Can I use a pot for speed and variable voltage?

[Michael2]

Hi all,

Can someone help me out please?
There was not enough room in the subject line to explain it well.

Here  is what I'm trying to do:

I am building a forum vibe (univibe) in an old volumepedal. I made a foot switch in the heel to select between univibe and volume. I swapped the original pot for the dual reversed 100k pot to adjust the speed of the univibe with my foot (it was a pain the a** to make it all fit). It works great. Since the original pot is gone I decided to make the volume feature optical. I placed a led, ldr and shutter. It works but for some reason not fully as desired. (I just cannot get the shutter fully cover the led or the led is not bright enough when I adjust the voltage with a dedicated trimpot, also the trimpot for the sensitivity of the ldr is not giving the best results).

Long story short: I was thinking that it would be great if I could use the speed pot also as a voltage divider for the led. I have no rows left on the 3pdt but a 4dpdt will arrive soon which give me an opportunity to do something with it to make it happen. Maybe power the pot/bypass one of the signals to the univibe? But then my question is: will the power that flows through the pot harm the univibe since the two leads that come off the pot go into the Univibe? Or is there a better solution? I have the feeling that I'm almost there. Still a noob so please bare with me

Btw: the pedal is running on 18v and I already have a buffer built in for the volume part.

I hope someone can help.
Thanks!

[antonis]

LEDs like to work with DC - Volume pots don't like it at all..

It's not going to be any damage but you'll "add" a variable DC offset on Volume output..
(pot prone to crackling..)

There are solutions for "isolation" (LDR/FET or CMOS) but they require space...

[Michael2]

Thanks Antonis!

Do you mean it will damage the pot at the end or that the audio will come out crackled through the LED/LDR volume setup?

There is still some space left in the enclosure, LDR/FET sounds promising, do you have an idea where I can find a schematic?


 

[antonis]

It will not damage the pot (as long as pot's power rating is higher than power consumed on it - some tens of mW in your case..) but the volume will come out crackled..
(like in case of heavy leaking output cap..)

I'm not experienced with LDRs/FETs "isolators" (never used one..) so I presume you have to wait for some other willing guy to come and clear up things..

[Michael2]

Alright, do you know where the crackled audio will be coming from? I'd like to understand :-)

BTW: the LDR is audio wise "isolated" from the LED and if the power to the pot is bypassed with the 4PDT switch when the Univibe mode is active it will not interfere with the Univibe, am I right?

The setup for the 3PDT atm is:
1. row of lugs: input to univibe/LDR
2. row of lugs: output from Univibe/LDR to pedal output
3. row of lugs: Cancel mode and ground connection to on/off led

Thanks for your help, I appreciate it!   

[antonis]

the LDR is audio wise "isolated" from the LED and if the power to the pot is bypassed with the 4PDT switch when the Univibe mode is active it will not interfere with the Univibe, am I right?

Now you lost me..

Do you mean to completely take off Volume pot from Unibe, via switch mode..??
(if yes, it should be fully disengaged - meaning all of three lugs..)

Or maybe I don't completely get your initial query..

[Kipper4]

Can we see a schematic of what you're trying to do?

Can you put a cowell over the led so it's more directional, to avoid unwanted ambient light within the box?

[Michael2]

Thanks guys,

good idea to draw it out, I'm not good with schematics but here is a drawing of the way it is hooked up now and verified to be working. A drawing of what I had in mind follows.
Sorry for the clumsy drawings and maybe the wrong terminology, as said I am a noob and English is not my native speech. I hope you can make something out of it 

@Kipper: I have tried everything regarding shielding/focussing the led and LDR, it is located in the hinge of the rocker for now. It is the best spot since totally dark but still not fully functional.

[Michael2]

And this is what I had in mind, totally not sure if I picked the right lugs of the pot and if this will be working in general...

[Michael2]

Anyone?

I have found a schematic by the way. It's at page 8:
http://basicaudio.net/univibe-ForumVibe-final.pdf

Thanks!