chance pi build

[baum7154]

What is circuit resonance if you lay your scope probe near the fet (not connected) with R damping and the input resistor disconnected?

I don't know, will have to go back and look at my notes. In ckt with damping and Rin connected it is 988Khz. Disconnecting those two resistors will be a pain as this is a fully integrated detector and requires disassembly. Might be able to activate my spare Chance board and run the test.

Dan

[baum7154]

You can simply trick it without software change using some “overclocking”. Use XTAL of somewhat higher frequency, 10-20% or even more, this will proportionally shift audio, filter constants and pulse width as well, not only sample time, but works just fine, I tried.

I see the max clock speed on this uC is 16 mhz so 13.5mhz is not out of the question. The only issue is can I live with the other side effects.

[green]

I don't know, will have to go back and look at my notes. In ckt with damping and Rin connected it is 988Khz. Disconnecting those two resistors will be a pain as this is a fully integrated detector and requires disassembly. Might be able to activate my spare Chance board and run the test.

Dan

Thanks. If the two resistors are acting in parallel I would expect the circuit resonance to be lower than what I thought you said you were getting. Maybe an error in my thinking how the circuit works. If it's difficult forget doing it, I was just curious.

[baum7154]

Attached is the schematic of the 2 stage amp I promised. Photos to follow of the original amp out to the A/D vs the new amp out to the A/D. Also photos of the circuit board and the assembled amp to follow.



This amp was designed as a 'plug and play' so that no significant changes need to be made to the CHANCE PI board. That said it plugs directly into the U5 IC socket assigned to the OP37 amp in the original design. The original R25 470K resistor and 1n4148 diode across it must be removed. Two 470 pf caps in series need to be installed between R21 (1K) and ground in order to create the 4th Low Pass filter (677 kHz)in the chain. This amp was designed on a 1.050" X 1.050" PC board with a .260" X .5" notch machined out of it in order to clear a 10uf cap on the original board. An additional ground was soldered from the CHANCE PI ground plane and the new 2 stage amp board ground plane. All other connections go through the contacts of the U5 IC socket. The schematic shows two offset adjustment pots, one for each amp, but I'm not sure they are really required especially if you have the OP37 factory trimmed for 10uv.

Regards,

Dan

[Tepco]

Few comments on this: are 9 and 38pF capacitors in feedback loop necessary? Try to omit 1K\470pF between two amps, keep it just from last amp to ADC. Individual offset adjustment for each amp is probably not needed, just on first one. Effects of 2 diodes on first amp feedback and 14 on second one (omit?) I don’t have 2x OP37 at hand to reconstruct this, just watching schematics. There are 3 explicit low pass roll-off constants plus original one, and both amps are prevented from saturation. Is this added later experimentally (and how performance is affected) or built in initially?


(I wrote this before last post edited, so filter stages are obviously intentionally added)

[baum7154]

Yeah I went crazy with filtering after seeing the noise without any filtering and they are so easy to implement that I did them all. I installed gold sockets for both of the feedback loops on the amps so it is easy to change gains, filtering, and diode limiting of voltage to keep the amps out of saturation. The 14 diodes in the second stage keep the amp out of saturation (about 10 volts)on large signals without harming the small signals...I think. The 2 on the first stage limit the positive going to 1.4 v. Both negative going diodes are as the original design to limit - to .7v. I'm not sure what you mean by the original filter. The values of these filters were determined experimentally in order to get best small gold sensitivity.

Thanks,

Dan

[Tepco]

Actually not “original” but someone added 220-470pF capacitor after original R21 resistor for same purpose in one incarnation (not on original schematic) I assumed this was built in already. Will be interesting to hear how these added stages affect performance. I tried before different 2stage amps, but not this way, typically 5532 or 5534+LM318 (on basis of availability), only built in ordinary analog integrating detector, not Chance. I give up any filtering, noise on scope looks nasty but not affecting analog machine, I limited only first 5534amp (LM318 is fast enough but more noisy for first stage). This is nice fine tuned upgrade for Chance, very interesting to see performance improvement. Excellent work. In combination with good coil design this can resurrect entire project

[baum7154]

