Hi

I’m building an optical guitar pedal compressor, based on the VTL5C3 optocoupler. Circuit below (VACT, lower right). The way a compressor works, it reduces the signal above a certain threshold. Below that signal level, it does nothing and passes it through. Is there a way to add an LED that turns on at that threshold level, off below?

 

IC1B seems to be driving the optocoupler's LED part (VACT_A). I imagine tapping into D4 junction D2 would give you the rectified audio signal at the trigger level. Feeding this voltage to possibly a transistor or comparator should give you an LED driver. This is presuming that putting an LED directly in parallel with VACT_A would interfere with its operation.
I hope this gives you an idea that you can explore.

 

I recently built a Velleman VU meter and modified it for more gain by adding a 330k in series with Rv1 I wonder if I could add this circuit (below) to the above compressor circuit, in a similar way to as you suggest.

 

VACT_A is current driven, not voltage driven.
Therefore the simplest way, which can also be easily reverted to its original configuration, is to use a LED in series with VACT_A. Probably the opamp has enough compliance to drive both.

If it affects the normal operation, then remove the extra LED and leave the circuit as before.

If this circuit were mine, probably I would install a 5 mA DC analog meter in series, and watch the needle dance with the music.

 

VACT_A is current driven, not voltage driven.
Therefore the simplest way, which can also be easily reverted to its original configuration, is to use a LED in series with VACT_A. Probably the opamp has enough compliance to drive both.

If it affects the normal operation, then remove the extra LED and leave the circuit as before.

If this circuit were mine, probably I would install a 5 mA DC analog meter in series, and watch the needle dance with the music.

That's a cool idea, very retro. I'd have to find a small one to fit in a pedal enclosure.

But the reason for the meter isn't for looks, it's to help set the compressor threshold. It's often hard to tell when a compressor kicks in by ear. There are a few sophisticated compressors that have LED reduction meters that are really nice to use. But from a DIY standpoint, I haven't found one... so I'm trying to figure it out myself.

 

VACT_A is current driven, not voltage driven.
Therefore the simplest way, which can also be easily reverted to its original configuration, is to use a LED in series with VACT_A. Probably the opamp has enough compliance to drive both.

LEDs have a Vf of 1.8-4 volts. is there a way to measure whether the added LED affects anything? Whether the op amp can drive it or not?

 

I recently built a Velleman VU meter and modified it for more gain by adding a 330k in series with Rv1 I wonder if I could add this circuit (below) to the above compressor circuit, in a similar way to as you suggest.
View attachment 291736View attachment 291737

I cannot see why it wouldn't work if the input is sensitive enough or has enough input voltage. The input to your bar/dot indicator is voltage driven.

 

Take a look at this optical compressor, the LED brightness goes up/down with compression. How does it do that?

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