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Offline AdrianMTopic starter
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« on: April 04, 2011, 09:11:52 am »
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Hello. I've decided to have a stab at designing a basic mono PI from the ground up (no pun intended). No previous experience of this particular bit of electronic kit, but I've read plenty about the principles so I thought I'd start by demonstrating the essentials to myself, and maybe document the process here.

My very first experimental setup consists of just 20 very loose turns of tinned, stranded copper wire with ptfe insulation (seems to make a pretty quick coil all the same) driven by a mosfet gated by a variable pulse generator. The damped flyback is clipped and amplified as usual and displayed on the scope. So now I can actually see the shift in the decay induced by a 50p coin at about 15cm on the 100mv/div range so I'd say it's a promising start Smiley

Now I'm just looking for inspiration on how best to follow this initial stage to process the decay into something that will give a sensitive detection. Obviously this particular coil ain't going anywhere Smiley

My final objective is to use a microcontroller (PIC or ARM) to generate the timing and do a share of the processing before displaying the results. I've got a fairly fast 16bit A/D loitering in the wings if needed but I'm a bit stuck on what to do with the signal next.... suggestions welcomed!

One thing I may be on the bleeding edge with here is a bluetooth link up to an Android smartphone for the final UI. This is stuff I've already achieved with other projects, and if there's some real processing poke required up the line, like FFTing then that could be done in the app itself.

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Offline GoldDigger1950
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« Reply #1 on: April 04, 2011, 09:32:24 am »
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If you want to use a PIC to control the pulse, consider using a digital potentiometer to control all the aspects of the signal using a 555 timer. Using even one timer can limit the processing power of the PIC should you later decide to design a discrimination circuit. Once the digital pot is set, you don't need the PIC to control it again unless you change the pulse in some way. I know the timers are separate inside the PIC but you will need to change the wave shape as well as the frequency to maximize your detector's efficiency.

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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.
Let's Talk Treasure!

Offline AdrianMTopic starter
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« Reply #2 on: April 04, 2011, 10:24:03 am »
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Good point GD. All parameters should be kept as tweakable as possible which is why I'll be using software PWM under interrupt for coil driving. I've got a fancy hardware PIC emulator which I think I'll use to get this part up and running next - although I'm a bit paranoid about 600V flyback getting anywhere near it or the attached PC !!! I will be sprinkling some 5V zeners around the interface just in case.

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Offline GoldDigger1950
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« Reply #3 on: April 07, 2011, 04:34:31 am »
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Look at some of the designs out there. They use two diodes, both reverse biased in a configuration unique to coil snubbing. This keeps the signal from going back into the circuit. The normal failure mode for a diode is a short. If that occurs, the high voltage is shunted to ground. If a diode opens, the signal can't go anywhere. The drop across a diode is a mere 1.5 volts maximum and this must be amplified. An isolation capacitor also helps by allowing only the AC component to pass from the first stage of amplification. The 600v is nowhere to be found, mate. Don't worry about it.

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« Reply #4 on: April 07, 2011, 07:34:26 am »
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This is going to be cool!  Shocked   Cool

{alt}

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« Reply #5 on: April 14, 2011, 10:38:15 pm »
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sounds like it will be


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« Reply #6 on: April 15, 2011, 08:55:04 am »
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Well?   What's up? Shocked

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Offline AdrianMTopic starter
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« Reply #7 on: April 15, 2011, 09:30:22 am »
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I've been spending more time on this than I care to admit to. So far I've achieved a fully differential front-end to eliminate the need for a screened lead which only adds to the parasitic capacitance of the coil and switched to a STM32F103RB for microcontroller.

The curious thing is, on the Geotech forums, someone there has been experimenting with a bifilar wound coil to Tesla's patented design and having lashed one together, I'm trying to nail down what it's all about. When I get some time I'll post up some more detail.

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« Reply #8 on: April 15, 2011, 01:18:11 pm »
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If I get you right the coil that you are going to make does work but the disadvantage is the hight of the coil mind you if it works well with your design it really doesn't mater.

All the best
Xavier

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So many questions so little time

Offline GoldDigger1950
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« Reply #9 on: April 15, 2011, 03:14:05 pm »
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Quote:Posted by AdrianM
I've been spending more time on this than I care to admit to. So far I've achieved a fully differential front-end to eliminate the need for a screened lead which only adds to the parasitic capacitance of the coil and switched to a STM32F103RB for microcontroller.

The curious thing is, on the Geotech forums, someone there has been experimenting with a bifilar wound coil to Tesla's patented design and having lashed one together, I'm trying to nail down what it's all about. When I get some time I'll post up some more detail.

Adrian: "Hi, my name is Adrian and I'm an addict!"

Us: "Hi, Adrian!!!"


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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.
Let's Talk Treasure!

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