Dear DIYstompboxer,
I am currently trying to add an indicator LED that should light up parallel to the led in an optocoupler. The circuit is basically bajaman's optical limiter that he published in the other forum (but my question is more of a general kind). How do I decouple these two leds, so that the optocoupler is not influenced by the indicator LED? Some internet research brought me to the two versions below. But I cannot get neither of them to work (either the LEDs don't light up at all or the limiting is influenced). Is one version correct? And how could I determine the correct values for the resistors given that I have no idea about the currents/voltages at the opamp's output?

Thanks very much in advance!
Lars

Original Circuit



Indicator added V1



Indicator added V2

Can you not just hang an led in parallel with the existing vactrols led?

V2 should werk, but take the R2 to ground, and use a decent big CLR with superbright led.

I'd use 10K for R1. You don't need a huge amount of current to turn the transistor on. This is a value based on experience not theory. I'd then adjust to taste if it doesn't work effectively.

R2 depends on your LED, and you do the usual calculation of (9V - forward voltage) to get the voltage drop across the resistor, and then use R= V/I with that voltage and however much current you want/need through the LED to get the final R2 value.

HTH,
Tom

I have no idea either. The point of that LED driver is not clear. Why a 2-diode drop from Vb? Where is Vb coming from, will it absorb the possibly hefty LED current? Can you even write all the idle DC voltages by inspection? (If you can't, then you don't know what it will do or how to make it more-like what you want.)

*In General*, you want two LEDs to do the same thing, you put them *series* (not parallel). Same current, same light.

Yes, this may fail when one is new bright LED and the other old-stock (the opto), and/or when one is for the eye and the other for an opto-R (may want different light levels for the same input). But let's start with simple series.




To go past your question: you have stolen the design of the LA-2a (down to the 2.7k), but shuffled some pieces. The "killer" change is the LED instead of the EL panel. The EL lights up either way, the LED lights up only in one direction of current flow. This may be "OK" for a pedal; I'd expect it to be annoying on full-orchestra and even some speech. The signal path gain pot should be *before* amplifier 1, not after (as you have it, POT and POT2 do similar things differently).

Thanks for your help. I tried out your recommendations and ended up with version 2, LED cathode to VB, R1 at 10R and no R2. When I tied the indicator LED to ground it was constantly on, no matter how large I chose R1. PRR's suggestion to put it in series seriously interfered with the vactrol LED (what I was hjoping to avoid by using the driver).
. 
As an indicator LED (which is really not even necessary for the effect to work) this works fine now.

One sentence from PRR puzzles me somehow (hopefully it is just the language barrier me being not a native English speaker):

Quote from: PRR on July 23, 2018, 03:16:41 PM
To go past your question: you have stolen the design of the LA-2a (down to the 2.7k), but shuffled some pieces.

In my first post I cited where I got the circuit from

Quote from: lars-musik on July 23, 2018, 07:37:32 AM
The circuit is basically bajaman's optical limiter that he published in the other forum

and I thought that would suffice (in combination with my re-drawing the schematic) to not have stolen it. In said contribution in the other forum Bajaman clearly explains in his first post that he got the design from the LA2A.

I suppose transistor base interferes with vactrols driving current. If you have a spare opamp section, you could try to buffer the voltage before R19 and use it to drive the LED. R2 would need some adjustment for brightness.



Disclaimer: I'm not an electronics engineer. Hopefully someone with more expertise will correct me.

ahh, see, your last post allows me to point out what an idiot I am. if the opamp's output as in V2 is sitting at V/2, and you run an emitter follower between the output and ground, it will put V/2 - Vbe across the led and clr. if instead the led/clr is taken to V/2, then no current flows, no light happens until a signal squiggles the output above V/2 [+ Vbe].

which is a long winded way to say in yr last dia w/ the oppie follower, the led/clr should again return to V/2 instead of ground.

> PRR puzzles me somehow

I did not accuse you of stealing my dog.

