Universal PI Micro

[bugwhiskers]

Hi Stefan,

Thanks for your interest in the project.

I hear what you are saying, that aproach would be very easy but I am trying to keep the parts used to readily available and cheap. Some of the forum members live in areas where even common parts are difficult to obtain.

With regards to noise, integrators as used in most PI's are affected by noise, whether it be from the power rails or induced.

The 64 sample approach has a lot of noise immunity I believe due to the fact the gate to them is only open for a very short period and because they are disconnected when not being charged or digitised which again happens very quickly. The prototype is so noisy I can't have my workshop radio on and yet the jitter on the counts is only 1 or 2 and because there is so many of them to average the noise will be well and truly cancelled.



regards
bugwhiskers

[bugwhiskers]

Following a suggestion by bbsailor the UPIM is now a Motherboard / daughter board system.

Over the next week I will be posting information and schematics for the 3 PCB's.

The Motherboard measures 3" * 3" (the reason for the small size will explained later). It holds the Micro (ATMEGA644 DIP 40 package), a programming header, a LCD screen header, MOSFET driver chip, minus battery *2 charge pump and conditioning circuitry to read the battery voltage and input push buttons.
The Motherboard has 2 bus sockets, socket 1 is for the coil drive and pre-amp circuitry and the second socket is for secondary signal conditioning, ie integrators or my switched multiple capacitor type.
The daughter boards measure 3" * 1.6" making the whole package very compact.

All micro source code to drive the display and daughter boards will be posted also.

At the moment I cannot source the micro so I am using an ATMEGA16 which has RAM limitations so the source code will change once the preferred micro is available.

The UPIM will not only make PI experimentation a lot easier but will provide an easy entry into the world of micro-control via the 3 essentials ie. input device (push buttons), micro-processor and output device (LCD graphic/text screen).

Stay tuned !

regards
bugwhiskers

[bugwhiskers]

UPIM Motherboard

The attached files show pics of the UPIM Motherboard at work, Schematic and PCB.

The schematic has the legend for the bus, what each bus line does.

There are a few components mounted under the micro. The board is single sided with just a handfull of links and not too tight to make at home like I did using Kinsten pre-coated PCB material.
If anyone wants the CAD files (Eagle light) I will post these also.

In the pic you will see a PCB in socket #1, this is the coil/pre-amp board, it will be populated tomorrow and I will post the schematic and PCB file. It is based on Carl's Dual/Single coil with 2 stage pre-amp.

Today I ordered 3 of the micros but they won't arrive until the end of April so the final programming can't happen till then.

All the chips (10 * 16 pin PDIP's) required to take 64 samples won't fit on the PCB. I have laid out a SMD version but that is a while off being made. The compromise is to have a 24 sample version, using 3 of the 8 chips required to take 64 samples. The artwork is ready and I just have to make the PCB and populate it. All going well there will be a 24 sample PI up and running by the weekend, I for one can't wait to see how the samples in certain areas of the decay curve change with different metal types.

Don't be too shy to ask questions

regards
bugwhiskers

[bugwhiskers]

UPIM Coil/Pre-Amp Board

The attached files are the schematic, pcb and component overlay for the UPIM Coil/Pre-Amp daughter board.

The circuit is based on one of Carl's dual coil designs. The changes are the MOSFET's, they are now "P" channel and the second stage amp is an AD8055 to provide the grunt to charge/discharge the sampling caps quickly.

Interfacing a micro to a PI is far far easier with "P" channel devices because the back EMF and decay signal are centred about battery ground. The only down side to this is generally P channel devices don't have as high voltage ratings and the RDS is not quite as good as N channel devices.

The MOSFET included in this design was suggested by bbsailor, it has a 250V rating. The prototype has an IRF9640 (200 Volt), 0.5 ohm RDS and works fine.

Tomorrow I hope to have time to make and populate the 24 sample board and the small PCB that holds the push buttons.

regards
bugwhiskers

[bugwhiskers]

UPIM 24 SAMPLER BOARD

The attached files are the schematic and pcb for the subject board. 24 samples isn't really enough but will have to do for the moment. If the results are encouraging in terms of discrimination it will provide the impetus for me to bite the bullet and get a SMD board professionly made. If the board were populated both sides there would be room for the chips for 128 samples. At 100 nS per sample (the fastest possible with a 20MHz CPU clock and 2 machine cycles per sample) digitizing the first 12.8 uS of the decay curve. Potential exists to perhaps take 64 samples of the early part of the decay and the other 64 evenly spaced further apart to take in the final stages of decay.

