Surf pi 1.2 damping resistor

[macross]

So after abandoning my surf pi project for two years i'm back and i want to finish what i have started. In the past i have wound 7 coils with different wire gauges and wire types, but i could not make it detect gold ring at all! Even when i touched it to the coil itself. Since i now have access to a scope i want to try and match-up coil with the damping resistor. But the problem is that i don't know which test points should be measured and what kind of waveform to expect. I have tried searching on the forums but could not find any information about that. So maybe someone could lend me a hand?

[homefire]

Page 6 here explains that basis. http://www.geotech1.com/pages/metdet...s/FastCoil.pdf

[macross]

Page 6 here explains that basis. http://www.geotech1.com/pages/metdet...s/FastCoil.pdf

I have read this pdf before, but i don't know which op amp to monitor with an oscilloscope and how the waveform should look like properly damped. I have made another coil now i'm able to detect gold ring, but at a poor depth. Air test shows that it can detect 1 euro coin at 18cm and that's not that great. Other people where able to detect 1 euro coin at 25-30cm using surf pi.

[macross]

Finally found what i was looking for! For reference.
http://www.geotech1.com/forums/showt...uda-and-SurfPI
Post #5 by tepco

[macross]

I'm still struggling to make surf pi "see" 1 euro coin beyond 18cm. I have made 240cm diameter coil with 20 turns using magnet wire 0.5mm in diameter. I know this wire is not ideal, but other people have achieved 28-30cm using exactly this type of wire. Strange thing is that no matter how big coil i make it performs same as smaller ones...

With my big coil connected i have tweaked damping resistor and the waveform now looks like this:


This is the limit. If i try to tweak more this happens:


Anyway i have no idea what else to try. No matter what type of coil i try i can't get past 18cm distance... Even smaller ones do perform the same as the bigger one.

[sinclairuser]

the first photo looks ok.

[Qiaozhi]

Anyway i have no idea what else to try. No matter what type of coil i try i can't get past 18cm distance... Even smaller ones do perform the same as the bigger one.

How long is the sampling delay, and how wide is the sample pulse?

[macross]

How long is the sampling delay, and how wide is the sample pulse?

Delay @tp1


As for sample pulse where should i measure it? Sorry i'm a bit new to this stuff.

[Qiaozhi]

Delay @tp1
As for sample pulse where should i measure it? Sorry i'm a bit new to this stuff.

The main sample pulse width can be measured at pin 13 U7a (CD4066). The secondary (EFE) pulse width is on pin 5. If you trigger channel 1 of the scope from the collector of U1 (2N3906) and probe with channel 2, you will be able to determine the pulse widths and delays for both samples.

[macross]

The main sample pulse width can be measured at pin 13 U7a (CD4066). The secondary (EFE) pulse width is on pin 5. If you trigger channel 1 of the scope from the collector of U1 (2N3906) and probe with channel 2, you will be able to determine the pulse widths and delays for both samples.

I hope i understood everything correctly. I have connected first channel to 2n3906 collector pin and with second channel probed pin 13 and 5 of u7a.

First channel 2n3906 collector second pin 13 of u7a


First channel 2n3906 collector second pin 5 of u7a

[Qiaozhi]

I hope i understood everything correctly. I have connected first channel to 2n3906 collector pin and with second channel probed pin 13 and 5 of u7a.

First channel 2n3906 collector second pin 13 of u7a
[ATTACH=CONFIG]32628[/ATTACH]

First channel 2n3906 collector second pin 5 of u7a
[ATTACH=CONFIG]32629[/ATTACH]

All looks good to me.

[macross]

Ok thank you! Good to know that everything is looking good. I guess i will have to experiment more with different coils. I will try to make flat spiral coil and see what performance i will get from it.

[Qiaozhi]

Ok thank you! Good to know that everything is looking good. I guess i will have to experiment more with different coils. I will try to make flat spiral coil and see what performance i will get from it.

Another possibility is that you have an error later on in the circuit. It might be worth checking all your components values are correct. This is a common problem.

[macross]

Another possibility is that you have an error later on in the circuit. It might be worth checking all your components values are correct. This is a common problem.

Went over all components everything seems to be in order. I have made flat spiral coil with 0.5mm magnet wire 230m in diameter and 1.6 ohms resistance. Now i can adjust delay pot all the way clockwise until i hear click and coil is still working so i guess i made it much faster (Measured with scope it's about 10uS). Depth at which 1 euro coin is detected has increased to 20cm and that's not that great... I'm starting to think that the real culprit here is piezo buzzer, because i can't hear first feint signal it goes off only when object is near the coil so with headphones this should improve a little bit more. Also detector now beeps when i touch the coil with my hand so i guess i will need shielding. Anyway i'm happy the way it has turned out. Now i need to work in the metal detector frame itself and off to hunt some treasure

[macross]

Finally i have found what was wrong with my surf pi! Now first feint signal can be heard of one euro coin at 30cm!! I checked every single resistor and finally found that R11 was only 12k while it had to be 22k. So i have replaced it and now it's working like a dream. Thank you all for your help!