XR71 Operation Help

		Quote:			Posted by hesamavr
	the design doesn't work as it should be. resistance and inductance are important factors. Impedance changes with a frequency, are you sure you are not mean inductance (L)?

If the circuit doesn't work, it needs to be fixed not redesigned. If you try to manipulate resistance in a COIL, it gets you nowhere and can cause a malfunction. Why? Because when you change the windings in a coil you change the impedance and that detunes the circuit. Read what I wrote again. Nothing has changed.

If people building these designs would simply stick to the design it would work. There are proven designs out there. There's no need to fiddle with things. Fiddling breaks things. When you break something, do not expect others to fix it for you.

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		Quote:			Posted by hesamavr
	OK, come on and help other people that have problems with their circuits. they've not changed anything of the circuit board really. it's a good idea that you stick too all posts in your forum. isn't it?

In almost every case it all comes down to a component that was changed, mate. This isn't a new thing. Every time someone comes along and tries to build one of the detectors here, they always substitute parts. Do you know what they use for a criterion? They use rumors and "So and so" said for their reason. Then they come here with a machine that doesn't work and expect us to troubleshoot it from half way around the world just based on their description of "it beeps all the time" or "I only get 20cm depth." Nobody should decide to build a machine of their own unless they are qualified to do so or assume all responsibility for it not working if they make errors.

Electronic building is not the same as building a model car or plane. You don't just glue all the pieces together and expect them to work. They parts must be oriented correctly, wound correctly, soldered properly and then there's the issue of static control. Many parts can be damaged just by being held in your hands without protection. And then along comes Johnny Fumblefingers asking what's wrong with his project.

You are new here. Welcome aboard. If you want to help others, go for it. But when I chastise someone for shoddy building practices or for giving bad advice, live with it because I know what I am talking about.

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		Quote:			Posted by Cornelius
	Golddigger . It is about time somebody told it the way it is  . Congratulaions   Cornelius

Thanks, Cornelius. I spend a lot of time here explaining theory to those who want to understand their machines and want to build their own. When it comes to building one from parts collected, I have little patience for those who don't pay attention to the answers they get. They want another answer. Recently, I advised a questioner not to substitute a FET into a circuit that called for a bipolar transistor. He listened and saved himself lots of grief. But then, he knows his limits and when he gets an answer, he pays attention or asks another question.

I wish everyone luck in building their own machine but with the price of an Ace 150 at under $200.00, it makes no sense to spend $400.00 on a less capable machine just because they built it themselves.

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Hello, everyone.

Just recently got interest for building some metal detector. At first I was confused with so many diagrams and configurations, and it is hard to chose one to built. After reading tons of forum's posts, I made decision to make Delta Pulse MD.

Used PCB from "Elbombaci" (Turkey) and seems to me that this one is just fine.

After just few days, project is finished (at least working device, not yet in box). At first I had many problems with solving problems about sensitivity and stability, but now thing is bit more sensitive, yet not very stable.

I found one error in circuit, and changed it. So now, "gain" pot has it's purpose - increasing sensitivity of detecting metals. Connection from LF357 to the "gain" pot should be galvanically de-coupled with 220nF capacitor (between output of that opamp and "gain" pot). It give chance to next stage to self-adjust to proper offset voltage.

I have still problem understanding some part of the circuit, especially auto-tuning part. Seems that there is more errors.

I have about 35 years of experience in electronic and good understanding of most of the analog circuits. But this one give me headache.

Anyone corrected that error of self-tuning?

Some photos and videos on Flickr:

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Regards,
m--

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P.S. This picture taken before completing all parts, so final MD looks little bit different.
So far, one error corrected, and now my Delta Pulse works much better

To be more precise about change in diagram, here is the picture:


PCB layout with change attached.

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		Quote:			Posted by steaua581
	Hello PowerLab, You are right this project in my opinion is far from over, the detector is verry instabil and still has errors.Herre is a pic wich seems to me like an error.....No grounding? ? For sure it needs improvements and also the component quality in order to have a good outcome...And as far as components made in China ...people have lots of trouble let me tell you... Regards Marius

There is "artificial ground" made by 1/2 of TL062 -> D6:A at pin 1. Two resistors of 100kOhm divide rail voltage (+E/-E) to half, and that opamp serve as current follower/amplifier. Two 1N4148 diodes in series made voltage difference between -12V battery and -E by factor of 1.4V. So, between +E and -E, there is voltage as follows: 12V-1.4V = 10.6V (this is the "rail" voltage for powering opamps -> +E/-E).

Note that +E is at the same potential as +V of the battery. -E is at 1.4V difference from -V pole of the battery.

Then, this voltage is split by that two resistors, current is amplified by D6:A, and there is that artificial ground. So, +E is 5V above that "ground", and -E is -5V below that "ground".

This is good trick to make +/- voltage supply for operational amplifiers. Or, else you have to mess with ICL7660 (DC/DC converter), and 78L05/79L05 (+/- voltage regulators).

There is few reason for instability:

1. Using ceramic capacitors for critical oscillating/timing circuit is very bad choice - ceramic capacitors change capacitance with temperature. Critical oscillator/timing capacitors: C1, C3, C4, C5, and C6.

2. RFI (radio-frequency interference). PI MD is actually transmitter/receiver. When in receiving mode, it is very sensitive to many broadcasting radio transmitters, power line interferences (yes, power line 110 or 220V is powerful emitter of 50/60 Hz interferences, but also it's harmonics and other pulses which come from various apparatus connected to power line mains - not to mention HV power lines).

3. Shielding of the whole unit is a must. All parts should has the same stray capacitance (between circuit board and metal case), otherwise it will be sensitive to proximity of your body or ground. Also, good thing is to make Faraday shielding of the coil itself and using coaxial cable between PI MD and the coil. Also, Faraday shielding of the box is essential for stability (even if plastic box is used, there may be added aluminum or copper foil inside box in order to make RFI shielding - Faraday cage).

4. Other reason as are errors in diagram. One must know that whole circuit has a lot of feedback lines, and each depend of others. Small variation at one point may lead to big variation at another. Especially at high amplification of the operational amplifiers (opamps).

P.S. Forgot to mention fifth reason of instability:

5. Coil vibrate audibly, so wires move at frequency of the pulses - which lead to changes in coil properties (by tinny changing in space between turns). That is important because immediately after "transmitter" mode, it begins to receive signal from metals. Coil is made of copper wires, so it is metal as well. Any fast movement of turns made interference and noise, which is then amplified by LF357 and TL062. To avoid that, it will be good to make coil stiff and rigid - by applying of some glue, grease or other chemicals. Or tightening by insulating tape.

Regards,
m--

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