Ave Crux

Substitute dual pot for LDR?
« on: June 14, 2018, 01:37:01 PM »
Hi folks, newbie question here -- can't find it anywhere else. But is there a way to substitute a dual potentiometer with a couple of LDRs?

It's much easier with just a single potentiometer--but somehow its just going over my head how I would do it with a dual pot...


JohnRoberts

Re: Substitute dual pot for LDR?
« Reply #1 on: June 14, 2018, 02:33:28 PM »
an issue trying to cover a dual pot will be tracking between the two LDRs.  Back in the 70's I did some bench work trying to linearize LDRs and found that I could use a center tapped LDR with the mid point grounded and use 1/2 in a NF loop to control the other half...

 It worked but was too complicated to be practical.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Ave Crux

Re: Substitute dual pot for LDR?
« Reply #2 on: June 14, 2018, 04:05:12 PM »
an issue trying to cover a dual pot will be tracking between the two LDRs.  Back in the 70's I did some bench work trying to linearize LDRs and found that I could use a center tapped LDR with the mid point grounded and use 1/2 in a NF loop to control the other half...

 It worked but was too complicated to be practical.

JR

hmm... i suppose i am not entirely concerned on making them linear -- i think to be more specific, i am trying to build this resonant lpf made by a member in a different forum (unable to properly register so i couldn't send the person directly a message)...

i will post the schematic, so you could see what i mean with what i am trying to replace; i think i'm trying to do some experimentational application on the cutoff (circled on the schematic) -- just mainly concerned on getting it to be controlled by a light source



JohnRoberts

Re: Substitute dual pot for LDR?
« Reply #3 on: June 14, 2018, 05:41:20 PM »
By linear I mean that you can control their resistance. If you don't care that the they wildly mis-track each other ignore my advice.

JR

[edit] disregard my suggestion, that circuit will not support one end grounded anyhow. [/edit]
« Last Edit: June 15, 2018, 09:22:56 AM by JohnRoberts »
John Roberts
http://circularscience.com
Tune it, or don't play it...

squarewave

Re: Substitute dual pot for LDR?
« Reply #4 on: June 14, 2018, 08:13:39 PM »
Yes. Definitely a dual pot will work equally well. Better for sure actually. Not sure why an LDR would be used in this case anyway.

Temperature comp. bias on the second transistor is actually quite clever.

The circuit itself will have relatively high distortion though (desired for guitar I presume).

shabtek

Re: Substitute dual pot for LDR?
« Reply #5 on: June 15, 2018, 07:27:16 AM »
Each section of the dual pot is just 2 connections, so its a rheostat rather than potential divider,
That makes it a little easier, though tracking/resonance will likely be wobbly somewhere in the LDRs range
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

Ave Crux

Re: Substitute dual pot for LDR?
« Reply #6 on: June 15, 2018, 10:44:42 PM »
Hey folks, so just an update on this,

I actually ended up using 4 LDRs -- 2 two pairs, and each pair pretty much has "3 legs" (left, left+right, right)

hooked it up according from the schem to replace the dual pot, works really nicely.

Trying it with just one pair however, results in some strange behaviour.. and only one of the two ldrs actually did anything.

I know some of you must be confused on why it needs to be an LDR -- long story short, I just needed a (relatively) simple LPF with reso, and I found that this one can run off a battery. Gonna install an array of these LDRs to distribute onto a big panel. got something to do with sound art installations and stuff.

But thanks for the inputs though, it did help with triggering some ideas

benb

Re: Substitute dual pot for LDR?
« Reply #7 on: June 16, 2018, 03:32:45 PM »
I posted this yesterday or something, unfortunately it was in the wrong thread (!) ... hope this helps:

Yes, just substitute the two connections to each pot with an LDR.

The trick (and somewhat repeating what others have said) is to make the LDRs' resistances track. There's a lot of variation between LDRs (they'll have different resistances for the same amount of light), so you should start with more than two (maybe ten), and find the two that match best.

Second is making sure both get the same amount of light in the circuit. Offhand I'd say put them physically next to each other and put a translucent piece of plastic as a diffuser over them, maybe an inch above, and have the volume between the diffuser and LDRs be light-tight so no other light can get in.


 

Related Topics

  Subject / Started by Replies Last post
21 Replies
3540 Views
Last post December 08, 2004, 05:41:28 PM
by SmG
13 Replies
2554 Views
Last post September 07, 2005, 03:38:23 PM
by SSLtech
3 Replies
1032 Views
Last post July 29, 2011, 07:54:25 PM
by dude24man
6 Replies
1735 Views
Last post August 09, 2012, 12:19:10 AM
by mjrippe