Null Detector based on Conrad Hoffman design check

[hozone]

Hello,

I would like to build a null detector based on the Conrad Hoffman design.
I've read the Conrad Hoffman article, find it here: http://conradhoffman.com/mini_metro_lab.html
And this interesting post here: https://www.eevblog.com/forum/projects/conrad-hoffman-null-detector/

The power stage it's built by using 4 AAA battery (6V), then a 5V LDO, then an op-amp as a rail splitter, so I'm going to use +-2.5V for the main circuit.
The op-amp I've selected is an AD8628, based on this @Kleinstein comment https://www.eevblog.com/forum/projects/conrad-hoffman-null-detector/msg3026766/#msg3026766
The only accurate resistor will be the 0.1% on the input divider.
Before going to test this circuit, do you think there is something wrong here?

Thanks

[Kleinstein]

The amplifier needs accurate resistors for both the input divider and the resistors setting the gain.
A Nullmeter is normally not about high accuracy in the scale factor, but good accuracy of the zero. So the resistor accuracy may not be that critical.

One may get away with fewer steps in the input divider (e.g. only 1:100 and maybe 1:1000). E.g. divide by 10 could be realized as divide by 100 and x 10.

I would make the input divider higher impedance, like 1 M if not 10 M.

The OP-amp for the rail splitter is not critical and should be a non AZ type.

[hozone]

Thank you!

I've just realized I've not attached the schematic. I've now edit my post, you can now find the schematic.

[iMo]

I would put a small resistor at the U2's and U3's output (like 33ohm or something like that).
Why such big 470nF in the feedback?
BTW., I had similar null meter in past with only one range (x1000 I think) and it covered all my use cases at that time..

[hozone]

Thanks @iMo
I get the reason after the U3, but even after the U2?

Another question is about the resistor accuracy, I've put 0.1% but maybe I can use less accuracy, cause on negative input of the op amp there are 1%, and they are trimmed by RV1, right?

I also would like to add an analog millivolt meter +-100mV on front on my case, switchable with banana plug.

[Kleinstein]

RV1 trims the offset, not the gain (only a very minor (e.g. 5 ppm) change in the gain).
With an analog meter movement an 1% or 0.5% accuracy could be good enough for the resistors. The analog movements are usually not that stable and a bit hard to read. An analog read out may want a trimmer for the gain. One could in this case consider an extra switch and series resistor for the movement to also get extra ranges as  x 3 or so - this could be even external to the PCB and still an option for later if wanted.
Today resistors with 0.1% are not longer that expensive and they may make some sense when using digital read-out.

A resistor (e.g. 50-100 ohm) at the output of U2 is needed, as most OP-amps do not like such a large capacitive load as the 2 x 100 nF shown.

The relatively large 470 nF capacitor at the feedback makes some sense to get some filtering action for the ranges with the highest gain. With a digital read out I would consider less capacitance there and also at the input.

The 1117 type LDO is one a kind of half way low drop type. There may be better ones to use more of the battery capacity (e.g. MCP1703 , LP2950,...).

[hozone]

Thanks!

I'm sorry but I don't get the modification I've to do for the analog meter, I've never use one analog. The series resistor and switch you are telling me are on the feedback network?

So:
- add 50ohm out of U2 and U3
- replace the LDO with something like TPMCP1703T-5002E

For digital meter usage:
- reduce C8, C9, C10 to 100n
- possibly use 0.1% for R5,R6,R7,R8 and R12,R11,R10. Just those ones?

For analog usage:
- all resistors can be 1%

[Kleinstein]

For the precision resistors one would have to include R9 too.

With the relatively low resistance of the divider the need for high accuracy is not that large. The 200 K resistor chain is quite some loading and this already limits accuracy a little.

The optional x3 range extension for an anlog meter would be between the shown circuit and the meter movement. A series resistor can reduce the sensitivity and this way extend the range by something like a factor of 3 to get a little around reading at only a small fraction of the range. Depending on the meter movement  (many come with a current rating, like 100 µA or 1 mA) one may need a resistor for the normal range too.  With a PCB this could be separate resistors for the output to the digital out and analog meter.

[Conrad Hoffman]

D3 provides protection one way but I think you need a second one for the other polarity?

[Kleinstein]

As shown in the plan D3 would only protect in one direct. However the normal version of the BAV199 is not common cathode, but 2 diodes in series.  The problem seems to be having the wrong symbol.

