My AGD analog detector work

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I have mostly completed my design work on my analog metal detector which is named AGD23-2 but will be updated to AGD23-3 for the final version. The AGD standing for Alaska Gold Detector. The goal of this project was to design an analog detector using modern components as my performance standard to compare later versions that could contain some form of digital signal processing. My requirement for this project was that it would not need a menu along with its required digital display. All control would be by rotary controls and toggle switches whose position could be determined by a simple glance at the control panel. Audio out alert tone output to be a low distortion sine wave that idle at about 400hz, be able to detect very small gold, and have the ability to drive a internal speaker.

The timing logic in the present design calls for the high speed version of the old 555 timer, the ICM7556ISD+. What I'm considering is to potentially replace the timing the c-mos logic with some sort of processor or even a FPGA that can accept rotary encoder inputs instead of the three linear controls that connect to the TMG connector at the top right of the diagram posted for an potential future upgrade.

The logic comes out with four time slots, TS-0 , TS-1, TS-2, TS-3 and TS-4. TS-2 being the total time of TS-0 and TS-1. Time slots TS-3 and TS-4 are not used in the current design and set the remaining time.

DecayEnd goes positive which comes from the TX board and provides a pulse only on the first set point crossing and activates analog gating.

JP1 on the DecayEnd input is provided so RX board testing of the RX board can be done without having a coil connected and a 100 or 1000 to one attenuation and signal generator is connected to the RX input.

The question I have If I were to replace the cmos logic with some other method what would be the suggested way. This is not a lot of logic and I think a rotary encoder is likely about the same costs as linear potentiometer. Its been a long time since I have done of that kind of work. I have a AVR starter kit but it may be out of date.

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Here is a picture of a earlier version to show the basic location of the logic on the RX circuit board. Towards the top center to the right of the larger vertical connector.

[GeoMax]

Hi,

nice structured drawing... and also the PCB makes a good impression. Specially the position of mounting holes for the PCB seems to be thought about prior placing components. I notice that you have placed the schematic symbols off-grid. Has this a reason? And which design program are you using?

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Hi,

nice structured drawing... and also the PCB makes a good impression. Specially the position of mounting holes for the PCB seems to be thought about prior placing components. I notice that you have placed the schematic symbols off-grid. Has this a reason? And which design program are you using?

Thank your for the complement. I'm not sure about your question of the schematic symbols being off-grid. I just did a quick screen shot of the diagram and not happy with the way it came out. If you are looking for component values the DipTrace program does not put those in the diagram but makes them available as a parts list in my configuration. I will post the diagram again in a different way along with its parts list.

I have been searching the internet for potential parts to replace the cmos timing section but have not found anything reasonable. Some FPGA components from Lattice Semiconductor look like a possibility since they have small footprints but they do not have the 10 volt output ability of the cmos parts. Most things that I have found just offers way more than required. The cmos timing parts do work extremely well so there is no real need to change anything but then upgrades are always nice if they do not cause much difficulty and offer increased flexibility. Some level shifting parts would require changing to different values due the 3.3 volt limitations of the parts I have been looking at.

I will post a better diagram and parts list today or tomorrow.

[Altra]

DipTrace program does not put those in the diagram

Hi AMA,

Diptrace does allow part values. Example double click on resistor "R1", this opens the "parameters" window next to "value", enter 100K. Then in the upper part of the box click on markings. In the value row click on common, show or hide. pick show. In the markings window you can also select the alignment.

You can do a global show or hide markings, under the main tool bar "View" and then "part markings"

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I have tried that now and it works, but uses what is already in my component data base which has more information than just the value. I did this initially to help clarify board population when I set up Diptrace. Now all this added information makes the whole diagram overflow with unreadable info.

I will need to correct this by moving the added info into its proper place, the component "description"

Thanks, for the information. It lead my to my own error in the use of the value parameter and its cure. Correcting this will make life a lot easier when looking at my diagrams

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Here is the updated timer page diagram in pdf form for better clarity and also its parts list in both pdf an txt format. Info in the txt file can be copied and pasted.

The timer diagram has not seen many changes since my development work started on the AGD23 project. The latest changes were made perhaps a year and half ago and changed the level shifters from 10 volt pulse output to 3.3 volt due to a change in the types of analog switches. Several unused ones were not changed and left at 10 volts. These may be changed in the next receive board update which is planned for this summer due the part to be changed will not be available until May of this year. However the diagram is already updated for this part change and the pcb layout changes have also been completed. This change required this parts rotation to allow for good trace routing on the board.

I will work on my component data base so I can supply diagrams with parts values. I can supply those in separate files until that work is completed. I had combined parts value and description into one item which worked fine for me but not when sharing information. So I need to fix that.

I will post information in logical steps that follow the actual analog signal paths. To start off some transmit pulses have to be generated. Those are handled by the ADG23 transmit plug in board board so I will post info on that board next .

