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While trying to get my project coil working today I made a surprising discovery. My coil was not working at all on small gold or an American nickle. It would pick up a silver ring quite well. So I started by trying to alter inductance by changing the shapes of both the rx and the tx coils. I had a range for both coils that I knew were acceptable. In addition, I had 2 other variables that I could play with. 1) the coupling of the 2 coils, and 2) the dampening resistor value.
On my detector, I have to turn it off for every adjustment or else it locks up, so this took quite a while. The results were that changing the resistor value between 1333 ohms and 2k ohms made no difference. Changing the inductance made some difference, but did not change the ability to pick up gold better like I had hoped. The big eye opener was changing the coupling of the rx and tx loops. Since my oscilloscope seems to be useless for me, I just went with trial and error. I finally got it to pick up smaller gold and the nickle fairly well. At this point my detector "chatters" or makes random noise unless I turn down the sensitivity. Raising the discrimination (or reducing the delay, I think) didn't reduce the noise.
The real shocker was that at some point I could pick up the nickle/gold quite good while at the same time the silver ring I was using for testing diminished a lot. This was very surprising. I made me wonder if there is still hope for a gold only detector? Now, I was still picking up the silver ring but at only half the depth. I didn't think that was possible.
Now, these were just bench air test without any shielding, so I have a ways to go. But at least I can pick up gold now and can start on making a case and sealing it up. I hope to shield with a 6K carbon twill. Not sure that will work but it will stiffen up my 46" long coil. I did test with the carbon laying across the top but it was not encased in resin so we'll see how that works.
The photo is actually my first attempt that quit working when encased in hot glue. I just posted it so you can see about what it looks like. I wrapped new coils for this newest version.
On my detector, I have to turn it off for every adjustment or else it locks up, so this took quite a while. The results were that changing the resistor value between 1333 ohms and 2k ohms made no difference. Changing the inductance made some difference, but did not change the ability to pick up gold better like I had hoped. The big eye opener was changing the coupling of the rx and tx loops. Since my oscilloscope seems to be useless for me, I just went with trial and error. I finally got it to pick up smaller gold and the nickle fairly well. At this point my detector "chatters" or makes random noise unless I turn down the sensitivity. Raising the discrimination (or reducing the delay, I think) didn't reduce the noise.
The real shocker was that at some point I could pick up the nickle/gold quite good while at the same time the silver ring I was using for testing diminished a lot. This was very surprising. I made me wonder if there is still hope for a gold only detector? Now, I was still picking up the silver ring but at only half the depth. I didn't think that was possible.
Now, these were just bench air test without any shielding, so I have a ways to go. But at least I can pick up gold now and can start on making a case and sealing it up. I hope to shield with a 6K carbon twill. Not sure that will work but it will stiffen up my 46" long coil. I did test with the carbon laying across the top but it was not encased in resin so we'll see how that works.
The photo is actually my first attempt that quit working when encased in hot glue. I just posted it so you can see about what it looks like. I wrapped new coils for this newest version.
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