New MPP test board project - TX Freq/PPS

[geoscash1]

I have started on a new mpp build mostly because it is an awesome project and works so well but this one will be a board for testing my coils and for other different tests and stuff.

After my 2nd build using KRinAZ's mods, I was never able to achieve a 5000 pps setting (although it performs excellent and beyond expectation) so want to know what else can be changed to be able to make R8 adjustable out to 10,000 pps. In my previous build I used a 50k trimmer in the R8 position but it would only allow me an adjustment to 3000 pps turned to its end.

This new one, with a 50k trimmer has limited me to 1900 so it is confusing as to why the differences between boards, maybe part tolerances and caps, not real sure I understand completely but hope to figure it out so I can learn more of how these things really operate.

After looking at the schematics for the TX oscillator, will changing R4 from the 10k to 15k and C6 from 47N to say 100N give the range needed? R7 is using a 5k for adjusting the TX main pulse and gives the needed range for that.

Any ideas or suggestions or what can be changed on this would be appreciated.

And just so I know I'm setting things properly and reading my scope correctly, setting the time division to 1ms and adjusting to say 5000 pps on my board should give (show) a dot/trace every 5 divisions, correct? like example pic below, set at 1ms/3000 pps with a dot/trace every 3 divisions. Please correct me if I am wrong.



Anyway, thanks for all the help, currently and previously. the memory isn't workin so well and meds seem to be screwing with my thinking so it is much appreciated when ever help is offered.

Geo

[Qiaozhi]

And just so I know I'm setting things properly and reading my scope correctly, setting the time division to 1ms and adjusting to say 5000 pps on my board should give (show) a dot/trace every 5 divisions, correct? like example pic below, set at 1ms/3000 pps with a dot/trace every 3 divisions. Please correct me if I am wrong.

I think you're getting yourself a little bit confused with the scope settings.

The pulse period is the reciprocal of the frequency (or pulses per second). Hence 1000pps with have a period of 1ms, 5000pps will have a period of 200us, and 10,000pps will have a period of 100us.

In your attached image you have a pulse occurring every 3ms. Therefore, the pulse rate is the reciprocal of 3ms. i.e. 1/3ms = 333.33pps.
In other words, to increase the number of pulses per second (pps), you need to decrease the pulse period.

[mschmahl]

You should change R7. Try different values and see how wide you can adjust the pps with R8 as trimmer.

[geoscash1]

I think you're getting yourself a little bit confused with the scope settings.

The pulse period is the reciprocal of the frequency (or pulses per second). Hence 1000pps with have a period of 1ms, 5000pps will have a period of 200us, and 10,000pps will have a period of 100us.

In your attached image you have a pulse occurring every 3ms. Therefore, the pulse rate is the reciprocal of 3ms. i.e. 1/3ms = 333.33pps.
In other words, to increase the number of pulses per second (pps), you need to decrease the pulse period.

Holy S***, you know I knew something didnt feel right or look right but could not remember for the life of me how to set that.
Thank you for the clarification, now I remember and was thinking exactly opposite of the right way.

Now I have it set correctly, which using 50us as the time table I have form every 5 divisions, also had forgotten I have a meter that measures Hz/Khz so double checked it as correct. Will add a pic in a bit..

This test board will be adjustable for testing purposes, right now it is set to 40us for the TX main pulse with the freq adjustable from 1000/10,000 pps set at 5000pps or 5.01Khz.

What exactly does R4/10k do or what is its actual purpose?

You should change R7. Try different values and see how wide you can adjust the pps with R8 as trimmer.

Hi Michael,

Yep, I did change it to a trimmer for adjustability, it is good where it is and gives me plenty of adustment for the TX main pulse. Now after seeing I was reading the scope opposite of how it should have been, I have that corrected as well and R8 gives me good adjustment as well for testing different freq/TX pulse settings.

Thanks for the clarification guys, much appreciated..

[Qiaozhi]

What exactly does R4/10k do or what is its actual purpose?

Actually, that's a very good question.

This connection between U4 and U6 is a legacy from the original Minipulse design. Looking at this part of the circuit more closely, I notice that U4 has VCC=0v and GND=-VB, whereas U6 has VCC=0V and GND=-5V. Which means that U4 is over-driving U6. With R4 (10k) present, this is no doubt to prevent U6 letting out the magic smoke; and it probably gets away with it due to protection diodes at the input of U6. At least I assume there must be some protection diodes there, as the MC14538BCP datasheet doesn't show the internal schematic.

Can you post a scope image of the signal on U6 pin 5?
I cannot check this myself, as I'm currently away from home.

[geoscash1]

probe pin5 U6

Actually, that's a very good question.
Can you post a scope image of the signal on U6 pin 5?
I cannot check this myself, as I'm currently away from home.

