AGD25.1 PI Detector Development Work

[ivconic]

...If you depend on videos to make decisions you are different then me...

No, of course not. Video however good and true; is the least important factor in deciding whether to start doing something or not.
William Lahr didn't make a single video, he redrew the schematics by hand, often sloppily... and yet I made most of what he drew so far.
I am asking to see the video for other reasons.
I explained most of the reasons in the previous post.
English is not my native language, I often don't write and express myself properly, I guess that's why you didn't understand almost anything of what I wrote in that post.
You already had one (or maybe a couple, I stopped following) topic on the forum. A very large and comprehensive topic. Look back how many people joined and participated in that topic?
And then find a topic on the forum like... let's say Delta Pulse. Look at the number of pages and the number of people who participated in that topic.
Do you get the point?
The forum is not a place for rather autistic actions (even in your case when you post good and interesting things). A forum is a place where people gather and discuss together.
A forum is a place of interaction. Cooperation. Give and take place it is.
Let me repeat this, maybe this time it will be clear to you?
A short video of your magnificent work in action in a room, on targets such as a coin, a piece of iron, a piece of aluminum foil... will that take up too much of your time? Will it cost you too much? Will it kill you?
No, it won't. It will take you 10 minutes at most. And that recording will tell us a lot.
The autism you insist on is very foreign to me.
Have it your way.
You wrote "tons and tons" of posts, "tons and tons" of claims.
You haven't given even 1 second of footage showing how your great work works in practice.
I'm sure 90% of the forum members agree with me.
But they are too cultured and don't want to stir up dust, but just watch quietly from the sidelines.
As for circuit analysis... as you say; I'll just say this: ha, ha, ha!
The works on this forum are the best proof that very often what works perfectly on paper; in practice it often produces clouds of white smoke... if not then in the mildest form it doesn't work at all.
The forum is full of years wasted on "phenomenal" ideas and "solutions".
Not to mention wasted money.
Okay, life isn't fair. Some of us can waste money and time like that and enjoy it. Unfortunately, some of us are not in that position.
Ok, this is my last post on any of your topics. What did we find in the end?
We discovered that 10 seconds of video will drastically jeopardize your precious time, health and financial condition.
So I won't insist on it anymore. Enjoy!

[dbanner]

I wouldn't describe the reverse engineered schematics drawn by William Lahr as "sloppy", far from it, they are very, very good, considering they were drawn by hand. And they are extremely accurate, except for a few errors here and there spread across several dozen or so schematic drawings attributed to him.

As far as demo videos go, I agree those would be nice to see. But I'm very satisfied just to follow the evolution of his work.

Besides, shooting videos takes up valuable time, which is just a distraction when engaged in high level cerebral work.

[Auto-Mation-Assist]

Today I finalized the new circuit boards for mounting the parts that will be on the front and rear panels.

The power connector and its associated on/off switch are separate and do not connect to the front panel circuit board. Wires are soldered to the in row holes from the front and routed through the black colored holes to the back side of the board shown. These wires will have pugs that route to various locations on the TX or RX boards. I usually use 24 AWG started wires from salvaged stripped 10 conductor cable which supplies 10 standard wire colors. The size of the front panel circuit board is 4 by 2.42 inches. Using this circuit board provides a reasonable attractive was of handing the wires and provides good flexibility to route the wires to the required sockets on the two main circuit boards.

The front panel circuit board:





The rear panel circuit board solders to the standard older five pin GX series connector. And contains all damping resistors and related switch for adding additional damping for a DD coil. This board has several 1 watt trimmers to handle the final damping adjustments. These trimmers are mounted on the other side of the circuit board so that they can be adjusted easily though holes in the back panel. These holes will normally be plugged by properly sized screws. A test point will be available to make adjustments without requiring removing the enclosure. The trimmers are 0.250 inch tall and 0.505 inch round. I decided to eliminate the TX/RX diode I mentioned in a prior post on this circuit board.