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This is a picture of my transmit board that provides TX pulsing and also decay end detection on the first crossing occasion. There are some minor changes planed that relate disabling pulsing if either or one of the 10V LDO voltage regulators go out of tolerance. The LDO's have output for this that are presently not used. This board plugs into the receive board and routes battery power to it. The receive board routes back +10V and -10V back to the TX board. On the receive board there is a isolated DC/DC converter that generates a regulated +12V source. Since the -10V regulator on the receive board would power up first it is delayed from doing so until the +10V regulator started and raising its buss voltage. There is also more load on the +10V buss than the -10V buss. The receive board 10V regulators have the current limits set to 250ma for protection. Capacitive coupling is used to the input of the mosfet driver in this version but other methods could with greater parts count. The mosfet drive input must only be driven high on a TX pulse. The mosfet I use is rated at 650V



The ADG23 presently has its TX pulse width set from 20us to 100us which is mostly for experimental use for various coils. I find system is set and quite happy at 36us. Caution should be used with pulse width since it can put large currents through a coil. Signal coming from the receive board are TX pulse and Pulse enable. Going to the receive board is Decay end whose position in time is adjustable via the DTD trim pot. Decay end is detected just enough early by a preset slight positive offset at the input of the high speed 5ns comparator. This allows for sufficient time to allow for trimming the decay end time and compensate for fixed natural delays in the receive from end. Typically this runs about 3.4us to 4us from the start of TX pulse depending on coils that I use. The 3.4us was with a DD coil. Best with my selection of coils is about 3.8us to 4us. Adjustment should be made while displaying the receive waveform on the scope while triggering on the TX pulse test point and displaying the Decay pulse from its test point and also the not yet discussed front end test point after the variable gain amp on the receive board..

I have attached my the diagram of the ADG23.1.B transmit board page 1 and 2 and parts list in both pdf and txt format.

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Just thought that I should upload some pictures of how I build the front control panel. It was sprayed with the Cerakote coating and then engraved on a small gantry cnc milling machine that I built many years ago for PCB work and has a high speed spindle. The front panels were machined from 1/8 inch aluminum and the PCB tray from 1/16 inch. Both of those were machined on my larger cnc knee mill. They came out reasonably respectful.

The front panel attaches to a extruded aluminum case. I had salvaged that metal from a old HP plotter years ago and the dimensions of that is what determined the initial size of the pcb board. Later the TX section was removed from it due to space limitations. The speakers I use are quite heavy and were chosen for their low resonance frequency. Having a large magnet in the enclosure has not been a problem nor have any problems shown up with multilayered ceramic capacitors that can act like a microphone when vibrating.

In the picture that show the back of the front panel you can see the loose connector that plugs into the TX board to supply switched power. Also visible is the 500ma isolated DC/DC converter. Next to it you can see the connector that routes power to the receive board, and to the lower left edge part of one of the LDO voltage regulators. To the of the DC/DC converter is the audio amplifier. The electrolytic capacitors are all anti vibration versions as can be seen by the plastic surround. Their solder pads are much more substantial.

[GeoMax]

Hi,

DipTrace.... very good. My favourite PCB program. With “components placed off-grid”, I mean that usually the schematic editor grid is set to 50mil. However, in your earlier posted schematic was the components placed somewhere irregular between the grid of 50mil. In the screen shot below, you can see in the status bar “Grid Snap: ON” in my DipTrace settings (bottom right corner). With the keyboard keys “FN + F11” you can enable / disable it.

Also you might run into problems if you do not mark unused component pins with an “X”. See example schematic (top left corner) below. DipTrace has an option for it.

[Carl-NC]

Just thought that I should upload some pictures...

That might be the nicest looking homebrew I've ever seen. Did you populate the PCB or have the PCB house do it? Looks like a paste/reflow job.

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Hi,

DipTrace.... very good. My favourite PCB program. With “components placed off-grid”, I mean that usually the schematic editor grid is set to 50mil. However, in your earlier posted schematic was the components placed somewhere irregular between the grid of 50mil. In the screen shot below, you can see in the status bar “Grid Snap: ON” in my DipTrace settings (bottom right corner). With the keyboard keys “FN + F11” you can enable / disable it.

Also you might run into problems if you do not mark unused component pins with an “X”. See example schematic (top left corner) below. DipTrace has an option for it.

I'm used to having grid sizes of 0.01 or 0.001 inches and would likely never use anything else. My diagrams have been using 0.001 inches. If you set the grid to 0.001 and display the grid and then zoom way in my system it appears to indicate that DipTracet may not be able to produce a grid of that size and snap to them. It is actually possible to place a wire close but not have an actual connection which should not be allowed if zoomed out. I figure that as long it can generate a circuit board without errors then all is good for now.

Placing the X for unconnected pins is good. I will look at configuring that since it may be an alert when a wire is placed but not actually connected as I mentioned or wire is deleted improperly and removes other segments that are then not noticed.

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That might be the nicest looking homebrew I've ever seen. Did you populate the PCB or have the PCB house do it? Looks like a paste/reflow job.

I do my own board population and order stainless steel paste application stencils when I place my board orders. I use AISLER for my board house. They do a great job.

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I have been working on my component data base configuration error (about 11 hours of work) and now working on updating the schematic to use the modified component files from my component library. I have the timing page completed with values now and continue to work on the other seven pages of the circuit diagram.

I have been notified that my latest order for a updated circuit boards is on the way and will need time to populate one and test it for proper operation. I expect them to arrive in about three days. I may be very busy for a while and thus not be able to reply to posts right away.

[ivconic]

I do my own board population and order stainless steel paste application stencils when I place my board orders. I use AISLER for my board house. They do a great job.

Great job, pcb outlook!
Using stencil is very good method.
JLPcb and lookalikes also offers a stencil as an option.
In the future I plan to try it on some of the pcb I will order.
AISLER, just visited their page, but didn't check the prices, are they competitive to JLPcb?
And last, as usual; can you show how your detector performs, some short video, something?