Universal PI Micro

[bugwhiskers]

Hi Sharky,

Where did you get the idea that the Evaluation board is running at 4MHz ? It's running at the full speed of 12 MHz.

The current screen is a common 2 line LCD display that is used to show signal strength and also allow the user to alter timings.

What do you consider is wrong with the latest published tried and proven pre-amp ?

regards
bugwhiskers

[sharky]

Hi

Hi any thing new on this Universal Micros, lots of Flash Ram and big screens, just 4Kb flash is more than enough for this application...u need to concentrate on 1st stage of the opamp....rather than the screen and 24 bit adc...wonder where u r clocking the micro @4Mhz !! this will be too low clock....does Any body has a better schematic with source code for a sensitive PI..

[bugwhiskers]

Revision

The attached .sch & .brd have a few revisions. The feedback diodes are now shown with correct orientation. Support has been added by way of headers for dual coils, either double "D" or seperate RX/TX coplanar types.
C20 & C1 should ideally be a low ESD type. D1 & D2 have also been changed for more readily available types.
The IRF740 is shown laying on the board and fouling C1 whereas it is actually mounted vertically.

regards
bugwhiskers

[bugwhiskers]

The quick answer is all the diodes in the feedpack path. In a few days I will post a revised schematic with those changes and a few others.

regards
bugwhiskers

Hi BW
Is that all diodes in feedback path or just the D9 and D7 ??
I follow this projecto of yours with good interest.
I hopes it sucsessful
Henry in Bulgaria

[bugwhiskers]

Addendum

The diodes in the feedback path of the pre-amp need to be reversed. Also, the diode marked "germanium" can be a schottky 1N5817 or similar.
The resistor between the MOSFET driver and the IRF740 can be deleted, it causes ringing.

regards
bugwhiskers

[bugwhiskers]

The attached .PDF files show the schematic and pcb that have been created with Eagle Light, the free version can be downloaded from the following link.

http://www.cadsoft.de/cgi-bin/download.pl?page=/home/cadsoft/html_public/download.htm.en&dir=eagle/program/4.1

JP6 is the header for connecting a LCD display with RN1 providing pullups for PORT0 of the evaluation board.
IC1, a 74HC4051 has eight caps for collecting 8 samples. JP1 is the header for the rotary encoder with the pullup resistors located on the satellite encoder board (Top right hand corner of pcb).
R19, R2 and Transistors Q1 & Q2 providing the level shifting required to drive an "N" channel MOSFET with the micro. D3 is a failsafe for protecting the micro if something goes wrong with Q1. When the micro is being programmed most of the inputs float, R7 is a pullup that ensures the MOSFET is off during programming.
L2 hasn't been tried but it is hoped a suitable choke can be found that kills off a lot of HF EMI.
JP8/JP10 is the header for the audio from the PWM.
The AD797 is a very low noise OP AMP and has some diodes in the feedback path which speed it up and also limit the excursions. If a germanium diode is used for D6 then the negative excursions caused by the TX pulse will be minimised allowing faster recovery.
C1, a 6800uF electrolytic supplies the current gulps required by the TX pulse.
The current setup requires 2 batteries. One battery of 8-10 volts to power the evaluation board via it's power socket and the other battery (absolute Max 16 volts) to power the coil circuitry. The postive of the coil battery is connected to the negative of the evaluation board battery and it's negative terminal is connected to JP2 with D1 providing reverse polarity protection. D2 provides a second negative positive protection for the LCD contrast with just a 1uF tantalum that is discharged fairly quickly by the LCD contrast adjustment pot. This setup is necessary as the LCD will eventually be destroyed if the contrast voltage is present when the LCD is not operating which would be the case if the power was turned off and the contrast was connected to the 6800uF cap.
All chips in the analogue part of the circuitry have their power supply pins low passed with 10 ohm resistors, tantalums and ceramic caps sited very close to the power pins.
The board has been designed to facilitate experimenting with 8 samples. There are risers on the board (JP10, JP7, JP5 etc) that take power and inputs/outputs to another satellite board that the user can make that hold filters/amps/ground balance/discrimination etc.
A predecessor to the current schematic was lashed up and worked well with very low noise. Many improvements/fixes have been made to this current version but have yet to be tested.
After the board is made and tested/finalised I will post the extensively commented software to drive it.
regards
bugwhiskers

[Alexismex]

HELLO BW,
Good work , I enjoy you take the IRF740 , to me it is the best , I experiment a lot with all kind of Mosfet and exotic Mosfet and because I live in a lucky place(in Mexico) near the big Papa USA and I have a cool access to Mouser , Newark, Digikey etc... to check components...
With the irf740 , I really don't know why? but the S/N is better ...etc...
Take your time with good experimentation , it is better
Have a good time and saludos from Mexico
Alexis.

