Bipolar PI design

[eduardo1979]

Hi Eduardo, I corrected the code in two versions, you can test if this affects the depth, one version is 80us and the other is 100us. the frequency potentiometer is no problem, the signal does not drop, you can increase the frequency potentiometer all the way.

Hi Algan.
Thank you for your effort and dedication. I've tried both versions. The depth increases, but the results are worse. At 100uS, the MOSFETs get very hot, as the coil saturates, and the IRF9640 P-channel MOSFETs have a breakdown voltage of about 200V and can fry. The 80uS version works a little better, but I'm honestly sticking with the original version, as I've managed to improve a few more things on the detector, and I think 50uS is enough for now. My charge pump doesn't drop with frequency because I modified it, and now it's stable regardless of the frequency applied. The changes are shown in the schematic. I replaced the two diodes with two 10K resistors and used Darlington transistors.

[Carl-NC]

Uh then it's a misunderstanding Carl!
That's how it behaves.

First, check that both demods are working correctly: IC5a should have a positive-negative target signal and IC5b should have a negative-positive target signal. If these look OK, then check the soldering of you GB pot, especially the pin to the ground channel.

[ivconic]

...If you rotate the GB pot fully to the Target channel, all targets should give a hi-lo response. If you rotate the GB pot fully to the Ground channel, all targets should give a lo-hi response.

The way you wrote this = the way my detector works right now.
There is no need to check anything on the pcb, everything is as you wrote; that's how it works.
But that's not how it should work!
What's the benefit of always having the same behavior on all targets?
Whether it go "wee-waa-wee" at all targets or whether is it "waa-wee-waa" at all targets; it's totally useless in both cases!
Do we understand each other?
What I expected and I think the only correct way for the audio to work is for iron to go "wee-waa-wee" and all other metals "waa-wee-waa" or vice versa, however you like.
Did you watch the video from Eduardo? That's exactly how it should work.

[eduardo1979]

Hi Ivconic. I'm still getting the impression that your detector's damping resistor isn't the right value for the coil you're using. When I first made my detector, I used a 680 Ohm damping resistor, and my detector didn't discriminate under any circumstances. I chose the resistor at random, without even connecting the oscilloscope. To my surprise, the same thing happened to me as to you. The detector didn't discriminate, and it wouldn't detect small targets under any circumstances. Then I connected a multi-turn potentiometer—which you can see in one of my videos is very large and blue—and started turning it until I got the right resistance value. It was 330 Ohms. Sometimes it's better not to have the damping be critical because targets get lost during sampling. If you'd like, I'll take some photos tomorrow of how I have the signal at PIN 7 at the preamplifier output.

[Carl-NC]

Did you watch the video from Eduardo? That's exactly how it should work.

The way Eduardo's detector works is exactly the same as the way mine works, except that his 4046 audio sounds better. Mine gives a hi-lo response for low conductors and a lo-hi response for high conductors. Iron can end up on either side depending on the nature of the iron, there is no way to make all iron respond in the same way.

[ivconic]

Taking into account everything you both wrote, the only conclusion left to me is that I failed to determine the damping resistance correctly.
Or that one (or more) opamps are not good.
This is especially the case when dealing with TL072 opamps. I make a lot of small modules for musicians and I noticed that there are a lot of fake TLxxx opamps on the domestic market.
But let's leave that for later. Let's focus on determining the damping resistor.
How is the damping resistor determined when we have such an H-bridge TX?
Eduardo, I have had a damping "jig" for a long time and have already done it many times on "regular" PI detectors.
However, on this detector I do not see anything irregular in the RX signal when I adjust the damping "jig" in a wider range.
I have a two channel oscilloscope.
On the first channel I connect either TxP or TxN as a reference (trigger) and on the second channel I connect VOP or VON.
Right?
When I did the GS4; I posted this video of the procedure:

[ivconic]

The main question at this moment is; how is this procedure done on a Bipolar PI detector?

[Carl-NC]

I doubt it's the damping resistor. Even when badly damped the early-late sampling mechanics don't change. Go back to post 302. Are you getting correct polarity responses on each channel before the GB pot?

[eduardo1979]

The main question at this moment is; how is this procedure done on a Bipolar PI detector?

Hello Ivconic.
I've attached a photo of my oscilloscope screen. You'll notice that my coil is well underdamped, as it didn't have a resistance less than 330 Ohms. In reality, I needed about 270 Ohms. I prefer a slightly underdamped coil to sample all the op-amp saturation points. You need to connect the channel 1 probe of your oscilloscope to the VOP output. It's not necessary to measure with both oscilloscope channels. Pay attention to the polarity of the VTXP to R8 and VTXN to R10 connections, as shown in the original schematic.
I hope your detector problem is resolved soon.

[eduardo1979]

Examples.

[Orbit]

I doubt it's the damping resistor. Even when badly damped the early-late sampling mechanics don't change. Go back to post 302. Are you getting correct polarity responses on each channel before the GB pot?

The problem not understanding how the balance of the land Geb actually works. I also don't think it has anything to do with the damper resistor...

[SHARK_MD]

Hello friends. I am from Ukraine, please tell me how to replace the chip 74HCT4316?

[Detectorist#1]

Hi SHARK_MD,

The simplest way to replace the chip 74HCT4316 is to use CD4066 + additional voltage shifters with transistors as the schematic modified from eduardo1979.
Read this tread from beginning or see attached information.

[eduardo1979]

This is the best way.
Detectorist 1 is right.
Plus, you'll avoid the glitches that occur in the demodulator.

[ivconic]

I think the main problem on my detector is that I soldered all the resistors backwards!