Over the next week I will be programming the beast.

regards
bugwhiskers

[bugwhiskers]

UPIM Phote

The attached pic show the UPIM with both boards in place.

regards

bugwhiskers

[Ray-NM]

Bugwhishers
If everything works as you think then that will be one awsome Detector. been following this thread for awhile. Get it built and let us know how well it works.
RayNM

[bugwhiskers]

Hi Ray-NM,

Thanks again for your interest in the project.
All is coming along well despite shortage of time due to paying work commitments. The routine that gets the current sample of each of the caps and compares it to the average for that sample is in place and working as expected. This routine makes the switched cap PI a motion detector, the faster you sweep over the target the greater the difference between successive samples.

The small PCB for 3 push-buttons is finished and today I will be writing the routine to read the buttons. Each button press generates a unique voltage that is read by one of the micro's AtoD inputs. Originally there was going to be 5 buttons but 3 take up less space and cost and is just as functional. The outer buttons = left or right(navigation around screen) or up or down(increment/decrement values) and the middle button = accept.

regards
bugwhiskers

[bugwhiskers]

Good news

I have worked out a way to get 64 samples and still use only DIP chips (not SMD). If you look at the previous photo you will see room above the CPU. I intend to make a board that "piggy backs" onto the sampler board and sits over the CPU. Six 74HC4051's and the associated caps just fit ! The front board will easily house the remaining two 74HC4051's, 74HC4053 & 74HC138. There will be room also for a dual OP amp to buffer the signals coming in and going out.

regards
bugwhiskers

[bugwhiskers]

Revised Coil - Preamp board

Attached are a revised schematic and pcb for the coil - preamp board.
The most important change is removal of the components for the second coil thus making room for a "poor man's" DAC. This DAC is driven serially by the micro and makes threshold adjust possible via software and the pushbuttons. It will be possible also for automatic adjustment via the AtoD reading a very late sample and the micro adjusting the threshold to ground potential. With the component values shown, swinging the DAC from 0 to 5 volts produces a +0.5 to -0.5 voltage change at the output of the last preamp.

It is very early days but with the 24 bit sampler board hooked up it was possible to see on the LCD screen graph a very distinct difference between fast targets (al foil, small coin, small nugget) and 2 quite different samples of ironstone (hot rock). It would seem that ironstone has a very distinct signature.


regards

bugwhiskers

[bugwhiskers]

Coil-Preamp pic

The attached pic is of the revised coil-preamp board.

regards

bugwhiskers

[bugwhiskers]

More pics

The attached pics are the reasoning behind the shape and size of the UPIM circuit boards. The plastic (polycarbonate methinx) are readily available, relatively cheap and light weight, they are also very tough and with the right sealing, rainproof too. The handle and fittings are also readily available and cheap.
The detector shown is an early design. The batteries can just be seen mounted on the underside of the box. The weight of the whole detector inc batteries and coil was just over 1kg, no steroid treatment required to allow a user to swing it all day.

regards
bugwhiskers

[gef12]

coil preamp

Hi BW
Looking at your coil preamp schematic I noticed 2 op amps .. seem to be running in parallel the AD8055 and NE5534. Is this so , is some sought of dual sampling going on here and then maybe averaging it out at a later stage. ??
Just curious. The finish product is looking ok.
gef

[bugwhiskers]

Hi gef12,

Thanks for your interest in the project.

The first preamp, the NE5534 has a gain of 1000 and provides most of the amplification. The AD8055's claim to fame is it is very fast and can drive low impedance devices like a cap that needs fast charging/discharging. The combined capacitance of the 64 caps is 64 * 0.1 uF or 6.4uF worst case which is quite large and must be done in 6.4uS at the fastest sample rate. Essentially the 2nd preamp is just a buffer so as to not load the first stage. Now that the offset adjustment is going to the 2nd preamp I will be able to have some gain on it also. Because the AtoD converter must be fed a signal that ranges between GND and +5 volts the offset circuitry must ensure the signal doesn't go too far below GND.

Because this is a project that is still on the fly there will no doubt be some changes along the way.

Hoping this explains your question.

regards
bugwhiskers

[Unregistered]

hello how soon is this to be in kit form?
any idea of cost yet?