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Offline bobasaurusTopic starter
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« on: January 27, 2021, 05:57:16 pm »
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I'm new to metal detecting, and may be more interested in building a DIY detector than actually using it.  I'm trying to make a partially digital metal detector using a sound card and DSP to do the detection and discrimination, based loosely on the Neco Desarrollo Smart Hunter and TGSL projects but modified to have a slightly different coil and built using whatever I have lying around.  I hope to eventually use a raspberry pi zero w/ usb sound card or a smartphone to handle the calculations.  I've built a direct drive transmitter circuit from an old audio amplifier IC, and wound the TX coil after making a jig:





Working on making an ac coupled non inverting amplifier for the receiver now.  Hopefully this works, I don't really know what I'm doing but it's fun so far.  

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https://i.imgur.com/0s8znFB.jpg
https://i.imgur.com/0n2JPe3.jpg




« Last Edit: January 27, 2021, 05:58:41 pm by bobasaurus »
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Offline GoldDigger1950
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« Reply #1 on: January 28, 2021, 12:04:21 pm »
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Looks good to me and I've built dozens of coils. Since the operating frequencies are relatively low, be sure to stabilize the coils after testing. Drench the coils in varnish to prevent the wires from oscillating. You can't see it but loose wires "wiggle" when they are excited. Early Whites coils had to be replaced because the coils, some of which oscillated just above audio, would scrape the varnish from adjacent wires in the coil and literally short causing inductance changes.

So, a thick varnish will keep this from happening and protect your coil at the same time. Modern manufacturers use coil forms and epoxy. Either is good. Epoxy can by dyed to give your coil some pizzazz. Either will add weight so don't go overboard.

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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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« Reply #2 on: January 28, 2021, 07:52:15 pm »
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Thanks, after taking this picture I made a lot more constraining knots and dabbed on some varnish with a q-tip between the pegs.  After drying overnight the coil was super stiff (the thick wire helped too).  I made the second coil yesterday, giving it a few more wraps to increase the inductance (not really sure if it's necessary, but all the other DIY metal detectors I've seen do this).  I figured out how to measure the inductance with an oscilloscope too.  Now to build the receiver circuit. 

Interesting point about the wiggling unconstrained wires.  I'll apply varnish fully over both coils now that they're off the frame to reduce the problem.  When I'm done I'll make a wood coil housing and pot it with epoxy. 

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