course, one in each of the two oscillators. One coil will be your detector coil, when it comes in proximity with metals, its inductance will change causing the the frequency to change in one of the oscillators, which is turn, will generate a difference frequency you can amplify and hear. Of course you will have to design your two oscillators to have the exact same frequency at the initial condition.

Hope this helps.

 

[neptune]

Do you want to come up with your own circuit, or do you need to make a metal detector circuit somehow? If the case is the later, read about BFO (Beat frequency oscillation). It is the easiest method to build a metal detector. You will be feeding two frequencies(from oscillator circuits) into a frequency mixer chip. You can set the mixer to give you the difference in frequency as an output. This difference will then be amplified and fed into a speaker. You will have two coils of course, one in each of the two oscillators. One coil will be your detector coil, when it comes in proximity with metals, its inductance will change causing the the frequency to change in one of the oscillators, which is turn, will generate a difference frequency you can amplify and hear. Of course you will have to design your two oscillators to have the exact same frequency at the initial condition.

Hope this helps.

Like this.
http://www.easytreasure.co.uk/bfo.htm
its pretty basic BFO but would it be good enough for an industrial grade metal detector ?

 

[neptune]

The main problem is going to be eliminating cross-coupling of energy directly from the Tx coil to the Rx coil. Any such cross-coupled energy is likely to be vastly greater than the eddy current energy you are trying to detect. So your signal will be swamped by noise.

oh i didnt thought about that !
ok if i filter out the cross coupled frequency out from receiver coil then amplify the detected signal it should solve the problem.
But again would freq. of detected signal be different from transmitt freq. or would the phase differ ?

 

[alec_t]

But again would freq. of detected signal be different from transmitt freq. or would the phase differ ?

IMHO the detected signal is likely to contain various frequencies, including the transmitted frequency and its harmonics, plus local resonant frequencies induced in objects by the transmitted energy. I'm sure there are experts on websites devoted to metal detectors who would better be able to advise you.

 

[sram]

Like this.
http://www.easytreasure.co.uk/bfo.htm
its pretty basic BFO but would it be good enough for an industrial grade metal detector ?

Industrial grade? No. But it actually works. I built one myself. It just works. I could detect coins underground.

There are so many circuits diagrams for the BFO method. Very easy to build, and cheap too.

 

[Mr RB]

(Like I said in your other thread) the BFO is easy to get it working as long as you can adjust it to get the beat right your ears do the rest.

Now you said "industrial metal detector"? What does it need to detect and in what environment? And will a human be listening or will it trigger machinery when metal is present?

 

[neptune]

it will only be used in hand held devices and door frame metal detector by security personel and human listening is permissible.
the smallest object detectable is car keys or coins.
I dont know at what frequencies it will be used. but it will use #26AWG wire for coil purpose. 3 to 4 inch diameter.

I also want to ask if stainless steel is detectable by BFO system ?

 

[neptune]

and how do i make comparator circuit any idea ?

 

[ericgibbs]

and how do i make comparator circuit any idea ?

What signals do you want to compare.? Sine , Square, DC levels.??
Frequency, Amplitude, resolution.?

 

[neptune]

i want to compare sine waves with freq. ranging from 1 khz to 20 khz. (comparing two different frequencies)
and i also want to compare there amplitude if possible.

 

[alec_t]

When you say 'compare' do you mean just detect if one is greater than the other (relatively easy), or do you want actual frequency and amplitude measurements (relatively difficult)?
How will you separate the signals you want to compare?

 

[neptune]

see comparison for me is detection of disturbance. a base signal will always be present on the line, if any signal is captured by receiving coil then it will add to the base signal. so a disturbance from the original base signal would be detected.

NOTE- i alsi want to know if eddy current is generated in a metal by alternating magnetic field does it generate magnetic field of same frequency as that of original signal which magnetised it ? or out of phase magnetic field (in opposition)

Amplitude would generally will always be less then or equal to base signals amplitude. so if freq. remains same then i want amplitude detection. if freq. changes i want freq. detection

 

[ericgibbs]

i want to compare sine waves with freq. ranging from 1 khz to 20 khz. (comparing two different frequencies)
and i also want to compare there amplitude if possible.

One way to compare two frequencies is at their zero crossing point.
This will give some indication of their phase difference.

For amplitude compare if their frequencies are the same and have the same phase shift a OPA can be used.

If the phase shift is not the same its a little more complicated.

 

[neptune]

i tried to search but couldnt get the answer, does freq. changes for eddy currents or remain same as of magnetising field ?

 

[ericgibbs]

i tried to search but couldnt get the answer, does freq. changes for eddy currents or remain same as of magnetising field ?

hi,
Its not a topic that I have specialised in, ie eddy current frequencies in ferrous cores.

But doing some quick reading it would appear that eddy currents and penetration depth are a function of the excitation frequency and can be at the fundamental frequency, harmonics or randomised.

Are the eddy currents in your detector being used to measure a parameter or an effect thats causing a problem.??

You may find this link useful.
**broken link removed**

 

[neptune]

Are the eddy currents in your detector being used to measure a parameter or an effect thats causing a problem.??

its the principle of metal detector. when alternating magnetic field hits a metal alternating eddy currents are produced which in turn generates their own alternating magnetic field in opposition to main magnetic field (out of phase), what i want to know is that do freq. also changes ?
since you wanted to know from me what type of comparator would i like (freq. comparator or phase comp. or amplitude comp.)

 

[ericgibbs]

its the principle of metal detector. when alternating magnetic field hits a metal alternating eddy currents are produced which in turn generates their own alternating magnetic field in opposition to main magnetic field (out of phase), what i want to know is that do freq. also changes ?
since you wanted to know from me what type of comparator would i like (freq. comparator or phase comp. or amplitude comp.)

In order to design a comparator, you will have to give me some idea phase shift that you are trying to detect and at what amplitude.

Regarding the frequency of the eddy currents are you confusing that with the phase shift between the excitation coil reference frequency and the search coil when a ferrous object is present.???

EDIT:
This link explains the operation of a simple metal detector.
http://www.easytreasure.co.uk/bfo.htm

 

[neptune]

yes i have seen that link and i will be going to make it.
i made a AND gate comparator to detect a magnetic field. but i have now a bigger problem.
How do i stop cross coupling between two coils ? as alec_T has questioned me in post #2