[hozone]

Thank you for clarifications!

I've modify the schematic:
- R5,R6,R7,R8,R9,R10,R11,R12 are 0.1%
- added 100ohm out of U2 and U3 (R16,R17)
- changed the regulator to the MCP1703 5V fixed version
- added a low battery indicator at ~5.3V almost
- updated BAV199 symbol and connections

I'll go for the digital one.

If this schematic pass your check I'll try the PCB prototype for this one

[Kleinstein]

Some reverse polarity protection at the supply may be a good idea, even of just a diode to short out a wrong polarity.
The low batt uses quite some current. With modern LEDs one could use less current and maybe use a slightly lower power version. The amplifier should not be that picky about the supply, so the limit could be quite close to 5 V.

The regulator may also want ceramic capacitor at the input side. It is at least good to have the option to add one.

[hozone]

Thanks!

- diode added

the low voltage supervisor is set to 5.3V cause the MCP1703 needs +0.3V in, so it's 5.3V

[retroware]

Have you given any thought to adding any sort of input bias current compensation? If one is using a null meter in combination with a high output impedance source such as the Fluke 720 KVD, then even the low bias current of the AD8628 can become significant - generating on the order of a microvolt or so of offset. Early meters such as the HP 419 didn't require this compensation given that they were using a pure resistive chopper.   Later meters like the K155 use a teraohm resistor to generate a small offset current. I've often worired/wondered about the drift in that part of the circuit.

[Kleinstein]

Adding a circuit for input bias compensation can absolute make sense. It is good to have at least the option - one can still decide not to populate a few reistors and the pot.
The input bias specs for the OP-amps are usually for the full common mode range, including the ranges close the the supplies, that can have higher bias. At a fixed center range and with well behaved source impedance the actual input current is often smaller than the spec limit, often even less than the typical value.
For use with a high impedance source it also makes sense to have a higher impedance for the divider - otherwise the divider would add to the noise.


For a digital read out it depends on the ADC used how much filtering capacitance makes sense.  SD or SAR (e.g. µC) versions can oversample the input relatively fast and near contineous. Here digital averaging / low pass filtering is preferred as it can be FIR type and thus fully settled in a limited time (e.g. 1 second or so). A classic dual slope ADC only average over part of the time (e.g. 1/4 for the classic ICL7106) and thus ideally would have quite some analog filtering in front for lowest noise.

[RolandK]

Power supply:
why not just use 4 eneloop divided in 2 groups. They deliver 2x 2.6V and and it's hard to get less noise. Just use a 2 pole power switch for positive and negative supply.

Circuit:
1.if you don't need high input resistance, you can use an inverting OP configuration. The operation point is then always at zero volts, so no common mode voltage and therefore better linearity.  (i think a µV meter is more useful then just a null-detector). Disadvantage of inverting configuration: low input resistance, e.g. only 10k if you have 10k input and 1 meg feedback resistor.
2. Use an OP with lowest input bias current, as it is drawn from the circuit to be measured. Input offset voltage may be trimmed.
3. Both inputs should see the same input resistance to prevent bias offset current. Beeing R1 input resistor and R2 feedback resistor at the - input, just parallel same resistors (1% is ok there) at the + input against GND.
4. Compensate bias offset voltage as proposed earlier. Just short the input and trim for zero output, leave it then switched on for 1 hour - your meter should not drift away.
5. The low pass filter at the input is against emitting chopper noise in the circuit to be measured, too. So adding a C against GND before the 2 20k may be a good idea.
6. Use a socket for your op-amp, so you can compare different types. Build it on a breadboard to see if it really works.

[hozone]

Thank you all,

so, I've a few questions. Please consider I'm more on the digital side, not so expert in analog.

@retroware, Kleinstein
Do you mean adding a voltage follower? Like some with an input resistor of 1k at +, and one on the feedback?

@RolandK
1. I'm going to use this for resistance measurement in Wheatstone bridge mode. Also I'm building this for experimentation.
2/3. You mean the MCP6001 used for virtual ground?
4. So, for calibration a) short +IN to GNDREF b) trim RV1 so that +OUT GNDREF is 0V c) check that there are not drift
5. Like another 100n, it should be 1500Hz cutoff when 1k (R8) is selected, 15Hz when 100k (R5) is selected

[Kleinstein]

The circuit as shown is a low voltage voltmeter / µV meter, not just a Null dedector. The main difference is that the null detectors don't care about the scale factor / accuracy and this way don't need calibration.

