Optocoupler tests

[RickL]

A couple of weeks ago I built a sequenced wah using the "forbidden" schematic that was briefly linked to. I had very mixed results using various optos that I had on hand so I decided to do a little experimentation. I ordered a couple of examples of each of the three grades of CLM 6000 that Small Bear sells (BTW they came unmarked so I don't know which is which) and two VTL5C1's.

I had on hand 21L915, VTL5C3, unmarked (but look like CLM 6000), CLM 8500 and various home-brews made with random photo resistors and LEDs. Don't bother looking for the CLM 8500, I salvaged them from something (an organ I think) a long time ago.

I got the best results with the VTL5C1 and CLM 8500. Almost identical results with perhaps a bit more high end from the VTL5C1. Next best, but not real good, was one set of CLM 6000. I'm guessing these are the ones with the lowest "on" resistance. All the other examples gave poor results of about the same quality - minor wah'ing but fairly muffled. The filter was not sweeping high enough to give any sort of quack.

I tried parallelling a variable resistor with the poor sounding examples but by the time I got the resistance low enough to get any high end from the filters I lost most of the sweep. This filter clearly wants more range than the parallel combination will give.

I've had good results in the past with the VTL5C3 and home-brews in other projects but this one, and the work-a-like posted in the same thread that I also built with almost identical results, seem to be fairly sensitive to the opto type. The VTL5C1 is described as being used for switching but if you're having trouble getting a project that useds an opto isolator to work you might want to try it.

[Jaicen_solo]

Looking at that particular schem, it appears to me that the opto is simply a variable resistance to ground.
I suspect that it would be possible to sub in a fet for the LDR, and control that with a more common pototransistor optocoupler.
It's not something that i've looked into seriously, but I can't see why it wouldn't work.

[robbiemcm]

various home-brews made with random photo resistors and LEDs.

I thought an optocouple was specifically an LED / phototransistor combination, or does it apply to LED / LDR aswell? I suppose it seems sensible since the name optocouple doesn't specify what it's coupled with.

[RickL]

Looking at that particular schem, it appears to me that the opto is simply a variable resistance to ground.

That's exactly the way it works. In fact when I first tried it with one of my home-brews and got the muffled response I replaced the photo resistor part with a pot wired as a variable resistor to confirm that the filter was working correctly. I never thought to use a FET as the variable resistance. I suppose you could replace it with the equivalent section of something like a Phase 90.

I thought an optocouple was specifically an LED / phototransistor combination, or does it apply to LED / LDR aswell?

You may be right. Optoisolator may be a more correct term. I hope the title doesn't confuse people.

The purpose of the thread was more to share my experiences with the various opto combinations than about the specific pedal. They get used in a lot of different types of circuits including phasers, compressors and envelope controlled filters. I've had fairly good luck using my small stash of CLM 8500 in circuits that wouldn't work correctly with other choices and I was pleased to find that the VTL5C1 worked just as well since I can get as many as I want from Small Bear.

[Jaicen_solo]

Any chance you could try out the FET/Optocoupler solution for all those of us on the wrong side of the pond? Costs way too much to ship an Opto from Steve unless i'm doing a big order, and they're impossible to find over here. I can however get loads of photo-transistor opto's, but i'm in the middle of a house move, so I can't breadboard anything yet.

[RickL]

I don't think it's as simple as replacing the optoisolator with a photo transistor or photo FET. If I understand the idea correctly the photoresistor/LED is replaced by a FET and some biasing circuitry. The source/drain becomes the variable resistance part of the circuit and the voltage at the gate varies this resistance. I don't think you can simply connect the gate to where the anode of the LED was connected.

I think the sub can be done but I don't think I'm likely to do it in the next short while. My main energies right now are going into a triathlon I'm doing on Sunday and finding a new house to replace the one I just sold.

[Jaicen_solo]

Fair enough! I've just bought a house myself, so all my stuff's packed up ready to move tomorrow, hence why I can't breadboard anything for a while!