I've been research metal detector designs for a few days now and I can't quite wrap my head around the amplification they are using for the first stage right out of the coil. Below is an example schematic from one of the paper explaining how to build a pulse induction metal detector:

Pulse Induction Metal Detector - Mark Stuart (1989)

I understand the most of the circuit, the problem I am having is understanding what the inputs and outputs should look like on IC4. As far as I understand it the amplifier is essentially amplifying the difference between the coil and the 9V rail. As far as I can tell the coil voltage should fly highly positive when TR4 opens. This voltage is clamped by D3 and D4 until it gets below the forward voltage threshold. At this point it is then amplified by IC4. The part I don't get is why they made it an inverting amplifier, because from what I can tell the output of IC4 should be pegged at 0 the entire time the coil is discharging since the voltage will always be higher than the 9V rail.

If anyone has insight as to what I am missing or how that section of the circuit works I would really appreciate it.

Here is the link to the full pdf paper which contains an explanation for most of the circuit: PI Metal Detector - Mark Stuart (1989)

  • \$\begingroup\$ The circuit needs to be redrawn, breaking it down into clean sections, with an idea towards understandability. This also means no "busing" of power to clean up wires that distract an understanding. I'm not willing to do that work. But reading the paper on the topic, I find that the basic concept for each cycle is quite similar to work I've spent years solving regarding phosphor thermometry. I'd approach this with a PIC DSP device and custom offset subtraction and decay linearization to find the slopes for discrimination. \$\endgroup\$ – jonk Jan 10 '18 at 16:42
  • \$\begingroup\$ Thanks, I haven't gotten around to reworking the circuit. I understand how a Pulse Induction detector works as a whole and I plan on using a micro-controller with a good DAC to process the decay pulse. I'm more interested in simply how IC4 opamp outputs a positive signal when it looks like it's wired as an inverting differential amplifier. \$\endgroup\$ – Wired365 Jan 10 '18 at 17:01
  • \$\begingroup\$ I know. I read through the article, looked over the schematic, and realized some things easily and right away. But that for a good description, I'd need to redraw it and then re-examine the result. Not willing to do that work, right now. \$\endgroup\$ – jonk Jan 10 '18 at 17:09

The schematic is easy for me to read but without knowing the resonant behavior of the coil I suspect this design could be quite different in the waveform from the original. It could be a discontinuous mode switching depending on construction details of L/DCR time constant being less than the switching time interval. The diodes act to clamp the high voltage spike and any resonant ringing by shunting the injected current to +/-Vf diode drop until it decays to 9V where the high gain output goes to a S/H on the tail voltage from a result in coil loading.

There are a number of circuit differences on implementation but it is hard to tell how performance differs from a quick look at the schematic.

The flux loop of the coil is affected by both conductive material with Eddy currents in its path as well as ferrous magnetic materials.

The original design used the main battery to drive the coil with a voltage doubler to drive the Op Amps, whereas this design uses the LMC7660 as a voltage doubler to drive the coil and biases the OA input to the main battery voltage of 9V.

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The output of IC4 is essentially amplifying the voltage from the search coil. IC4 has a gain of ~1750, the diodes limit the output to 1V. VR2 is probably a DC reference adjustment.

The output of IC4 looks something like this, but the output would be clipped and the scale would be different. At some point the circuit detects what the voltage is after the pulse (blue arrow) with IC3 and then changes the tone output.

enter image description here

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  • \$\begingroup\$ I hope you add more. That's just a replication out of the PDF the OP provided. It's useful. But I think the OP is looking for a detailed explanation that goes beyond the already useful discussion in the PDF. Just my opinion, though. D1 and D2 performed obvious functions that were clear even before I read the text. But the details of how this works and why the design is exactly as it is will take a little work to write about. \$\endgroup\$ – jonk Jan 10 '18 at 16:52
  • \$\begingroup\$ I would spend more time on it, but usually people don't even upvote. "the problem I am having is understanding what the inputs and outputs should look like on IC4." I believe this answers the OP's question. \$\endgroup\$ – Voltage Spike Jan 10 '18 at 17:02
  • \$\begingroup\$ @laptop2d Sorry I guess I should reword my question a bit better. The question I was trying to ask is less generally what does the input look like and more why does a positive spike, as depicted in the paper, on the negative input to the opamp and the positive 9v rail on the positive input result in a positive output that is later integrated and used to trigger some threshold. Am I interpreting that opamp wrong or is there some inversion happening that I am overlooking? \$\endgroup\$ – Wired365 Jan 10 '18 at 17:28
  • \$\begingroup\$ I've never cared about upvotes after getting the basic abilities. I owe everything I am to people who cared enough to talk with me and help me learn, who provided libraries and paid for public education, and who gave me a world's worth of infrastructure. I pay that forward by giving of myself, in return, to others who are willing to work a little in return for my work. But it was only my momentary thoughts I added. If that strikes a chord, fine. If not, that is also fine. \$\endgroup\$ – jonk Jan 10 '18 at 17:29
  • \$\begingroup\$ I meant it only as a positive encouragement and hope you don't imagine I feel anything but positives that you help with an answer, at all. It's all good to me \$\endgroup\$ – jonk Jan 10 '18 at 17:29

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