Use optocouplers to make a 9-gang pot?

[jubal81]

I posted a question about this on DIYstomp, but I thought I'd put it to you guys, too.

I'm planning a big project where I need to control 3-4 boards with the same 3 pots.

After a lot of research, I've discovered there's not going to be an easy answer to this, especially since I wan't to keep it analog.

The best idea I've come up with so far is to use each pot in a voltage line to control a series of circuits with octocouplers. Similar to what TGD did with his Rock 'n Control: http://tech.thetonegod.com/rockncontrol/rock.html

My thought is that if I daisychain a series of Rock n' Controls I can create a virtual 9-gang pot.

Does this have a shot of working or am I nuts?

Also, anyone have an opinion on these cheapie Octos?
http://www.futurlec.com/Photocells.shtml
http://www.goldmine-elec-products.com/prodinfo.asp?number=G15396&variation=&aitem=1&mitem=1

Sure, I could easily make my own but ... 99 cents!

[JakeFuzz]

Are you trying to control 3 parameters simultaneously in 3 different circuits? Are all the circuits going to be running at the same time or only one at a time? I am assuming you want them to run simultaneously. I think the photo-coupler would be an okay way to go. You would probably have the three controls set up as variable base bias resistors for 3 LED's/drive trannies which would be wrapped with 3-4 LDR's which go to their respective circuits. The problem with this is you will only get the variable resistance action of a standard pot but not the fixed resistance action which many circuits rely on.

Also check out things like packaged photodiodes/triacs/transistors (the 4N series). I have used the 4N35 in somethings and they're super cheap!

[jubal81]

The setup I've outlined would control all the boards at the same time, but I'd use a rotary switch to control which one of the three boards is in the signal path.

For the most part, the things on the board I need to control act as potential dividers and not simple variable resistors. If one of the controls happens to need a simple variable resistor, then that's a LOT easier to set up.

The only schematic I've found that seems to do what I want is the Rock n' Control. I don't know how to get a better setup using photo  diodes/transistors, but I'll look at anything to somehow do this more simply with fewer components.

[JakeFuzz]

Hmm, I see what you are doing. I think Tonegods method is going to be your best bet, but driving 18 LED's from the same power supply might not be ideal. Another idea might be using digital potentiometers, You could set up a simple voltage divider pot (or even a rotary encoder  ) for A2D conversion in a simple microcontroller which interfaces with as many digital pots as you like. They even make 4 channel digital pots but I think they are all the same resistance values. The pot control is digital but the signal path remains analog. If you're really good at programming you could even program in different pot tapers that you could switch between on the fly using a wave table, that would be cool!

[jubal81]

Just been looking up digital pot chips and ... 

Looks like a great solution, but I have no idea how to control them or program them.
Got any links to educate myself?

[jubal81]

Well I found these modular pots, but they're $10 each and I don't know how many gangs come with them.

http://www.mouser.com/ProductDetail/Vishay-Sfernice/P11S1V0FLSY00504KA/?qs=sGAEpiMZZMuarXojuOTsc%2f2bhTU8JDi5GY%252bgZkyQdqk%3d

[JakeFuzz]

Well I found these modular pots, but they're $10 each and I don't know how many gangs come with them.

http://www.mouser.com/ProductDetail/Vishay-Sfernice/P11S1V0FLSY00504KA/?qs=sGAEpiMZZMuarXojuOTsc%2f2bhTU8JDi5GY%252bgZkyQdqk%3d

That looks exactly like what you need. I didn't even know they made those. Looking at the datasheet I am assuming "modules" means the number of resistive elements/decks? So you can get up to seven with one pot it looks like. That's the way to go if you can find someone who stocks them. Another way would be to disassemble a few pots and see if you cannot somehow make the elements and traces run off of a common shaft whether it be a new custom shaft or tacking the existing shafts to each other. Ill crack one open and see when I get home. 

[JakeFuzz]

Hmm scratch that last idea. I found one and opened it up. Mechanically it is possible but wouldn't be very easy. You would need to make a frame which secures the three resistive trace platters while a central square shaft rotates the wiper elements. You would also need some kind of collar to hold down the wiper elements to their respective platters. That wouldn't be a fun project.

[jubal81]

I think I found a cheap way to try out the octocoupler idea.
I found these lots of 20 for $2 each - untested grab bag kind of thing.


http://www.goldmine-elec-products.com/prodinfo.asp?number=G17898

I also got a pile of cheap transistors and trim pots. I think the total came to $12 or so.

I'll play with them and see what I can make. If nothing else it's a pretty cheap toy to experiment with and learn a bit since I don't have much experience using optos.

And if it doesn't work, I'll probably try some cmos switching.

[rullywowr]

Just curious how you made out with these optos?  Do you think they would work in the Tremulus Lune circuit?

-B