Vactrols are magic!

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!


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.


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 :slight_smile: i’m not sure.


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 :slight_smile: .

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 :slightly_smiling_face:

Exactely, a pot connected like an attenuator, and i add also another led & resistor for the panel :wink: