Yo, this is a usefull schematic from the vacpak by synthrotek (bouh), for all the hackers who like to used vactrols to add cv control on (almost) everything !! It would be cool if someone can make a stripboard version for the beginners who can not read a schematic (maybe I could try to make it later) , because this circuit is very cool, I use it in a lot of modules or hack/bend circuits who need more controls!
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Interesting. I wonder what this would look like with a tl071 and 12+.
Did something vaguely similar in my Megamodule:
I mean not that similar in design… but a similar idea.
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No problem, I guess it will work fine too. You can use it on your 5v rail if you have one in your modular psu, without using a regulator
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Of course it look like the same thing.
Your design is interesting too, I save it for later ! Thank you !
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Would you mind explaining the function? I’m learning electronics slowly!
What is D1 for? Is it to protect against the voltage getting too high for the LED?
What is C1 for?
Do you need any further buffering after the LDR?
in the Glossary :
" Vactrol : A light dependent resistor (LDR) placed near an LED to add voltage resistance. As the LED gets bright, the resistance goes lower."
It’s to make some CV IN on module.
So if you put a Vactrol connected to a Pitch potentiometer of a VCO, with an external voltage (an LFO for exemple) you can change the pitch.
More you send voltage more the led will shine and more the LDR let the voltage pass between the 2 pin of the pot. So more the pitch go up.
Sorry for the rest, D1 C1 … wait for good explaination 
i’m not sure.
EDIT :
The schematics proposed are complete, but personally I use a resistor, a led, a potentiometer and an LDR and it works very well too (just a few tests to do for the value of the resistance for best result )
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Sorry I didn’t word it very well I meant the function of D1 and C1, I learned about Vactrols from Sam’s videos 
.
For your simple design you are using resistance to stop the LED burning out and a pot to control strength of the CV?
Thanks for the help
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Sorry i misunderstand 
Exactely, a pot connected like an attenuator, and i add also another led & resistor for the panel 
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Hey @analogoutput , would you mind explaining how you came up with this circuit? I read a bit on northcoast synthesis and found that your circuit is similar to (D), but I don’t understand the two diodes D1 and D2 (I assume it’s some kind of reverse voltage protection, but couldn’t that be done with a anti-parallel diode to the LED?)
Why the additional 1k resistor? And tC1 is for some kind of stabilization/filtering?
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Basically I came up with it by modifying it from some design I found online. D1 is a Zener intended to limit voltage into the op amp and hence the current through the LED. D2 is to prevent reverse biasing the LED. There certainly are other ways to do both. I don’t know why my comment talks about 0–10 V CV when the Zener limits it to 5 V, maybe it’s left over from before I added the Zener. C1 and R7 were from the design I was copying and I guess they form a low pass filter to keep noise down.
Thanks for the Northcoast link, valuable.
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Quite a high frequency filter though. 160 kHz. I’d think if you were bothering to use a filter you’d use one with a frequency much closer to the upper limit of the signal you expect, which for a CV is several orders of magnitude lower. Maybe it made more sense in the context of whatever I lifted it from, cargo cult style.
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I really enjoyed building the Vactrol VCF. It’s so different to other VCF’s and if you experiment with the type of vactrols you put in you can get some very cool and very weird results. As I mention in the article it also has some great percussive possibilities.
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I dont know if I’ve seen a stripboard annotated like this before - but I’ll be darned if it isn’t helpful!
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Yes the Eddy’s stripboards are allways well done and very detailed.
Hey! im trying to add a vactral to this RC low pass filter i have tried many places on where to put the LDR but all seem to just crackl or hum and not do its job any help would be great
- Jack
i would try here
i suppose you know that for making a vactrol you need an LDR and also a Led (+ 1 resistor to protect it), and “normaly” when the led don’t bright there’s no changes on the value
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Yeah I got the vactrol to work on all three of the oscillators I made by connecting it to leg three of the potentiometer and the other side to ground however I didn’t think to put the a resister after the led thanks for reminding me haha.
I’ll give the position a try tomorrow as I have inhaled enough solder fumes for one day.
Cheers
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That position didnt work it only created a hum. any other ideas?
(Edit) my bad i didnt have enough light for it to work hahah. Thanks Dudit all works now 
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