LDRs as pots?

[tbryanh]

Here is a touched up version of the LDR Variable Resistor.

[tbryanh]

For the LDR Pot (Figure 3), I am thinking that maybe the feedback resistor and capacitor should be in series rather than in parallel. (See items 26 and 28 and items 27 and 29).

I am just guessing, but I was looking at some other op amp circuit for a power supply regulator, and it showed the capacitor and resistor in series for what appears to be a similar situation.

[Don Moose]

For the LDR Pot (Figure 3), I am thinking that maybe the feedback resistor and capacitor should be in series rather than in parallel. (See items 26 and 28 and items 27 and 29).

I am just guessing, but I was looking at some other op amp circuit for a power supply regulator, and it showed the capacitor and resistor in series for what appears to be a similar situation.

Certainly seems reasonable - like I said, I'm out of my depth there.

Do you have a SPICE program you like? (SuperSpice by Kevin Aylward is a pretty nice one) Since I move to Mac, I haven't found one that really works for me.

[tbryanh]

I never used SPICE. I went to the website and checked it out. A circuit simulator.

http://bwrc.eecs.berkeley.edu/Classes/IcBook/SPICE/

[tbryanh]

For the LDR Pot (Figure 3), I am thinking that maybe the feedback resistor and capacitor should be in series rather than in parallel. (See items 26 and 28 and items 27 and 29).

I am just guessing, but I was looking at some other op amp circuit for a power supply regulator, and it showed the capacitor and resistor in series for what appears to be a similar situation.

Now that I understand the other op amp circuit better, I think that for the LDR Pot, the feedback resistor and capacitor should be in parallel as shown. this setup should be added to the LDR Variable resistor too.

[Gregg]

Hi,

After I built that LDR pot /answer #63/ for testing purposes and did some DMM measurements to see if it's going to behave like a real pot I finally found some time to test it in a real tube amp circuit. I connected it as Gain pot in a typical SLO100/Mesa DR type of high gain preamp. The result? It works. First I didn't like the response of the pot but then realized it was a linear one. Changed it to log pot and it worked OK. It was set to be a 1M pot. At center position it reads 102k which means it's a 10% taper log pot. I'm planning to run some measurement to see how close to a log pot this LDR pot behaves but that will be just out of curiousity because I'm satisfied with it's response. After all this is a guitar amp not a Hi-Fi project.
I ran the circuit without the time correction filter in the feedback loop of the opamp without any side effects that I could notice. No delay, no oscillation or anything else that I could hear. I will test it in the clean channel as well to make sure no side efects are present.
For this test I used Perkin Elmer's VT935 series of photocells which along with other types I got from Allied. There are other better candidates for this job to be tested.
Because when ON these photocells measure 1.5-2k there's still some signal leaking out when pot is at zero position. This can be fixed by using in the lower position other type of photocells with lower ON resistance - some Vactrols for example.
However I'm having some doubts about using these LDR pots in a regular Marshall type tone stack. Although they are rated at 100V peak I'm not sure how will they handle the voltages in a real situation. Mesa used them after the first stage where the voltage swing is not high unlike the Marshall TS where I measured ~60Volts across the MID pot /when fully open/. I gues the best way to find out is to use them in a real TS.

[tbryanh]

Excellent job. I would like to duplicate what you are doing. If you didn't mind posting a schematic and parts list, that would be cool.

Thanks

[Gregg]

This is how the current schematic looks like:

[tbryanh]

Thanks for posting the circuit. Very brilliant. I’ll be spending some time to fully understand it.

I was wondering if you had a split supply (+ 15v and –15v) for the Op Amps, or are you using a single-ended (+ 15v only) supply?

I am curious what you are using to draw the diagram? It looks real good.

I modified your circuit a little bit (see attached) to show the idea of using the Perkin's Elmer VTL5C Series Vactrols. The series has Vactrols with on-resistances that go well below 100 ohms. Some of them can handle up to 200v too. http://optoelectronics.perkinelmer.c...toisolator.pdf

I hope you don't mind me modifying your diagram a little bit. Great circuit you made. Thanks again for posting it.

[Steve Conner]

My Magic 8-ball says "Don't connect LEDs in parallel, series is better"

[Gregg]

Thanks for posting the circuit. Very brilliant. I’ll be spending some time to fully understand it.

The circuit is actually not mine - what I did is just to put the pieces together.
I think the schematic for a Gain/Volume control using a VTL5Cx/2 series Vactrols should look like this:



I used +5V single power supply.

The schematic is drawn with sPlan. Check out also Sprint Layout for PCB drawing:

http://www.abacom-online.de/uk/html/produkte.html

[Don Moose]

I modified your circuit a little bit (see attached) to show the idea of using the Perkin's Elmer VTL5C Series Vactrols. The series has Vactrols with on-resistances that go well below 100 ohms. Some of them can handle up to 200v too.

2 gotchas here -

1) as Steve mentions, diodes (including LEDs) don't parallel well at all, you need to put the LEDs in series (and use a higher supply voltage on the OpAmp) or give each LED its own series resistor.

2) The LED datasheets I've looked at allow a 3:1 range in brightness for a given current. Unless the P-E LEDs are matched, you're going to have to match your own pairs.

Otherwise, well done and best of luck!

[tbryanh]

2 gotchas here -

1) as Steve mentions, diodes (including LEDs) don't parallel well at all, you need to put the LEDs in series (and use a higher supply voltage on the OpAmp) or give each LED its own series resistor.

2) The LED datasheets I've looked at allow a 3:1 range in brightness for a given current. Unless the P-E LEDs are matched, you're going to have to match your own pairs.

Otherwise, well done and best of luck!

For 1) Yeah, I didn't think about that much. I was just getting a concept out quickly. Thanks for the finer points.

For 2) Yeah, I was figuring you would have to match your own pairs. I was not thinking there would be a 3 to 1 range though. Ouch! Hopefully the Vactrols are tighter than this.

[tbryanh]

The circuit is actually not mine - what I did is just to put the pieces together.

Yeah, well thats still really cool anyway, and thanks again for a great effort.

[tbryanh]

2 gotchas here -
2) The LED datasheets I've looked at allow a 3:1 range in brightness for a given current. Unless the P-E LEDs are matched, you're going to have to match your own pairs.

Maybe if the diodes are kept in parallel each with their own series resistor as mentioned, the values of the series resistors could be tweaked to try and make the diodes match.

There is also the issue of the light dependent resistors matching too.

I am thinking that the matching may not need to be real precise. If they are close, it is probably good enough.

If you are Mesa Boogie and you want all of your Triaxis's to be interchangable with each other, then precise matching might be needed. But for the typical one-of-a-kind amps we make, precise matching is probably not be needed.