Let's made a PC-base metal detector with usb interface !!!

[Aziz]

and I'm diving into transistor amplifier design at the moment, trying to understand/get a transfer function for Aziz's ultra low noise amplifier would be great if I could adapt it to a 3.3 or 5V power supply with an 1 Vrms output swing

Hi sled,

this is a trivial job. First thing is to find and to use a relative low noise, low voltage, rail-to-rail bi-polar op-amp. There must be some convenient out there.
The BJT stage can be dimensioned for almost every voltage (3V - 5 V). The supply voltage must be regulated of course. The supply voltage filter can be made via LC filter with minimum voltage drop.
That should satisfy the requirements.

But be careful. The BJT-amplifier has a high bandwidth and tend to oscillate when it isn't frequency compensated (bandwidth limitted). Most (fast) HF BJT's are quite difficult to tame. So its more convenient to use the standard (power) amplifier transistors as mentioned in the schematics.

Cheers,
Aziz

[sled]

Thanks! I'm currently looking at this schematic here: http://www.janascard.cz/PDF/Design%2...amplifiers.pdf

First thing is to understand how it works with all the transistor biasing and feedback and calculate some transfer functions just for fun (at least I know what it is good for, unlike all the transfer functions I had to calculate in exams )

Do you think the BC337 is a good choice? That's one of the transistor they have at my local supplier here (Conrad) the other ones I'd have to order and get ripped off by ridiculously high shipping costs

[Aziz]

Thanks! I'm currently looking at this schematic here: http://www.janascard.cz/PDF/Design%2...amplifiers.pdf

First thing is to understand how it works with all the transistor biasing and feedback and calculate some transfer functions just for fun (at least I know what it is good for, unlike all the transfer functions I had to calculate in exams )

Do you think the BC337 is a good choice? That's one of the transistor they have at my local supplier here (Conrad) the other ones I'd have to order and get ripped off by ridiculously high shipping costs

The PNP variant BC327-40 is a good alternate. Three of them makes an ultra-low-noise amplifier.
Well, the BC327-40 costs only 4 EUR cent here.
Aziz

[Silver Dollar]

Hey guys I got those boards if anyone is interested. The ones from here;
http://ebrombaugh.studionebula.com/synth/stm32f4_codec/

I had them made from his gerber files so they should be exactly the same.
Pics



If anyone wants one PM me. It's $6 for one or $10 for 2. We just started using the F4 at
work. I have 3 or 4 projects going right now so it might be a bit before I start developing this board.

[bobasaurus]

I'm resurrecting an old thread here, but I'm very interested in this project and would like to build one myself to tinker with the DSP involved. Trying to parse through 72 pages of information is challenging... I have some questions and ideas I would love to get feedback on if any of you are still around.

Is this the latest simple schematic?

https://www.geotech1.com/forums/atta...2&d=1396434351

Why is the TX coil set up as an LC oscillator if you're also driving the coil from the sound card? Is the sound card output frequency the same as the LC resonant frequency? Seems simpler just to drive the coil directly without a cap involved.

Any reason why L out and R out are tied together? Are they both the same frequency?

Is there a version of this schematic that uses amplifiers for the TX and RX? I'm picturing having an op amp non-inverting amplifier with a bjt or fet to drive a higher load for the TX side. Should both of these amplifiers be AC coupled? If so, dealing with the virtual grounds is a bit confusing to me. I feel like this has already been done by someone, I just can't find an actual schematic.

Where and how would you attach flyback / snubber diodes for protection?

Do computer microphone ports have two receive channels these days? Most laptops (and phones too I believe) now have a single "headset" 3.5mm TRRS jack, which I think only has a mono microphone attachment. On computers with separate 3.5mm TRS jacks for headphone and microphone, does the microphone jack have two input channels?

I'm going to try drawing this up into a schematic and post it for feedback if that's okay.

[bobasaurus]

I just found this paragraph from Aziz:

"Once the TX coil is in resonance, the sound card is only delivering the loss of the resonant circuit and keeping the LC resonant swinging. With high Q of the resonant circuit, we do not need much power for this."

So the sound card is just keeping the LC circuit resonating, and as a result not much current is being drawn? If an op amp high current driver was used instead with no capacitor resonance, would it draw a lot more current since it's not using the em field to keep the oscillations running? Seems like the resulting frequency would be more accurate the op amp direct drive way though, no RC instabilities to worry about. Might not even need to measure a reference if you work out the delay.

[bobasaurus]

I found a picture of an amplifier built by Aziz, but the schematic link is dead. Anyone have the schematic handy:

https://www.geotech1.com/forums/atta...2&d=1211991272

?

[6666]

I found a picture of an amplifier built by Aziz, but the schematic link is dead. Anyone have the schematic handy:

https://www.geotech1.com/forums/atta...2&d=1211991272

?

Have you got the page link ?

[Carl-NC]

So the sound card is just keeping the LC circuit resonating, and as a result not much current is being drawn? If an op amp high current driver was used instead with no capacitor resonance, would it draw a lot more current since it's not using the em field to keep the oscillations running? Seems like the resulting frequency would be more accurate the op amp direct drive way though, no RC instabilities to worry about. Might not even need to measure a reference if you work out the delay.

Yes, the TX coil is resonated to maximize energy efficiency, without the tank cap then the power consumption is much higher. If you drive the coil with a sine wave then the tank cap does not alter the frequency, but if it is not matched to the frequency the efficiency will be reduced and the signal possibly distorted.

[bobasaurus]

My DIY detector based loosely around this and about three other open source detectors is coming along. I have it mostly built now, need to finish the glue-up then mount the electronics.

[bobasaurus]

Here are the wooden parts of the body:

[Carl-NC]

That's some nice woodworking.

[bobasaurus]

Thanks, it's my hobby. All joinery is mortise and tenon or dovetail, avoiding using metal fasteners (at least outside of the control box) for obvious reasons.

[Riss]

Hi friends,

to get an idea, how the transmitter works, just see the graphics below. It is divided into four phases. If the mosfet is switched during the recycling phase on again, the power efficiency increases further. The voltage drop of the internal body diode can be omitted and there is only a voltage drop of the mosfet on resistor.

This one is the most efficient pulse induction transmitter I have ever seen. But it is not a resonant circuit.

More infos about this transmitter will follow. Stay tuned..

Aziz

I soldered and experimented with this simple circuit ( VMOS , diode, capacitor, coil )- in typical PI mode - 150 microsec. pumping current into the coil , shutdown the VMOS , the capacitor reverses the current and after the energy in the coil is returned to the supply- then pause for ,, listening " about 1.2 milliseconds long . The principle and scheme is the same , as in the thread ,, PI metal detector with energy recuperation " from deemon (06-04-2012 )- circuit_4.gif- https://www.geotech1.com/forums/atta...1&d=1338765051 -section A-B-C-D- oscillogram of the current through the coil . I had my suspicions and they were confirmed -immediately after the discharge of the coil in the power supply damping oscillations occur with the resonant frequency of the coil ( 1,5 milihenries ) and capacitor to reverse the current ( 33nF ). I tried 3 types VMOS , even the bipolar MJE13007, 10nF capacitor (a drowning man grasps at a straw ) - there is no salvation from the damned damping oscillations ! I dropped everything - I have things from which I will receive money ...