Using optocouplers as variable resistors...

[mrsage]

I was thinking about optocouplers the other day, and decided it would be really cool to put one into a circuit I was building instead of a pot. For the sake of reference, the circuit I was building was a Kay Fuzztone, and the pot works as sort of a volume/tone control. I posted sound clips in another thread, but basically I would either rock the pot at a steady rate (which is perfect for an optocoupler kind of thing) or I would have it in the full toe-down position.

So since I'm still a bit green when it comes to coneceptualizing these types of things, I thought I could just stick an optocoupler in there. Easy, right?

Then I realized I'd need to make it turn off and on.

Oh yeah...that part....

So my first thought was to copy part of the tremulus lune circuit...then I thought that seemed a bit complicated when all I want to do is basically have the LED turn on and off.

But the Tremulus Lune is the only effect I've built that involves modulation, so I wasn't sure how to approach this challenge at all. Well, I also built a Phase 90, but that's different, isn't it? Or  could I use concepts from that instead of an optocoupler?

So anyway, what do I want to do here? Is there an easy way for me to get a simple sine wave pattern out of an optocoupler to use it as a variable resistor? Or will I essentially have to build a tremolo-type circuit into my current Kay Fuzz?

Like I said, I haven't built much modulation...and I didn't really pay attention to the theory behind the circuitry when I was building it. Where should I start looking?

[moosapotamus]

Yes, you need an oscillator to drive the LED in your optocoupler. The LFO in the Lune is a great option for this, very tweakable. That two opamp configuration is pretty typical, too. There are probably some simpler designs out there. There are definately some more complex ones. But, even a 555 timer based oscillator is still going to start off with the dip-8 IC package, plus whatever other components are needed.

I would go with the Lune LFO if I were you. Try putting the LDR leads in parallel with the pot (tie the ends to lugs 1 & 3) instead of replacing the pot.

~ Charlie

[ExpAnonColin]

You don't need the WHOLE tremulus lune circuit, but it is a good reference.



If you copy the "LFO stage" and "Rect LFO" stages, and use your optocoupler's LED leads for the LED, you'll have a blinking, and quite controllable, optoisolator.

-Colin

[NaBo]

Please tell me you've seen and read this!!!

[mrsage]

Wow...wealth of information here guys.

Should keep me busy for a while...

Thanks!

[mrsage]

I would go with the Lune LFO if I were you. Try putting the LDR leads in parallel with the pot (tie the ends to lugs 1 & 3) instead of replacing the pot.

What would that do?

Allow me to control the oscillation with either the pot or the LED?

That could be cool...

[idiot savant]

ive done someting like moosapotamus said, using this LFO from commonsound.com, its good, if you don't need all the extra controls from the trem lune LFO, just speed and depth.

come to think of it there's prob. a ton of ways to do it.

http://www.commonsound.com/phaseur/auxlfo_modulate.pdf

[mrsage]

Okay, so here's another question...

How do I know which optocoupler to use?

There are a ton on smallbear and mouser...Do I have to try different ones until I find one that works, or is there another option?

Should the resistance on the photoreceptor match the pot resistance I'm looking for? For instance, since the kay fuzz calls for a 50k pot, should I be looking for a photoreceptor with that kind of resistance?

Or does the lfo circuit take that kind of thing into account?

[idiot savant]

i really dont know, i think the one i used was a clarinex c6000 or someting crazy like that, maybe one of those nsl32's...

i just got a package of these:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G15396&variation=&aitem=1&mitem=1

someone on here mentioned them, they are cheap as hell.
now i just gotta find something to use them in.

[ExpAnonColin]

PS moose, I swear I posted the tremulus lune schem before seeing your post, how cool is that?

-Colin

[ExpAnonColin]

Should the resistance on the photoreceptor match the pot resistance I'm looking for? For instance, since the kay fuzz calls for a 50k pot, should I be looking for a photoreceptor with that kind of resistance?

Yes, plus or minus.  This kind of thing can be aided by resistors in series.  Get out your multimeter and put the LFO on square and SUPER slow and measure away.

-Colin

[mrsage]

Okay, I'm just about ready to try building the Aux LFO circuit mentioned earlier ( http://www.commonsound.com/phaseur/auxlfo_modulate.pdf )...

Any ideas on how to actually implement it into the design I have for this Kay Fuzztone? I'm sure I can figure it out for myself, but I figured I'd ask for other input as well. I'm just wondering because the pot obviously has three contacts, but the optocoupler will only have two.

[80k]

the ultimate way to use the photoresistor is also the easiest way... just mount it on the outside, and then play in the dark with a flashlight or a strobe light

I took my homemade simplified triwave synth to a few buddies and put on a show with a strobe light in the dark.  they were totally blown away  it's the best laser gun synth sound.



Top photoresistor controls pitch, and side photoresistors control speed.

[ExpAnonColin]

Okay, I'm just about ready to try building the Aux LFO circuit mentioned earlier ( http://www.commonsound.com/phaseur/auxlfo_modulate.pdf )...

Any ideas on how to actually implement it into the design I have for this Kay Fuzztone? I'm sure I can figure it out for myself, but I figured I'd ask for other input as well. I'm just wondering because the pot obviously has three contacts, but the optocoupler will only have two.

Things are going to get harder fast when you need a potential divider and not a variable resistor.  You are going to need to make two different LFO circuits-one inverted (via an op amp follower is best in my opinion)-and wire them together at the middle, and then use the side lug for either lug of the pot.  Look at the Tremulus Panneur for ideas.  Just use the two LED/LDR combos on that one.

-Colin

[mrsage]

Whew!

That's good to find out before I dove in and got frustrated with myself. I had heard the term "potential divider" thrown around before, but didn't quite realize that it referred to the "other way" pots are used...(besides being used as variable resistors)


On the Panneur, it looks like the two LED/LDRs are identical...what do you mean when you say one is inverted?

http://www.commonsound.org/panneur/panneurscheme.gif

Is it more difficult than simply making two LED/LDR circuits and wiring two of their legs together?

[ExpAnonColin]

Note the inverting followers before each LED, when you are using it in sync.  If you think about it, the resistance from the center lug to either side lug is always max resistance minus the resistance to the other side log-it's the opposite.  In order to get that in an LED/LDR combo, you need to invert your LFO for the second LED.

-Colin

[mrsage]

Okay...

That makes sense conceptually, but I can't find any information on how to invert one side. I'm sure it's there...I just don't know where to look for it. Most of the info in the archives refers to inverting or non-inverting opamps, but doesn't really explain how to make that happen...

I've been pretty much paint-by-numbers until now, so this is one of my first forays into actually trying to do something that isn't spelled out for me ahead of time...

How would I go about inverting the LFO? I built one LFO based on the phaseur auxiliary LFO design posted earlier...that seems to be working fine. Now I need to build an identical one, only inverted, right?

How do I go about doing that? Is it as easy as running wires to switch the + and - legs on either side of the opamp? (e.g., flip legs 2 & 3 and legs 5 & 6 of a 4558 chip: http://www.semiconductors.philips.com/acrobat/datasheets/NE_SA4558.pdf )

[mrsage]

And while I'm at it, can you point me towards some resistor theory? You said that I could change the resistance of the LFO by using resistors in series...how? Won't that just increase the overall resistance?

Can you point me to a link where I can school myself on that topic?