Geofex Neutron question

[Le québécois]

What is the expected resistance range of the LED-LDR in this pedal. More detailed question below.

I build my own optocoupler with blue LED and LDR and breadboarded the neutron filter. The in and out opamp work when I bypass this non true bypass pedal, therefore this part work but I have distortion when the filters are engage (all three of them – LP, BP and HP). I really suspect the resistance range of my LDR. What should be the range of variation of the resistance coming out of my two optocoupler. They are in parallel with 220k resistor so I assume they should vary between 0 or close to this and up to 220k or even 500K. This would give a range between ±10k and up to 220k. Mine never exceed 30k even with very strong picking.

thx

P.S. Yes I know, I should buy vactrol

[R.G.]

They are in parallel with 220k resistor so I assume they should vary between 0 or close to this and up to 220k

If an infinite resistance were paralleled with 220K, the parallel resistance would be 220K. So the range is lowest-LDR up to 220K they *can't* go over 220K, no matter what.

Mine never exceed 30k even with very strong picking

That's because you're using LEDs and LDRs that make the resistance be too small. Blue LEDs are notorious for making a huge amount of light, so your LDRs are probably always turned on too much. I would swap for dimmer LEDs, then see what happens. Maybe you need to increase the series resistance with the LEDs to cut the current they get a bunch.

Also, the Mutron circuit was designed at a time when the only LEDs had forward voltages of 1.5-2.0V. Blue LEDs are 3-4V. You could be having sudden turn on problems with the voltages on the LED driver.

Measure the dark-condition resistance of your LDRs. If it's in the 100K to 1M region, the LDRs are OK, and the problem is the LEDs. Some LDRs don't get that high, so that could be another problem.