There is a balance between voltage noise on one side and input bias / current noise. So the choice of amplifier depends on the expected source resistance. The AD8628 is not the lowest bias, but usually still quite good and more like an intermediate choice.
For high source resistance, like 1 M and more I would prefer an max4238 or LTC2050.
This would only make sense when the input divider is also higher resistance (more like like 10 M or 100 M instead of the current 200 K).
Something like at least 1 M would already make sense with the AD8628, if not planing for high source resistance.
For a low source restance, if low noise is important an OPA387 / OPA388 can be better (but would need lower resistors in the protection to make full use of this).

The modern AZ OP-amps don't come in a DIP case - so a socket is not really a choice. It could also add thermal EMF and is not ideal.
To a large part this is a cheap circuit and would be more like building a 2nd version populated with a different OP-amp (and maybe different resistors) if needed.

The question is if one needs / wants compensation of the bias current. This circuit part may not be so obvious and there are different options (e.g. photocurrent, extra follower (e.g. MCP6001) from the inverting input of the AD8628 + shift + trimmer + a large (e.g. 1 Gohm) resistor to the input side). One would also need a higher resistance divider to "measure" the bias current.

Having the same resistance at both inputs of an OP-amp was valid for BJT based OP-amps to reduce the error from bias currents.
Things are diffferent for AZ amplifiers. Here the input currents are not equal on both side, but more of opposite sign. So adding resistance to the other side would only make things worse.
There is however a point in getting symmetry in the impedance at higher frequency (e.g. 1-100 MHz). In the curren plan the capacitors at both sides that make it low impedance on both inputs. Symmetry here can help with the switching spikes and charge injection related to this.

It makes no sense to switch to an inverting configuration. With low voltage DC this is a bad choice. If at all it may have some advantge with high voltage and AC and electronic range swiching, but these point's don't apply here.

[hozone]

Thank you @Kleinstein

I've read your post about the op amp suggestion on another topic in this forum, this is why I chose the AD8628

I'm going to use for precise resistance measurement so maybe I'm not going to need bias compensation. But maybe adding a simple bias compensation could help.

What do you think about implementing the simplest solution of a resistor followed by a trimmer from the negative input of the AD8628 to GNDREF? Should do the job?
If I've understand well, supposing a Ibias of 50fA (30 to 100 by datasheet AD8628). Vcomp = Ibias * Rcomp. If I want to compensate the voltage drop caused by the bias current of 1mV. Vcomp = Ibias * Rcomp. So Rcomp will be 20M.
Can i suppose 1mV or is too much?

[Kleinstein]

In may 2 tests so far the input bias with an AD8628 was quite a bit lower (more like 5 pA range including some CMOS switches in one case they could add or compensate). The specs may include the unfavorable regions close to the rails when the leakage current from the protection diodes no longer compensates. The input bias can also depend on the capacitance / impedance at the inputs.
With those low currents there is the problem that many parts are not tested very strict and the spec limits are more test limits not actual part properties. The typical bias is here the more relevant, but even that could be on the high side (e.g. including the worst case over voltage and possible improvements (tweak some capacitance / gate area) over time, not reflected in the DS).

A simple form for the input bias compensation would be a large resistor (e.g. 100 M, ideally more, but larger resistors get more expensive) from the OP-amps input towards a small voltage (some +-10 mV or so). Chances are one would have one of 2 resistor populated to have either a positive of negative compensation range and sign for the voltage.
This would add some loading (still small compared to the divider) and some biasing current (even if with a significant input voltage the overall current flows the other way). 10 pA would be 1 mV over 100 M ohm. 100 M would still be large compared to the resistors at the divider and not change the divider very much.

The bias compensation would only make sense if one can also test the bias and thus have a larger impedance (e.g. 1 M or 10 M) for the input divider. 1 pA with 1 M would still be only 1 µV and in the range of thermal EMF at the resistors. With only 200 K for the divder it does not need much thermal EMF at the divider to get more unwanted current.

Ideally one would do a polarity reversal for the excitation voltage in the bridge. This would average out both offset voltage and a bias current effect.

With high impedance sources in mind, one could consider using PP type capacitors for the filtering at the input. These are a bit larger, but have less dielectric absorbtion than the more common polyester film capacitors.