The resistors I have chosen to use are quite large due to having higher wattage ratings than those typically used, and are none inductive wire wound types. It is not required to use these types and metal film types can be used as well. Some board space could be saved by using lower wattage resistors like the metal film types that handle part of the final adjustment damping towards the bottom right of the circuit board.There is a slot to hold the ferrite core in the TX path in place so it does not move around once a bit of adhesive is applied.

The TXC solder points handle the TX signal, and the voltage sample for determining a the desired decay point of the coils decay wave form. The RXC connection point has one ground wire and two connections for the receive signal. These all route to the RX board. The rear panel circuit board is 3 X 2.1 inches.



I placed the order for these two simple boards with PcbWay today.

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I'm slowly progressing of testing the prototype transmit and receive boards I posted pictures of earlier. I did find that the TX board had several footprints that were corrected but linked to wrong parts. I wired around these two problems and corrected these for when I place the order for the final circuit boards. I was able to program the STM32 MPU with no issues and is operating properly. While looking at the circuit board layout for the components of the rear panel circuit board and the direction of the wires that exit the board that these were not optimal and required more length than necessary. The rear panel board is working properly and does allow me to properly adjust the damping of the coil. I have designed a new rear panel circuit board that is slightly smaller and also routes is interconnect wiring to better directions for attaching its plugs to the sockets on the TX and RX circuit boards.

Picture to the new rear panel circuit board.
The 5 pin coil connector will be machined to allow the complete rear panel board to be removed from the actual metal rear panel. I have ordered a 16mm x1 tap and die set for this work and will use our lathe to do the required machining.





Here is a picture of the TX and RX boards plugged together back to back and the normal position that the rear panel circuit board will be in.


As you can see the TX wiring is heading it the wrong direction with the prop type rear panel circuit board.

The switch on the rear panel board selects Mono or DD coil loading. DD has more coil loading based on my tests that show that my DD TX coils have less inductance than my Mono coils. However I'm not sure if that is always the case. I have Carl's Third Edition "Inside the Metal Detector" book but have not looked to see it it mentions the typical inductance differences between the two types of coils and how that affects damping adjustments.

Also if you look at the 5 pin coil connector you can see the stop ring that is normally on the outside its mounted panel that need to be machined down to allow for threading the area underneath it. I added a ground wire to the outer shell of the connector for my prototype testing since there is no actual rear metal panel to provide the ground for the connectors shell.

The new rear panel circuit board has the correct spacing for the connectors pins so that the circuit board mounts more closely to the connector. This will provide a better adjustment range for the switch and connector to the actual rear panel.

[Auto-Mation-Assist]

I have been working on some updates for my AGD25 detector since I found some problems that needed to be fixed and have worked on the solutions required to fix those and developed new circuit board layouts so that the proposed changes can be incorporated. In the past I had posted some information on my rear panel circuit board that contains all most all the coils damping circuitry based on just using carefully selected resistor values but these do not allow for easy adjustment. Also in the past I posted some circuit ideas on how to adjust the amount of damping by using active circuits and 25 turn trimmers. That circuitry worked fine but I wanted to make some improvements the active circuit that I have used in the past and thus developed a new way of accomplishing this in a way that minimizes injected noise.

The AGD25 detector underdevelopment has coil voltages very close to simulation voltage shown below.


The 36us location on the bottom left is the point where the TX pulse ends. In order to detect very small gold the high voltage needs to decay as smooth as possible to zero volts no later that 3.8us from the 36us TX pulse end. By small gold I mean 0.015 grams at more than one inch or more during air testing, one inch means that it is good but not great. If we look at the flat line just before the 40us marker it looks real good but then the vertical scale on the left is not to good for looking at mV or uV levels so lets expand it to something useful.



This picture is the the same as the expanded parts of the prior picture between 39 and 41 microseconds. The picture also shows the adjustment range of the new active circuitry in five present steps. The goal here is to make the waveform flat near zero volts without moving the crossover point too far to the right with this polarity of coil decay voltage. In the picture above this occurs at about 39.6us after the TX pulse ends. Lets expand this a bit more to see the actual time frame available for the desired sampling period.



