Help me with a resistance problem

[soggybag]

I am trying to add an LFO to a PT-80 delay that I have made. I've used the Commonsound LFO connected to photo coupler. It isn't working so well. I'm sure it could work better. The photo coupler has a resistance of a few K up to about 10M. I think I need to raise the value of the pot a bit and possibly add a ressistance in series with the photocoupler.

I'm open to ideas if anyone thinks that this would be the wrong approach. This was the simplest method I could come up with.

Here's a diagram:

[brett]

Hi
The optocoupler is very non-linear, which will cause some headaches.
Can you disconnect it and check the actual range of resistance that it is producing?
You might want to consider using the delay control in series with the optocoupler.  However, with the LFO turned off, you'd then need a switch to bypass the coupler coz it would have very high resistance.
cheers

[soggybag]

Thanks Brett that makes a lot of sense. On paper it seemed that the range of the opto-coupler would work well. But in practice it seems that it spends too much of time on the low end, and I get very short delay times. I tried switching the pot to 250K and this gave better results.

Is there anyway to calculate the resistance in this case? I'd like to get a range of 1k to 100k for both the opto coupler and the delay pot.

I keep thinking a resistance in series with the optocoupler might help and maybe a larger pot... I'll have to give it a try when I have some time tomorrow night.

[brett]

Hi
try adding a pot/trimpot as as a variable resistor before the coupler LED.
That'll give you control over the brightness and resistance range.
You're lucky - it's easier to dim the LED and get more resistance than the other way around.
cheers