What type of ADC is planed for the display part ? This may effect how many ranges and what accuracy makes sense for the divider / amplifier.

[donlisms]

I think for balancing a resistance bridge, an analog null meter (with a needle) still makes a lot of sense.  I think it's harder to balance with a digital display. 

[Kleinstein]

For the display it depends on the needed bandwidth / speed that sets the noise level. With relatively fast cases the analog display can be faster. With a slow BW, which may be needed for lowest noise, a digital reading can be better. There is also the option to have digital reading via an ADC and still get a more analog like graphical display.

[RolandK]

@hozone: With 4 Eneloops you have +/-2.6V and GND ready, no virtual GND is needed.
 - on/off switch - +2.6V
|
Battery
|
Battery
|
 - GND=GNDREF
|
Battery
|
Battery
|
 - On/Off switch - -2.6V

As voltage surveillance you can add a low current LED on the supply side which draws less current. E.g. a red LED goes off at about 1.9 V. Trim the current consumption with a resistor that this side needs slighly more current.


@Kleinstein and Conrad Hoffman: Thank you for sharing your experience, allways a good read.

[hozone]

Thanks all,

I will use it mostly for resistance measurement (like the FIG7 sample here: https://conradhoffman.com/MML%20files/2_null_p5.jpg).I will not handle high impedance, and I stay with low bandswidth I suppose.
I'm going to use the accurate multimeter I've around here, that is a Brymen BM785.

@Kleinstein
Sorry for the dumbness of my replies.
Because I will not add large impedance on input divider, I does not need the bias compensation. To have the kind of compensation you are talking about (the one of the 100M to +-10mV) I have to build up a 10mV, right? That I don't have now.

@RolandK
That's a way. I can not find 1.3V cell, but rechargeable batteries are 1.2V, so maybe I can get it to +-2.4V. Find attached the schematic. Maybe that will be better than the 5V regulator + op amp virtual ground.
As for the monitor, trimming the resistor will not be easy, I'm thinking about a zener application, but the TL431 will cost almost the same.

[iMo]

With 2 packets by 6 position switch you may arrange it such you will measure the battery voltage in the first position, and then 100mV/10mV/..10uV ranges clockwise - as you usually start with the highest range (in reverse order as you have it today).

[hozone]

Thanks @iMo

I don't understand, why have I to measure the battery voltage in different position?

[Kleinstein]

Alkaline cells and fresh charged NiMH cell can be a little over 1.6 V. 4 x1 .6 V would be a bit on the high side for the AD8628. I would suggest using diodes in series for the reverse polarity protection. These would reduce the voltage enough to stay below 5.5 V.

[iMo]

..I don't understand, why have I to measure the battery voltage in different position?

It is up to you how you wire the switch, sure, my idea has been to organize the switch positions from "Battery"->100mV->10mV->..10uV in clockwise direction (considering your switch pins numbering goes in reverse)..

[hozone]

Thanks

@iMo
Now I get it!
I've to check the switch I've here.

@Kleinstein
I thought NiMH where 1.2, indeed you are right, they can go up to 1.6V. I've put two SS14 (0.5 drop each), so that even if batteries are 1.6V I will reach 5.4V.
Do you think is better the NiMh cell configuration or the 5V regulator + op amp virtual groundone?

[Kleinstein]

One could also still use a regulator for one side. There is not real need to have the ground exactly in the center. This does however help with the zero trim.
I would prefer the version with the virtual ground and regulator - it is just more stable supply.

[donlisms]

If there is already an input offset adjustment (RV1), couldn't you just disconnect the supply from the Wheatstone bridge and include the voltage due to offset current in the overall offset adjustment?  The bridge is essentially just a resistor across the null detector input in that state, yes?

[Kleinstein]

If there is already an input offset adjustment (RV1), couldn't you just disconnect the supply from the Wheatstone bridge and include the voltage due to offset current in the overall offset adjustment?  The bridge is essentially just a resistor across the null detector input in that state, yes?

That is possible, though it takes the additional step for the offset adjustment for each resistor values separate. With a digital readout this may not be so bad, as one could subtract the offset digitally and thus have a relatively fast zero.