I think that the best place to start sampling the waveform would likely be at 39.6us and ending at 40.2us. This is a short sampling period and analog gates following the RX pre-amp may have a slow turn on and thus not be able to pass the initial true voltage present for the first 100 or 200 microseconds. This can be avoided by starting sampling earlier but that may lead to circuit overloading due to the fact that the signal voltages rise sharply when moving start of sampling more to the left. If coil damping is not near perfect as possible circuit overloading can almost be guaranteed if your goal is to find small gold.

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I have simulated the signal that the first gain stage in the AGD25.2 will see while using the active damping circuit that I have shown the simulations for in my prior post. The AGD25.2 first gain stage will again use FET input OpAmps because these have input currents in the picro amp range. Many very low noise OpAmps have significant input currents which aggravates DC offset control, cause bias on the clamping diodes and also present a fairly low impedance load when compared to FET input OpAmps. I recently tested that gain stage of my AGD24.2 detector and it shows that the electronics in the detector has a threshold for detecting at 60dB below 1mV at the input of the first gain stage while the detector is in test mode, or about 1uV. This included the old input resistive and diode clamping network and feel there is no need to use anything else.

Lets have look at the expected output of the old resistive and diode clamping network that feeds the both FET input OpAmps.



The initial large spike shows there is a short duration period of clamping diode current which they have no problem handling but will cause a temperature rise. With 30dB of first stage gain it can be calculated from this chart when that stage comes out of saturation. The first stage of the AGD25.2 detector operates at +13 and -13 Volt regulated power supplies. I would consider the clipping output point of the first gain stage to be about 80% of this or just about 10.4 volts plus or minus. The voltage passed to following stages is gated and based on time for both channels. Thus the clipping points of those stages can be calculated based on gating loss and stage gains for each channels time slot.

Now lets have a more more detailed look at the actual voltage levels in the primary time slot.



With the active damping network adjusted properly the voltage levels entering the first gain stage may be similar to those shown in the picture above. The slight negative going point is desirable and is an indication that the coil is not over damped and thus extend the decay time more than necessary. If over damped this negative going point will become positive going. Depending how your coil is pulsed your polarity may be reversed from what is shown.

The graph above shows the coils decay voltage by itself. If you look at the low going point it is at bout the -400 micro volt level. Compare that to the next picture in which I inject a 250mV 1Mhz signal into the simulated coil with a 2 Meg Ohm resistor and note the difference in the level of the negative going point.



You can see that the negative going point has increased to -500uV from about -400uV. This indicates that we can use changes in the level of this point to detect very small objects like sub gram gold if we gate the signal to a following stage at the proper time. The design of a detector will determine when to start passing the signal on to the next stage as will its sample width. If you cold synchronize on just the negative going points at both side 300uV level you would have the most sensitivity. Unfortunately that is a really narrow window and thus may actually need to be between 1us and 3us wide and the be averaged out to come up with and averaged voltage which reduces sensitivity. Now set this average, usually known as a ground level as a reference and compare any changes to it to generate some kind of alert tone.

In the AGD25.2 detector primary channel has a width that is variable from 1us and 3us. The secondary channel width is 10 times the width of channel 1. Spacing between channel 1 and 2 is also front panel adjustable.

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My prior post today has shown some graphs to help visualize what the active damping circuitry in the the AGD25 versions is expected to do. In my prior work with my idea a active adjuster damping network using trimmer pots has shown that it is usefully and worked fine in actual built and tested circuits. My early work on a active coil damping circuit placed the added circuitry on the high voltage end of the coil. The new design places the active circuitry at the low side of the coil to eliminate having to deal with the high fly-back voltage. By doing this it is anticipated that there will also be a reduction in added injected noise as compared to the original design. It should be noted that the AGD25 versions use none inductive wire wound resistors to reduce resistor induced noise and improve temperature related stability for all primary fixed damping resistors. These are 2.5K and 10 Ohms on the diagram below and each has a 3 watt rating.

On the diagram you will also see a symbol for a trimmer pot that adjust damping to compensate for loss of clamping diode conductivity during the final decay part of the coil, and which feeds the FET input Pre-Amp. There are some added small value capacitors that are there to simulated OpAmp input capacitance. Using a DD coil will require a DPDT switch and add a few parts to equal the load that the RX clamping circuit would normally supply. DD coil trimmer is shown as R8 and would be in parallel with X1 and its output switched to X6 pin 1 via one side on the DPDT switch. All the added clamping diodes provide reference voltages in MONO or DD mode that allow damping to to be adjusted very accurately.

