Replies to Minipulse Plus

[Polymer]

To remove the SAT permanently and use non-motion, you can swap C15 (470nF) for a 100k resistor, and R29 (1M) for a 470nF cap. (See original MiniPulse schematic.)

Or - you can probably simply add a switch in series with R29 to toggle between the two modes. I haven't checked this as a solution, so you'll have to try it.

Hi George,

I tried this with a double toggle switch. The static mode does not work, no audible signals at all.
When switching back over to sat-mode the MPP works just fine again.
What might be going on here?

Here's my "translation" of the schematic for that into a wiring diagram. Maybe something is wrong!?



I also tried an on/off switch in series with the 1M resistor like you mention.
It works in a quasi static mode with constant reduction of detecting distance.
Usable for around 15-30 seconds, too short for my pinpointing adventures.
That is why I would like the first mentioned possibility to work.

Any ideas?

How do I delete the attached picture below? "Attached Images" ...

[Xtrem]

Not so easy, offset problem in the next stage, (061) "zero" button is necessary

[Polymer]

Not so easy, offset problem in the next stage, (061) "zero" button is necessary



[ATTACH]46466[/ATTACH]

Thanks Xtrem. I do not want to install an extra pushbutton for this ... will be difficult.

I might try toggling to a larger resistor when going in "static" mode. eg. 5-10MOhm.

Or I might try what I believe Carl wrote "... important to have low input bias current ... " the opamp LMC6082AI has only 4pA, so might work for this.
But am just getting into reading datasheets so it would be great to know if someone has already tried it or is this nonsense?

I am not looking for constant static mode, but one that works at least for a few minutes without losing much depth.

[Davor]

061 may do it for SAT, but 061 in a previous stage is the one feeding the offset and 1/f noise. Picking an opamp with a low 1/f corner frequency for the circuits prior to SAT will improve things a lot. Bipolar opamps tend to have a significantly lower 1/f corner.

[Qiaozhi]

I also tried an on/off switch in series with the 1M resistor like you mention.
It works in a quasi static mode with constant reduction of detecting distance.
Usable for around 15-30 seconds, too short for my pinpointing adventures.
That is why I would like the first mentioned possibility to work.

I think your best option would be to use the same quasi-static mode as Crossbow.
This circuit uses a 470nF capacitor (same as C9 in the MPP REV-E schematic) with 3 resistors in series going to 0V (in place of R6 1M).
The 3 resistors are 2M2 (fixed), 1M (Auto-track external pot), and 100k (fixed). There is also a switch across the 2M2 resistor to change between Dynamic and Static modes.

Or you could try option 2 as per the original Minipulse circuit, which can be seen here -> https://www.geotech1.com/forums/show...066#post183066
Note that the feedback resistor of U4a is 4k7, so the gain of this stage in the original circuit is much lower.

[Polymer]

061 may do it for SAT, but 061 in a previous stage is the one feeding the offset and 1/f noise. Picking an opamp with a low 1/f corner frequency for the circuits prior to SAT will improve things a lot. Bipolar opamps tend to have a significantly lower 1/f corner.

Thanks Davor. I don't really understand what you are saying in practical terms. Isn't it the 062/072 op-amp that is feeding the integrator?

Is right to understand it like this: the 1/f noise "fills" the integrator faster than say the leakage current, hence is more important to get rid of?

[Polymer]

I think your best option would be to use the same quasi-static mode as Crossbow.
This circuit uses a 470nF capacitor (same as C9 in the MPP REV-E schematic) with 3 resistors in series going to 0V (in place of R6 1M).
The 3 resistors are 2M2 (fixed), 1M (Auto-track external pot), and 100k (fixed). There is also a switch across the 2M2 resistor to change between Dynamic and Static modes.

Or you could try option 2 as per the original Minipulse circuit, which can be seen here -> https://www.geotech1.com/forums/show...066#post183066
Note that the feedback resistor of U4a is 4k7, so the gain of this stage in the original circuit is much lower.

Thanks Qiaozhi, I have been playing with the Crossbow version of the "static" mode you mentioned. It works quite well ...

Am having a closer look at the original MPP circuit now. Is this the part that you mean with the swapped R 100k and C 47nF as compared to the MPP E 470nF & 1M?

[Qiaozhi]

Thanks Qiaozhi, I have been playing with the Crossbow version of the "static" mode you mentioned. It works quite well ...

Am having a closer look at the original MPP circuit now. Is this the part that you mean with the swapped R 100k and C 47nF as compared to the MPP E 470nF & 1M?

[ATTACH]46571[/ATTACH]

Yes, that's correct. The original MiniPulse circuit does not provide a self-adjusting threshold, as the 100k resistor and 47nf capacitor form a low-pass filter, whereas the MPP uses a 1M resistor and 470nF capacitor as a high-pass. If the Crossbow's quasi-static mode works for you, then that's probably the best option. Reverting back to the original MiniPulse configuration (with the reduced gain) with not only lose depth, but will also require constant manual threshold adjustment.

[Albdelafar]

Hi
I have a question about settings on MPP rev-E.
Is it normal that my speaker ring when my Audio (threshold) is set to 0V ?
I tried to disconect the coil to avoid interference but is stil ringing i tried to replace the 8ohm speaker by a buzzer and it's ringing too.
If i whant to stop the ring i need to down the treshold very low.


I suppose this is not normal, can someone confirm ?
Best
Alban

[Albdelafar]

can anyone confirm that this situation is abnormal ?! The detector works but because of these audio threshold settings my detection depth is really poor.

[Qiaozhi]

can anyone confirm that this situation is abnormal ?! The detector works but because of these audio threshold settings my detection depth is really poor.

This is not normal.
I suggest going through all the checks at each step of the build process to identify the fault.

[Albdelafar]

Thank you very much for the reply !! It is my second board of minipulse rev_e and I have the same problem on both but higer on this one who is the second that i build to do experiments on diff?rent pre amps. All steps are good until step 8 or I find -4 and +4v and variation when a taget is near the coil. So I suppose that my issue is on the audio stage am I right ?
Thank for the support !!

[Albdelafar]

I took measurement on my oscope with audio treshold low enought to cancel buzzer to ring. The second was took when it start to ring continously. All this experiment was made without coil CH1 on TP1 and CH2 on speaker positive.

[Qiaozhi]

Thank you very much for the reply !! It is my second board of minipulse rev_e and I have the same problem on both but higer on this one who is the second that i build to do experiments on diff?rent pre amps. All steps are good until step 8 or I find -4 and +4v and variation when a taget is near the coil. So I suppose that my issue is on the audio stage am I right ?
Thank for the support !!

I took measurement on my oscope with audio treshold low enought to cancel buzzer to ring. The second was took when it start to ring continously. All this experiment was made without coil CH1 on TP1 and CH2 on speaker positive.
[ATTACH]54720[/ATTACH]
[ATTACH]54721[/ATTACH]

If the DC voltage at TP1 can be varied by the Threshold pot between +/- 4V, then the problem does appear to be in the audio stage.

[Albdelafar]

Yes it is. There is no problem on step 8 treshold pot voltage is between -3.9 and +4,1.
So do you have an Idea on what could be wrong on audio stage ? Capacitor I used are polypropyl?ne and resistor are 1% 0,5w all tested before soldering to pcb.