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 Forum index » DIY Hardware and Software » Developers' Corner
voltage controlled resistor
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mrand



Joined: Mar 30, 2014
Posts: 27
Location: Ottawa

PostPosted: Sat Jun 14, 2014 6:49 pm    Post subject: voltage controlled resistor
Subject description: how to implement voltage controlled resistance
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Hi all,

Can someone recommend some resources for learning how to implement a voltage controlled resistance? I've read various things about optocouplers but am having a hard time coming up with any practical suggestions.

I'm trying to add a cv input to modulate the "tune" of a simple drum circuit. I'm using a transistor as a kind of voltage controlled resistor, to vary the critical tune resistor, which works, except the control voltage I apply goes right through the circuit and is heard at the voice output.

I've posted the same question, more or less, here: http://waldorf.electro-music.com/forum/topic-35059-0.html&postdays=0&postorder=asc&highlight=

This is more of a learning exercise than anything, so any suggestions would be appreciated!

-C
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gdavis



Joined: Feb 27, 2013
Posts: 159
Location: San Diego

PostPosted: Sat Jun 14, 2014 8:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

The question of how to add voltage control to a circuit is asked often. The reason you don't find a clear cut answer is because there isn't one. It depends a lot on the circuit you're dealing with and requires a good understanding of the components being used, whether they be transistors, OTA's or vactrols (optocouplers).

People often turn to vactrols since the basic concept is simple: the resistor side of it can be dropped in place of a fixed resistor, the resistance varied with a current on the LED side. The difficulties arise in the response being very non-linear and not very accurate. There is a lot of variation from one vactrol to another and it can be tricky to tweak it to get the response you desire.

The simplest way to convert your voltage input into a current for controlling the vactrol is a resistor in series (similar to an ordinary LED current limiting resistor) but you'll have to experiment to find the best value for the response you want and make sure your CV source can supply the current (or buffer it).

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JovianPyx



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PostPosted: Sun Jun 15, 2014 9:01 am    Post subject: Reply with quote  Mark this post and the followings unread

The H11F1 (also H11F2 and H11F3) ICs are special opto-isolators that contain an LED that shines on a MOSFET transistor. In theory, it works similarly to a vactrol, but the datasheet claims fairly high linearity (99% I believe - whatever that means).
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mrand



Joined: Mar 30, 2014
Posts: 27
Location: Ottawa

PostPosted: Mon Jun 16, 2014 6:25 am    Post subject: Reply with quote  Mark this post and the followings unread

I found an optoisolator in a bag of random parts someone gave me and promptly cooked it... oops
Going to see about the H11F1 or maybe try rolling a makeshift one.

Thanks for the help!
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L´Andratté



Joined: Sep 23, 2012
Posts: 60
Location: Hamburg, Germany
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PostPosted: Wed Jul 16, 2014 1:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Read all starting from page 62!
http://skpang.co.uk/datasheet/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf

You would only need one transistor to drive it, 2 to drive it exponentially with a very good frequency range.
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mrand



Joined: Mar 30, 2014
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PostPosted: Tue Aug 26, 2014 1:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Someone once suggested that I could use an OTA. I didn't understand how, but just stumbled on the Ray Marston Nuts and Volts article on OTAs (part 2) which includes na lm13700 circuit to achieve voltage controlled resistance. I haven't tried it yet but am posting this for anyone else that comes upon this question.

http://www.nutsvolts.com/uploads/magazine_downloads/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf[/url]
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gdavis



Joined: Feb 27, 2013
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Location: San Diego

PostPosted: Wed Aug 27, 2014 11:24 am    Post subject: Reply with quote  Mark this post and the followings unread

mrand wrote:
Someone once suggested that I could use an OTA. I didn't understand how, but just stumbled on the Ray Marston Nuts and Volts article on OTAs (part 2) which includes na lm13700 circuit to achieve voltage controlled resistance. I haven't tried it yet but am posting this for anyone else that comes upon this question.

http://www.nutsvolts.com/uploads/magazine_downloads/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf[/url]


Those circuits are in the LM13700 data sheet as well, but the Nus and Volts article is a nice resource for gaining a better understanding of OTA's.

The first VCR circuit using 1 OTA is only good for AC signals and applications where one end of the resistor is connected to ground.

The floating VCR using 2 OTA's is intriguing, but probably more complex (uses more components and higher cost) than what is usually necessary in most synth applications. I've never seen it used, I think I saw someone try it at one point without success and went on to a simpler solution.

I was tempted to use the floating VCR to add voltage controlled resonance to the Yusynth moog ladder filter, but instead just went with a basic single OTA based VCA in the feedback path. Works well and frees up the other OTA to use as the main output VCA.


It would be interesting to see it used. Might even be fun to try and make a small module that can be used more or less as a simple drop in replacement for a resistor.

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mrand



Joined: Mar 30, 2014
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PostPosted: Wed Aug 27, 2014 10:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Heh, you seem to be coming through with answers to all my questions, gdavis, thanks!

Quote:
It would be interesting to see it used. Might even be fun to try and make a small module that can be used more or less as a simple drop in replacement for a resistor.


Something like this? http://www.doepfer.de/A1019.htm

I saw that a few months ago and was wondering what one might use it for, aside for development purposes.
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gdavis



Joined: Feb 27, 2013
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PostPosted: Thu Aug 28, 2014 9:41 am    Post subject: Reply with quote  Mark this post and the followings unread

mrand wrote:
Heh, you seem to be coming through with answers to all my questions, gdavis, thanks!

Ha, ya, I've been through these same questions myself while working on my synth project, so I've put a fair amount of thought into it already.

Quote:

Quote:
It would be interesting to see it used. Might even be fun to try and make a small module that can be used more or less as a simple drop in replacement for a resistor.


Something like this? http://www.doepfer.de/A1019.htm

I saw that a few months ago and was wondering what one might use it for, aside for development purposes.


Actually I was thinking of just a small board, like your idea in the small utility circuits thread, that could be hacked into any existing circuit where you wanted to make a fixed resistor voltage controlled. But otherwise ya, same idea except OTA based instead of vactrol.

For a moment there I forgot I was on the modular synth forum and that "module" already has a specific meaning here. Embarassed

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