Sure can, although during my test with the freq I installed a 15k in place of the 10k at R4 so the scope shot may not show what you are looking for.

the scope is currently set to 2v div/20us div. let me know if I should change the scope settings to help with what you want to look for.



Looks to be the same as the tx main pulse...

[Qiaozhi]

Sure can, although during my test with the freq I installed a 15k in place of the 10k at R4 so the scope shot may not show what you are looking for.

the scope is currently set to 2v div/20us div. let me know if I should change the scope settings to help with what you want to look for.

[ATTACH]37557[/ATTACH]
[ATTACH]37558[/ATTACH]
Looks to be the same as the tx main pulse...

Thanks.
However, I should have realised that you were working on a new build. I really wanted to see the signal with U6 installed in order to check if there is any loading. From a quick LTSpice simulation (with a simple bipolar transistor as the input and no protection diodes) the signal is limited to 5V amplitude. But it definitely needs the series resistor to work correctly. It's also ok with 15k.

[geoscash1]

Thanks.
However, I should have realised that you were working on a new build. I really wanted to see the signal with U6 installed in order to check if there is any loading. From a quick LTSpice simulation (with a simple bipolar transistor as the input and no protection diodes) the signal is limited to 5V amplitude. But it definitely needs the series resistor to work correctly. It's also ok with 15k.

OK, cool. we will come back to this point as I progress thru the steps and retry the scope shot just to see on board.

[geoscash1]

Finished steps 1 thru 7 now and everything is right on que and working perfectly. Thought I'd have it finished today but am missing some parts from my parts bins that I must have forgot to replace so will be waiting for a few days to button it up.

will post some pics after completion and add more scope shots for others to use as reference as well for their projects.

[geoscash1]

Finished - Modified Minipulse Plus #3

Well found some parts to complete the board so she is done and working like a champ without fine tuning yet. Did reset the TX main pulse to 30us with freq set to 3000 pps for starting point to initial testing.

First tests without tuning give:
US Nickel = 9 inches
14k Ring = 8 inches
1.2 gram nugget - 4 inches
.45 gram nugget - 1 inch

Will fine tune with test coils of 5x9 Razorback, coil spec is 312uH @ 2ohms shielded, second test coil is 8x10 DIY 3DSS, specs are 309uH @ 2.3ohms, 600v teflon 7/32 silver plated 24awg unshielded.

Some pics:

[geoscash1]

Scope shots

Some un-tuned scope shots, they me be out of order or not in build sequence but they are labeled and show what they should:








This board has R7/R8/R10/R11 using trimmers along with sensitivity pot added at R33 and pot added for SAT at R29. Also added dip single socket adapters for R1 & R3 for fine tuning purposes.

Will add more pics and results after fine tuning with multiple coils. Will be wrapping a new 3dss 7-1/2in round with 175/46 2 layer litz as well, so will be using 3 different coils to find the best of the bunch. Then will build it out after the best combination is finalized.

Gotta mention again, a great project with excellent documentation not only being a good learning platform or tool but an awesome PI detector to boot.... Thanks for making this a great and fun project for some of us, the electronically challenged...

[geoscash1]

First tests before fine tuning. Not bad for right off the bench. So far set the TX pulse to 30US and Freq to 3000pps with sample delay set to 6US. Tested with 8x10 3DSS 24awg 600v teflon coated 7/32 silver plated wire wound coil.

[WM6]

Good work, Geo, wish you a lot nice findings with.

[geoscash1]

Good work, Geo, wish you a lot nice findings with.

Thanks WM6, hopin to find some good stuff too.....

Second coil test with a 5x9 Razorback shielded coil, its specs are 312uH @ 2ohms (couldnt remember in the video...lol). Still haven't tuned other than setting initial TX main pulse and freq and adjusting the panel delay pot so its pretty good right off the bench. Averaging around 10 to 11 inches on a US nickel and right at 10 inches for a 14k ring, 5 inches for the 1.2 gram test nugget. Detection on my little .45 gram nugget is a bit low (right at 2 inches first detection) so will probably fine tune the coil series resistor R3 and tune the damping resistor to try for better results although the results so far are pretty nice.

[geoscash1]

New coil wound and tested

Laid out and made a new 3dss coil form today. It is 7-3/4" in round and wound with 175/46 litz wire w/double layer cover, specs are 310uH @ 2.1R (31 turns total w/ 36" continuous twisted leads). Board is set to 6us delay with tx main pulse at 32us and freq @ 3.0kHz or 3000 pps.

Seemed to be pretty good for the size and close to or about the same as my 8x10 3dss and a bit better than my 5x9 razorback as far as small gold goes. Am going to make another 3dss form tomorrow, either a 9" or 10" round, haven't quite decided yet but leaning towards the 10".


Bench air tests as follows:

US nickel - 11 in.
14k gold ring - 10 in.
1.2 gram nugget - 5 in.
.45 gram nugget - 3 in.