The coil I made for the Russian Pirate, I tried on the SurfPro(tacoma56 square-board). It gets a hammering signal on my 1.8g sample of flour gold at 5 inches! I thought my sample was 2.4g but I just re-weighed it, so I've been getting slightly better results than I thought. The coil is 6 inch, 3 rounds of Cat5 wire,twisted around itself, ends all joined in series. From what I can gather twisting lessens the self capacitance(?) making it more sensitive.
This is the Pirate.
Even though this detector doesn't have discrim, it will see a 5c piece at 6 inches- double the distance of my Tracker IV. !
I downloaded a free program from THunting to minimise pics for that site -it works excellent. This first pic was minimised. Something for our Mods to consider??
Also I have now worked out my PCB express and Sch Express programs (also free downloads), so here's a couple of diddles- I re-drew the Pirate board for DD option- haven't tested it yet, but I'm reasonably confident all the traces and components are right. Anyone thinking of making PI detectors, this is a good starting point to understand the workings of more complex machines. R13 and the pot I added near R3 are for sensitivity- I'm just about to work out where I need another pot(just replace R?) to make it variable frequency- hopefully then I can get it seeing gold.!!
And this is a super-simple IB based on Beau Schwabes inductive proximinty sensor- if you google metal detector circuit it's the utube vid in images about the seventh row down. I made this detector circuit ages ago with BC547/557s, but it was a bit unstable-I'm hoping the original 2n3904/6s will solve that. NOT TESTED YET EITHER !!