Actually not “original” but someone added 220-470pF capacitor after original R21 resistor for same purpose in one incarnation (not on original schematic) I assumed this was built in already. Will be interesting to hear how these added stages affect performance. I tried before different 2stage amps, but not this way, typically 5532 or 5534+LM318 (on basis of availability), only built in ordinary analog integrating detector, not Chance. I give up any filtering, noise on scope looks nasty but not affecting analog machine, I limited only first 5534amp (LM318 is fast enough but more noisy for first stage). This is nice fine tuned upgrade for Chance, very interesting to see performance improvement. Excellent work. In combination with good coil design this can resurrect entire project

Thanks for the encouraging words TEPCO I value your opinion and collaboration on this detector along with all the others who contribute to this effort to make CHANCE PI a worthy build. The reason I chose to follow this through is it has great potential to be a great detector with sophisticated features in spite of the lack of source code. Maybe one day SIlver Dollar will remedy that short coming. I'd just be happy to be able to step down to 5us delay someday instead of the present 8us. Your suggestion to increase clock speed may be the answer to that.

Thanks again,

Dan

[Silver Dollar]

I still have plans to make a "Second Chance". It looks like I will have to rewrite the software completely.
I have collected the data for the peripherals and disassembled some of the code but need to get mine
working to see how the menu's should show.

I ran into a bit of a stumbling block with the fuses.
This is what they should look like;



And this is what my software shows me;



I figured out all of them except is BODLEVEL 2.7V? And how do I set BOOTSZ1 and BOOTSZ0?

[baum7154]

I still have plans to make a "Second Chance". It looks like I will have to rewrite the software completely.
I have collected the data for the peripherals and disassembled some of the code but need to get mine
working to see how the menu's should show.

I ran into a bit of a stumbling block with the fuses.
This is what they should look like;

[ATTACH]29729[/ATTACH]

And this is what my software shows me;

[ATTACH]29730[/ATTACH]

I figured out all of them except is BODLEVEL 2.7V? And how do I set BOOTSZ1 and BOOTSZ0?

I'm sorry I am no help on this but keep up the good work! I'm sure there is help on this forum to answer some of your questions.

[baum7154]

Photos of the 2 Stage Amp PC board in various stages and installed in CHANCE

The following are the build pictures of the 2 stage amp board:



The last photo is the board installed in the detector housing on the 'Mother Board".


Dan

[baum7154]

Here is the photo of the BEFORE original design CHANCE Receive amp out to the A/D. Note the 16us decay.



The photo below is the 2 Stage amp out to the A/D. Sorry for the trace bloom and the trace ghost on the left.



Dan

[baum7154]

I mentioned in an earlier post that I'd take the modified CHANCE PI out to my prospect area this past Monday and give some field reports but I got side tracked by another 'build' for my son. Will probably be able to take it out this coming Saturday. Will report findings then.

Dan

[Tinkerer]

Here is the photo of the BEFORE original design CHANCE Receive amp out to the A/D. Note the 16us decay.

[ATTACH]29736[/ATTACH]

The photo below is the 2 Stage amp out to the A/D. Sorry for the trace bloom and the trace ghost on the left.

[ATTACH]29737[/ATTACH]

Dan

Check if the FE discrimination still works.

[baum7154]

Check if the FE discrimination still works.

Discrimination of Iron was always a major part of my testing in the process of this amp development. It does work as well as it ever did. An Iron bottle cap, one of the worst targets for falsing, is initially detected about 2" to 3 " farther out than discrimination 'kicks in' but as the signal strength increases discrimination defines it as 'Iron' in both a high pitch audio and on the VDI . Detection of this bottle cap begins at about 12 inches with my 8" coil in air. VDI indication at levels below 6 units of signal strength are unstable and jump from left to right gradually settling on the right end as signal increases. However the instability of the VDI itself at low signal levels is an indication that the target is likely to be iron. Along with the high pitch components in the audio output when over iron, these are the clues that the operator can use to become more effective in discriminating a target. Gold is always presented as a steady low pitch tone solidly on the left of the VDI display. As always the operator can and should learn to use a detector to best advantage.

Dan