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vactrols?
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prophei



Joined: Jan 27, 2007
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Location: san francisco, ca
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PostPosted: Thu Oct 11, 2007 8:29 pm    Post subject: vactrols? Reply with quote  Mark this post and the followings unread

how do you generally hook up a vactrol to handle cv control of something? i am most interested in being able to add the vactrol for cv control in addition to a pot, not replacing it. i have a handful of these here and am dying to learn more... any help is MUCH appreciated!

thanks!
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Randaleem



Joined: May 17, 2007
Posts: 456
Location: Northern CA, USA

PostPosted: Thu Oct 11, 2007 11:26 pm    Post subject: Re: vactrols? Reply with quote  Mark this post and the followings unread

Hi Prophei,

A Vactrol is a variable resistor. So you hook the resistor part up the same as if it were a pot. An additional one in the case you stated. So if you're using an opamp as a summer, then the LDR side of the Vactrol gets put in place with its own connection to a voltage source and a scaling resistor (to determine how much effect it will have overall on the summed total) on the way to the summing node of the opamp.

In other circuits it is placed inline wherever you want a variable resistor. Treat it just like a pot in these cases. Adding parallel/serial r's can shape the response if you like.

The other side which controls this variable resistor is nothing more than an LED or incandescent lamp (depends upon your vactrol type).
Any circuit suitable for varying the intensity of a Lamp or LED will work.

You want to use a circuit which will vary the intensity of this light source according to the desired output, and the available input. IOW, you may want to invert the lamp operation (gets darker with increased CV). You may have to limit the range of the intensity(scale it) to keep the VR side of the Vactrol in line with the resistance needs of its portion of the circuit.

The key thing to remember is that the vactrol turns off more slowly than it turns on. Sometimes MUCH more slowly. LDR's (Light Dependent Resistors) have light memory to differing degrees, and this is one of the main differences between various Vactrols. How quickly they "turn on" is the other major spec. difference. Obviously max/min resistance is the third primary concern. See the factory spec sheets for mfd. Vactrols. For home-rolled types you just have to experiment.

Hard to say more than this unless specifics are detailed. Many variables involved. Have a look at the classic Vactrol circuits available online (Buchla is a well-known source) and experiment with your breadboard. That's the fastest way IMO to get a handle on these guys.

Kind regards,

Randal
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Scott Stites
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PostPosted: Fri Oct 12, 2007 7:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Buchla schematics would be, at best, a confusing resource to refer to if you're just getting into optocoupler control. The control circuits have some very interesting arrangements Don put in to get a specific response out of them, which makes them appear fairly complex. That's not to dissuade you from studying them, it's just to let you know there are ways of controlling optocouplers that aren't as complicated.

I learned the most from studying Grant Richter's Wogglebug3 schematic. For linear control, really all that is needed is a transistor and a few resistors. All of my linear control circuits are extrapolated from that one elegant example.

My Loony WSG page is plastered with putting Vactrols across pots in order to get voltage control of the WSG functions; there are also a few resources there as well:

http://mypeoplepc.com/members/scottnoanh/birthofasynth/id16.html

For expo control, I just use a standard expo transistor arrangement to control the optocoupler. The Mutant Filter uses such a control, though for just putting a pot under voltage control, probably not necessary.

On the linear control, don't let the emitter resistor go too low - it basically determines how much current will go through your optocoupler's LED, and you don't want too much current going through, which can pop the LED. I'd stick to around at least 1K or so. Maybe more or a little less.

Cheers,
Scott

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prophei



Joined: Jan 27, 2007
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Location: san francisco, ca
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PostPosted: Fri Oct 12, 2007 7:57 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks for all the help guys! i was recently lucky enough to end up with some of these vactrols and have been dying to start experimenting. your insight is definitely appreciated!
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