With a separate offset and bias adjustment one could have the trim part stable. Especially with AZ amplifiers the small resitdual offset (a few µV) and also the bias is quite stable. So no need to adjust very often.
For a PCB, I would plan with both. One can still decide later to populate or not to populate.  Higher value resistors for the input divider would be good anyway, at least a factor of 10. 1 M resistors are still available as thin film types with low TC / tolerance.

[hozone]

I was trying to simulate this circuit using LTSpice to understand the changes I have to do for the input bias compensation. I admin I don't yet understand where I have to put compensation resistors 
But I've find something strange, for sure it's my mistake but I don't understand where.

Find attached the .asc for LTspice. and Run simulation
Shouldn't I have 10mV at output when set as 10mV in input?

[iMo]

Your amplification is 1+20000/100 = 201x..
10mV * 201 = 2.010V

[Kleinstein]

The gain setting are only approximate to the round numbers (good enough for analog and digital may have to do some math anyway).

The simulated circuit is still missing part of the divider. So the 10 K or so towards ground.

[hozone]

 

Thanks! 10.09k to ground solves it. Also at 10mV setting, 1mV in is 10mV out.

Now, as for the bias compensation, you mean adding a space on PCB for eventually populating R9 and R10?

[retroware]

You probably want a trimmer in the bias circuit so you can adjust it.  See for example what Keithley did:

http://www.ko4bb.com/manuals/192.31.192.5/Keithley_155_schematic_restored.pdf

To adjust the offset bias current, one needs to be able to measure it.  Keithley takes advantage of the fact that the input resistance of the their meter in the microvolt range is 1M. They measure the voltage across this resistor (input to the meter is open) in order to measure the bias current.  See section 4.2.5 of the K155 manual for the procedure they use. If your input is resistance significantly less than that, you'll have a harder time measuring small bias currents.

[hozone]

So, something like adding a couple of test point between the two R9 and R10 resistors in the ltspice schematic and the space for a trimmer on bottom of each one.

[Kleinstein]

One only needs one large resistor, possibly even more than 100 M. The choice of sign would be more in front of a trimmer. So maybe a 10 K trimmer to ground and some 1 M to either the positive or negative supply. The large resistor would be from the wiper. To reduce the range without changing the trimmer maybe an optional resistor in parallel to the trimmer.

[hozone]

I'm not sure I've understand correctly,
You mean like the LTSpiece attached? (R5 and R9 are the 10k trimmer) R12 is the optional parallel resistor.

[Kleinstein]

That is about the circuit I had in mind, except for the link to short out R9.

[hozone]

Thanks!
Monday I'll put it in KiCad and share it here.

[hozone]

Find attached the schematic.
If it's all ok, I think I'll try to build one. First I'll check the footprint of the rotary switch that I've buy.
Let me know.

[Kleinstein]

The low bat detector still uses quite some power. A simpler version would use the 5 V regulator as a "reference" and than compare something like 45% of the input voltage to the 2.5 V.  For this one could use an OP-amp  (e.g. use MCP6002 together with the virtual ground).

I would still want a footprint for a ceramic capacitor in front of the regulator. Low impedance at the input side is usually helping the regulators.

The capacitors C8 and C9 should be low leakage types - so more like film types that are usually easier to get as THT parts. Beside leakage, also the dielectric absorbtion from X7R or similar capacitors would be an issue. If SMD is really needed this would be C0G types and these would likely need a larger footprint, maybe a little less capacitance. C10 is a little less critical, by I would still prefer a film type here, just to be on the safe side (X7R may need a little more settling time).

The bias compensation may want an extra footprint for an alternative to R23, but to the negative side. The bias can have either sign.
The resistors R3,R4,R5 may want a larger foot-print (at least 0805 ideally even more), so they can better withstand transient voltages.

Ideally the meter would have a way to indicate clipping, e.g. from mains hum. So some kind of comparators to check if the output exceeds some +-2.3 V at any time. A slow DMM may not notice this.

[iMo]

MCHP recommends COUT = 1 μF Ceramic (X7R), CIN = 1 μF Ceramic (X7R) for the 1703..

[hozone]

Thanks,

- added ceramic 1u out of the 5-V LDO, and on input side too
- C8,C9,C10 replaced with Polypropylene Film Capacitor (CBB)
- added a selector for the Vref of Bias Compensation
- changed R4 and R3 to 0805
- added a +-2.1V almost peak detector

Later I'll check the low voltage detector.

SMD has been here for simplicity, I mean manufacturer build and assembly SMD for low cost

[hozone]

Updated schematic and layout attached, still have to check the low battery voltage detector.

[iMo]

R25/26/27 - why not a bit higher values? Like 10x for example?
Hopefully the blinking "Peak" LED will not disturb the equilibrium there.. 
PS: is it possible to make the Peak comparator with a single opamp?

[hozone]

I've review my simulation on falstad.com (attached), with the 1N4148, 0.7V drop almost, maybe it's better to change values.
I've +-2.2V detector changing R26 and R27 to 47, and R25 to 20k.
So it may work better.

[iMo]

Another variant of the Peak Indicator..
With R1 you may set the threshold..
 

[hozone]

That's interesting, seems the difference is that your design has a + indicator as well as a - indicator.
Can you please share the LTspice file? I'll pack it with the detector files.
Later I'll share the KiCad source too, when all is done.
Find attached the .asm file for the 5V low monitor. As Kleinstein stated current consumption is pretty high, 4mA almost. Need to investigate an alternative way maybe.

[Kleinstein]

4 mA is indeed pretty high for battery operation. The alternative is a comparator / OP to compare the 2.5 V (half of 5 V) to some 45% of the input voltage. This could getaway with some 200-250 µA when using a MCP6001/2 and less if needed.

[iMo]

That's interesting, seems the difference is that your design has a + indicator as well as a - indicator.
Can you please share the LTspice file? I'll pack it with the detector files.
Later I'll share the KiCad source too, when all is done.
Find attached the .asm file for the 5V low monitor. As Kleinstein stated current consumption is pretty high, 4mA almost. Need to investigate an alternative way maybe.

The wiring is different, see yours and mine.
Attached the sim file. Mind it is a sim only, I would highly recommend to try it in HW whether it works fine. Also blinking the LEDs might be "dangerous" for your design - the weak virtual ground might cause issues with 1mA pos/neg pulses.

[hozone]

Thanks!

For the low voltage detector I'm moving to MX810, the one with 4.6V threshold, and I'll put it after the LDO. I've worked with MAX809 before (for other purpose), they work good. Do you think it will be ok even there?

I've notice the peak detection in your circuit changes regarding the led used, and op amp used. Find my asm attached. I've to check my design with LTspice too. For my purpose I think there will not be many blinking.

[iMo]

The behavior of my null indicator changes based on
a) the opamp is rail2rail or not,
b) the opamp can work at +/- 2.5V
c) the LED diode forward voltage - with only +/- 2.5V your LED's Vf has to fit somehow (based on the color the Vf is from 1.6V to 3.5V, afaik).

PS: I doubt your above indicator may work as there is none feedback manipulating the input threshold levels..

PPS: Recommended reading:
Jerald G. Graeme: Applications of Operational Amplifiers, Third-Generation Techniques, McGraw-Hill, 1973, p.112
 

[hozone]

I've to investigate about that specific implementation, but I can confirm you I've one built in another project that works. It's RC4558 based, +-15V rail and it peaks almost +-8.5V.
Note: posting here I've notice this +-2.1V implementation has +- rails inverted, I've to swap them.

[trobbins]

Thanks for the thread - it has inspired me to start preparing a null detector to drive the original galvanometer in a nice 1950's PYE wheatstone bridge I tweaked last year. 

I had been using the mV range on an Aneng AN8009 cheap DMM in lieu of the galvanometer for much better auto-ranging and discrimination, but I think a bit of interface circuitry based on an AZ opamp could well do a much better job, and could be easily powered by 5V from an external USB battery brick.  The application doesn't need accurate FS voltage ranging, and the load is likely a few tens of uA (although I haven't measured what the meter FS current actually is).  I'll try and keep the original initial/final keying of the galv, and retrofit the final key with a 'super final' press-button.

[Kleinstein]

A power bank could add quite some EMI issues. With some care the circuit can be relatively low power and well get away with direct battery supply with a linear regulator, as shown. The OP-amp can also work with less than 5 V so even a single LI cell can work.

[hozone]

In a couple of weeks I think I'll take this PCB to production, I'll keep you updated.

[hozone]

Forget to post here the peak-detector based on my design. Op amp is 741, and like the iMo design, changes in the Led and the Op amp makes changes in the R5 detector resistance.
One option could be reuse the 100k trimmer already used in the BOM in place of the R1,R5 or use one trimmer in place of R5 and a fixed R1.