*In Audio*, "stealing" ideas is a long and honorable tradition. New-Cloth designs are incredibly rare, almost unknown. Everybody "studies" and "builds upon" the work of others.

Just as bajaman has clearly seen the LA2a limiter plan.

However it is important to "steal the whole dog", not just parts of the dog.

While I get what you're saying Paul, I don't think you've phrased that in an ideal way for a non-native speaker.

Lars, what he means is that everyone re-uses and changes existing designs - we call this "borrowing" or "stealing", but it's neither of those things in reality. And no dogs are involved! - "stealing the whole dog" means taking the whole design and not leaving any important parts out.

Bajaman took the LA2a design and changed/tweaked/modified it, you took that design and tweaked it a bit more. Someone else here will probably take what you've done and alter it again. That's how things work.

"The folk tradition of effects electronics"

Gonna squeeze that lemon til the juice runs down my leg....

Quote from: tubegeek on July 29, 2018, 10:56:25 AM
"The folk tradition of effects electronics"

Gonna squeeze that lemon til the juice runs down my leg....

*Someone I am playing with says* "I wrote a new Blues song".
*Me* "No. You didn't".

So, I finished the circuit, boxed it -  and will unbox it soon and fill the enclosure with something different. Maybe I've made a mistake somewhere, quite probably PRR's right as usual and the circuit is just not as it it should be. In short: In lower settings somehow it ruins my sound and turning it up it starts to distort.... I am just not the man for any dynamic pedal, be it a compressor or a limiter.

Thanks to you all for helping out and for your explanations regarding stealing and dogs!

Thank you for the follow-up.

Maybe off topic, however it's vactrol question O:)

What kind of vactrol do you use for Bajaman Limiter / Demeter Compulator? I'm currently having trouble with the BL, cause it seems to be distorting sound a little, and I presume that my vactrol is causing this.

I never got an answer to this. In the meantime, I tested all the vactrols I have with this circuit and this is the conclusion:
- the best is NSL32, almost no distortion
- the second placed is LCR-0202
- the third is some VTL5C10, marked with those letters and PRC, so I'm not sure if it is the original one or some kind of copy.

All the others distort the sound, and I actually don't like those "best" 2nd and 3rd either. Strange circuit.

Hi, Rankot.
I just built this effect.
I could hear some distortion, but when I swapped the first LF353 for a 5534, the distortion went away.
I could tell it wasn't coming from the vactrol, because even without compressing, the distortion was audible (not with every kind of input sound, but with percussive sounds and bass heavy content I could hear it).
Maybe you could try with a different IC?

... And to answer your original question : I got the best results with a home-made vactrol (5mm orange led + GL5516). The LCR0202 gave me worse results, and my other vactrols were out of spec, so I didn't even try.

Quote from: Kipper4 on July 23, 2018, 10:06:49 AM
Can you not just hang an led in parallel with the existing vactrols led?

no, you can't, changes the brightness response. depending on the Vf, it van even change to the notorious 'always dark' response.

cheers

Quote from: snk on April 17, 2021, 06:23:48 PM
Hi, Rankot.
I just built this effect.
I could hear some distortion, but when I swapped the first LF353 for a 5534, the distortion went away.
I could tell it wasn't coming from the vactrol, because even without compressing, the distortion was audible (not with every kind of input sound, but with percussive sounds and bass heavy content I could hear it).
Maybe you could try with a different IC?

... And to answer your original question : I got the best results with a home-made vactrol (5mm orange led + GL5516). The LCR0202 gave me worse results, and my other vactrols were out of spec, so I didn't even try.

Thanks a lot. I will try with homemade vactrol, have few GL5516 in stash. I will also try with 5532, although my build uses TL072.

Why orange LED, when GL5516 datasheet says it's most sensitive to green light?

Did anyone made some kind of nice and not intruding indicator circuit for such opto compressors? I was thinking of using two op-amp voltage followers for both opto LED and indicator LED. What do you guys think about that idea?