High Range Metal Detector Circuit

[Cre Moonin]

UPDATEchanged the schematic as I realised I had the power supply wrong; the whole thing is powered directly from the 7-12V supply. Which brings up another issue: the MAX951 as recommended by the project's author is rated 7V for normal operation, absolute maximum 9V. Powering it at 12V will destroy that part.

Doing so, my eye was caught by the unusual OpAmp circuit. Why is the output only used to drive the inverting input, not an Arduino input? This in turn led me to look closer at the OpAmp, which is not your regular OpAmp. It has a comparator for output stage, so it either outputs 0V or Vcc. No in between. Replacing it by a regular OpAmp is probably not going to work.

L1/R9/Q1 is the detector. Q1 is switched on and off creating an AC current through L1, the switching off causes a positive voltage spike which is dampened by R9, R13, D4 and D5. The presence of some metal object should increase the magnetic field and increase this spike. C2 couples this spike to the inverting input of the OpAmp. The OpAmp's comparator output turns high, charging the C2 through R10 until the voltage is so high that the negative input goes above the positive input and the OpAmp output will turn low again. The time the OpAmp remains high on the output I believe is related to the height of the spike of L1.

Now to me it looks like this OpAmp can NOT be replaced by a regular OpAmp (the output may swing rail to rail it won't be fast enough); it MAY however be possible to replace it with a regular comparator like the LM339N. It is open collector, so only driven low, but a 10k pull-up to Vcc wouldn't make much of a difference considering that 2M feedback resistor.

The OpAmp as shown in the Fritzing uses the standard pinout of a single OpAmp, as indeed the LM741 is. The MAX951 as linked to in the parts list however is a dual OpAmp (and a highly unusual one at that, as it comes with a comparator output and built-in voltage reference).

Tomorrow my buddy Alex is swinging by with an addition or modification to the circuit. He is a trained electronics engineer so I hope he will solve it. He said the same thing about the op-amp not working because it was being used as a comparator and not an amp at least not only an amp.

So either the OpAmp number in the schematic is wrong, or the schematic itself is wrong. Which is it? We don't know - until the author of that post comes in to clarify it's anybody's guess on what it should be.

Really seems to work and has a good sensitivity,

I was wondering if anyone can help to improve it for better performance .

I used a 20 cm coil of 60 turns with a 0.4 mm copper wire.

The project was built on an Arduino Uno - all the electric stuff is the same as described.

Can anyone advise?

Hey. Sorry folks. I have been busy this week with my day job. Have not had much spare time to invest in figuring this out. I will fill you in. My friend Alex brought us a scope to borrow and tested the circuit. We have the same problem you are having with it Goose0. We can trace all the paths but we do see a spike, and a pulse...but the spike is not enough to trigger the response from the Arduino, so we are pretty sure that his schematic is wrong. There is no telling how he messed with his final product after creation or what he has tweaked or changed...or if it is a transposition error from his paperwork to fritzing. Any number of possibilities. It may be as simple as our version of the Arduino. I am reading about bootloaders and chipsets and find it absurdly hard to keep up. In fact, with my current nano v3 I have to run "old bootloader" to get any code to load.

BUT, my friend Alex has taken the circuit I created with him, and is working on figuring it out. He is very good at that stuff, but takes him a little time. He may completely remake the circuit or give up who knows. I will know in a couple days.

Anyway. I did find the schematics for the "pirate" a russian made PI metal detector...and supposedly what this project was based on roughly...though the similarity stops once you look at the schematic. I am thinking of going down that road if the results of the current circuit no worky. It looks fairly simple, and does not require any programming...or arduino for that matter.

Hi

I am taking a last shoot at this:

Could it be that he used an active Buzzer for the project?

Since he is just turning the output to High- and not sending any pulse or pwm signal?

Is this why we failed?

I have not been able to put much energy into this since my work picked back up and has been keeping me busy. My buddy has been periodically giving me updates on what he has discovered and as of last update he said it is working now, but with some considerable circuit changes. He burnt up 2 or 3 nanos before getting it right, because the pulse is unpredictable. So, I am not sure if his final product will be very sensitive or not. I am awaiting his response currently, and hope to have it back in my hands so I can start making a proto board from the bread board.

Essentially this is the schematic I believe that the OP of the project used...and he utilizes the Nano as part of that schematic...and makes the necessary adjustments...and I think that may be where our problem is. He made adjustments but never explained them and gives us a finished product with little substitutions and I had a hard time matching exact parts...

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- multiplexing: I can use multiple multiplexed coils (other built-in channels are already used in my application); shall I switch off the current before switching the coils (external mux on a single LDC channel)

Your sensor coil has a large inductance value but the AC resistance may also be quite large. You can use the calculator tool below to estimate the electrical parameters of different sensor configurations. Generally it is best to optimize for Q factor. Hundred kHz range should work well.

Sorry for not having put here all the details; adding some comments I would read the presence of coins (euro project) in a box which base is around 25x100mm. If possible I would detect a single coin presence. So this is a empty box coin sensor; do not mind to the small sensor (as full sensor):

I mean that in case of rectangular coil shape not only the lowest side measure is the matter. So If I want to detect at 10/15mm distance the presence of a 10mm diameter coin which is the suggested coil shape? Is multiple coils the easiest solution?

1) Sensing range of the inductor is not a function of the sensor frequency: I have plastic parts on the sensor; probably the frequency is involved in different absorbtion (and propagation) because of different materials proximity.

2) With your sensor having a SRF of 400kHz, you should not exceed 300kHz as your operating frequency: OK I have to verify the SRF; I think is more than 400kHz. This is the frequency I set by a 270pF capacitor.

3) sensing 10-15mm of target range with a 25mm sized sensor will require a higher resolution device - especially since the target is smaller than the sensor (this reduces the signal also). LDC1614; probably S/N is not good enough. Anyway, I will verify testing the suggested LDC.

4) When switching sensors using an external mux, you can switch while the sensor is operating, but you may have a voltage spike that can damage some muxes.I understand; I've not particular fast timings. I probably will put 0 current or switch off by I2C (if possibile) before muxing. I do not know how this is performed in the application TIDA-00509

5) For your application where you wish to identify the presence of a coin, you may want to use an additional channel as a reference to compensate for environmental shifts, as your signal is quite small. What is the qualitative frequency shift target in case of normal enviroment noise? 1 to 10%?

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