[BillA]

"if it was easy the top of the range detectors wouldn't cost so much"
lol
ah BW, those are not the words many (most ?) will want to hear
the goal is to knock out all iron with a $550 detector
as an engr I am quite comfortable with high performance = high price,
not a deterrant at all if the performance is there

do keep at it, your stds seem appropriate
BR, BillA

[bugwhiskers]

Hi Ray-NM,

I lashed up the circuit posted earlier and the low frequency noise (0-10Hz) was horrendous. This necessitated a major rethink and a chip recommended by Dave Emery, the AD797. It's low frequency noise is very low.
The other changes are the MOSFET is now an IRF740 with a pair of transistors doing the voltage translation so the mosfet driver can be driven by the micro. The noise is now much lower and I am working on making the pre-amp a multistage filter otherwise the sample caps will have lots of EMI noise on them (prevention is better than cure).
It's a long slow process but then again if it was easy the top of the range detectors wouldn't cost so much.

regards
bugwhiskers

[Ray-NM]

bugwhiskers
You have been very quiet lately.
did it work so well that now you are out finding gold and coins?
RayNM

[bugwhiskers]

Attached is a colour .PDF of the PCB to make it a little clearer.
Since the earlier post I have added circuitry to condition the battery voltage for reading by a spare AtoD and also added more de-coupling caps.
Added a protection diode for the main AtoD in case it's input goes negative, the back EMF catcher zener polarity error has been fixed also.
There are about 17 jumper links (reddish lines) which is unavoidable in such a compact single sided board.
Added some test points for attaching the CRO probe as well.

regards
bugwhiskers

[bugwhiskers]

Getting close.

Attached is the current schematic and pcb .PDF's.
By using a "P" channel MOSFET the back EMF can be used to create the -5V required by the OP amps.The components to the right of IC2 (79L05) are used to achieve this saving a lot of software overhead and also reducing noise.

The next step is making and populating the board and finalising the software.

regards
bugwhiskers

[bugwhiskers]

Hi Richard,

Thanks for your interest in the project. The adjustment pot is now gone. It's purpose was to create a null with no target present, it can also be achieved in software by micro adjustment of the position of the second sample.

Later today I will be posting the revised schematic which is the whole circuit.

The new circuit uses a rotary encoder (digital pot) for making adjustments like setting the null for various coils.


regards
bugwhiskers

[rWAVE]

Greetings bugwhiskers,

I am really enjoying the development process you are going through (hope you are too) and look forward to new discoveries unearthed by your efforts. As inspiration is NOT gained from some recent, Unregistered posts, the following "urban legend" seems more appropriate:

One story goes that "Thomas Edison failed more than 1,000 times when trying to create the light bulb". (The story is often told as 5,000 or 10,000 times, depending on the version). When asked about it, Edison allegedly said, "I have not failed 1,000 times. I have successfully discovered 1,000 ways to NOT make a light bulb".

As my only MD experience is with an ACE 250, I am not familiar with the setup and calibration of a PI detector, prior to "swinging". Based on your design, could you briefly mention the process needed for your design? For example, when you arrive at your hunt area, you turn the detector on and then . . . ?

Your posting of the discriminator board schematic on 07/03/07, you state that R22 is used for a no-target zero adjust. Is this an adjustment required regularly, thus needing front panel control or is it part of a final calibration once the complete unit has been built? Since this is a uC-based project, it seems that after proof-of-concept prototyping is done, R22 could be replaced with a uC-controlled digital pot for auto-zeroing.

I wish you continued success on this project.

Regards.
Richard