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Re: My Home Built PI Detector
Posted by: kt315
Date: November 24, 2017 12:44AM
again foil shielding? who suggest you to use the foil? do you need a protection from EMI? why?

Re: My Home Built PI Detector
Posted by: Smitty II
Date: December 13, 2017 03:56PM
Idefix,

Actually the coil shield is not a complete circle. On the opposite side of the coil from the connection to the cable there is an break in the copper shield. The break is insulated with black electrical tape and the ending of the shield winding wrapped up on to the tape insulation. You are right if the shield is a complete circle it becomes the same as a shorted turn and lowers the coil "Q" drastically and the coil will not work.

Smitty II

Re: My Home Built PI Detector
Posted by: Smitty II
Date: December 13, 2017 04:02PM
Kt315,

The shielding is NOT an EMI shield. It is a Faraday shield that shield the coil against ground capacitance. Without the shield the ground capacitance and grass and bushes will cause the detector to sound off. EMI is still an issue where there is a lot of electrical and electro magnetic noise.

Smitty II

Re: My Home Built PI Detector
Posted by: kt315
Date: December 17, 2017 06:26PM
i see THE FOIL on your coil. FOIL! its meant EMI PROTECTION. for Faraday one you have to use GRAPHITE powder.

Re: My Home Built PI Detector
Posted by: Smitty II
Date: December 28, 2017 07:15PM
kt315

The foil shield is a Faraday shield that provides capacitance shielding for the coil. The foil is not continuous as there is a break under the electrical tape on the opposite side from the cable connection. Believe me it does not shield out EMI. It doesn't shield the detector from computer noise and various EMI noise in town and around the house. It will even detect solar flares at max gain. You can call it whatever you want but without the copper shield, the coil is unusable, as it sounds off from everything that the coil come close to or touches. It will even sound off if you bring your hand close to the coil. I sure wish it did shield out EMI because that would sure make it run a lot less noisy here in town. If it shielded out EMI you could not detect any targets as the detector generates an Electro Magnetic field pulse and then detects the Collapsing of that field in the metal targets.

Smitty II

Re: My Home Built PI Detector
Posted by: Eric Foster
Date: January 01, 2018 04:28PM
Hi Smitty,
You are correct in that your shield is a Faraday shield and also prevents ground capacitance effects. You will find that it does reduce EMI but only for frequencies higher than about 5MHz. With the coil vertical, where you get maximum rf pickup, disconnecting or removing the screen will have a noticeable effect if you look at the high frequency noise on a scope and a wide band preamp. A detector coil works on the magnetic component of an rf field rather than the electric component as a normal wire or rod antenna does. In fact I use a 14in Coiltek mono coil as an aerial for my workshop medium and long wave AM radio as it cuts out a huge amount of interference. The best type of coil for reducing interference within the pass band of the detector is a figure of 8 winding.

Eric.

Re: My Home Built PI Detector
Posted by: Smitty II
Date: January 22, 2018 04:52PM
Eric

Thanks for your input on the coil shielding. I am in 100% agreement. The swamping out of the ground capacity and the effects of vegetation is the primary effect of the shield. Without the shield the coil is almost impossible to use.

My PI does really well on small gold targets since I designed a dual gain receiver amp to reduce the gain as the amp approaches saturation. Preventing saturation eliminates the saturation delay and allows me to sample in the. 5 to 6us range to improve tiny gold target detection.

Thanks again Eric!

Smitty II

Re: My Home Built PI Detector
Posted by: Mike D.
Date: March 09, 2018 01:49PM
Hello Smitty II,

I am new to Find's forums, and have a little background in digital electronics.
You seem to have quite a bit of E engineering experience.
I am trying to learn more about Op amp circuits and play with oooold detectors of the transmit/receive low frequency types.
I have purchased a few 'Vintage' era detectors to take apart and examine - draw up the schematics and try to understand.
Would like to work on some 'surface mounted component units', but find multilayered boards hard to trace.
It appears that much of the Tech Forum section here is heavily involved in PI type units.
Does anyone still dabble with transmit/receive units?
I have millions of questions.
Is your 'Home Built PI Detector' your own design?
Thanks.
Mike D.

Re: My Home Built PI Detector
Posted by: Mike D.
Date: March 11, 2018 04:50PM
Your detector looks great!
May I ask how or where did you acquire the circuit board - fabrication?
I am looking on the internet for detector kits and have found thousands of videos on various stages of construction, but no mention of where to purchase them.
Any ideas?
Thanks.



Edited 1 time(s). Last edit at 03/11/2018 04:53PM by Mike D..

Re: My Home Built PI Detector
Posted by: Smitty II
Date: March 20, 2018 05:02PM
Greetings

I laid out the PC board and had some made. I have some extras!

Smitty II

Re: My Home Built PI Detector
Posted by: Smitty II
Date: March 20, 2018 05:11PM
Mike

Yes! It is basically my design, but like most of the home built detectors out there, it has distant roots in other designs. About 50% of the circuit will not be found in any other detector.

My circuit boards are single layer through hole boards. I do not have any SMT boards.

Smitty II

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