All parts required for this circuitry including the GX16-5 coil connector and a ferrite core is on the rear panel circuit board.


Here is the simulation diagram that I used for circuit development.



And my proposed AGD25.2.ACTIVE.RP printed circuit board. Measures 2 X 3 inches

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As you can see that many of my posts have to do with TX coil damping since it is very important to get this as perfect as possible, which in turn requires ways to make adjustments easily. The circuit board layout that I posted in my last post hopes to accomplish my desires. On this circuit board only the surface mounted parts are accessible once assembled because everything else is mounted on the back side including the ferrite core and the connections that directly connect to the GX16 coil connector. The circuit board thus provides some shielding from those parts that handle the high coil decay voltage. There will be 0.500 spacers between this board and the back panel. This is fine but not so good for the GX16 connector which needs to be modified for a rear mount to allow the circuit board to be removed.

To do that the pin assembly itself will be removed from a standard GX16 connector and placed in a new surround. For my application I have machined several of the required mounting surrounds for the coil connector to allow it to ave its lock washer an nut on the outside of the front panel from material I had on hand. I made some drawings to work with that show the dimensions that I used to make these and posted these here since they could be helpful to some one else who need a rear mounted connector instead of the usual front mount. I could not locate a true rear mount GX16 connector.

The dimensions I used are these. Not by any standard but they worked fine for me while machining.

[Auto-Mation-Assist]

My last post showed the dimensions for making an rear mountable GX16-5 male coil connector using the pin insert from a standard GX16 connector for my AGD25 detector that uses a read panel circuit board that handles most coil damping components and the mono-dd switching. I initially modified some stock existing connectors, and the modified ones were usable but really not that good due to the sloppy fit of the nut over the flattened areas if the back of the connector. This is mostly due to the rear outside of the connector body is made real sloppy and wobbles badly in relation to the threads at the front. To work around this something more accurate needed to be made.

Here is a picture of the parts I machined to replace the outside of a normal GX16-5 connector with male pins using the prior posted dimensions from the AGD25 detector. The black pin assembly, nuts and lock washers are from normal GX16 connectors. The normal locating pin is not required but can be added if desired by using some small diameter round stock and holding it in place with adhesive. Insert it in the indentation of the black pin assembly and apply adhesive to hold it in place before inserting the pin assembly into the new connector body.

This modification allows me to remove the attached circuit board from the rear metal panel of the detector and thus have the ability to access the components mounted on the back side of the circuit board without having to un solder the coil connector.

[GeoMax]

Interesting...
But why not make a small circuit board, insert an original GS16 connector into the housing, and then solder the GX16 connector onto the small board? The small board has a different standard connector type that fits into the actual backplane. That would also work well and would be significantly cheaper to manufacture. The only disadvantage is if you want to replace the GX16 connector it self. Then you have to desolder the small board.

By the way, the angled GX16 connector is also available for soldering circuit boards. See the image below. Perhaps that would be an option for you as well.

[GeoMax]

You should have a closer look at the angled GX16 connector. If you would cut the connector pins right before the 90 degree angle you can solder them right onto the PCB. The rear PCB you can mount with the connector the same way like you wanted.

[Carl-NC]

Not quite sure of what you are doing, but there are GX16 connectors that mount directly on the PCB:

[Auto-Mation-Assist]

Not quite sure of what you are doing, but there are GX16 connectors that mount directly on the PCB:

Thanks Carl. I'm just working around a problem of mounting this connector in a suitable way because none of my normal parts sources have had a connector like your picture listed which was surprising since PCB mounting is rather common. My solution will work perfectly but does take considerable machining time. I will try to find the part number for the connector you have shown and get some in stock for future work.

GeoMax, It also looks like that your suggested solution may may also work if the distance between the panel the connector is mounted to to allow 0.5 inch distance to the PCB circuit board after cutting the pins.