Optical volume in crybaby shell, questions regarding LED control and design

[Myampgoesto12]

First off, I'd like to take my treadle controlled tremolo to a new "level"-- heh. I'd like to use the stock wah pot as the scratchless wah control, to manually control volume if possible,( on to that at the bottom of the post), in a crybaby shell, and with some switching also as the rate control for an EA trem kit ill get at a later time.

On to LED/LDR volume control. Is there a reason that the Little Aligater uses a voltage divider composed of LDRs ( just like a potentiometer), instead of a single high ohm LDR directly to ground, such as various diy tremolos out there?? Also in this particular circuit there is a third LED placed physically behind the LDR in the signal path, not the LDR going to ground, why is that there? A means to make the resistance as low a possible to allow more signal to flow when at max volume position?

Here's the schem



Why not use just a single LDR with an led dimmed by the shutter? Would that work as well or as predictable as the voltage divider LDRs?

[PRR]

You need TWO variable resistors for good work. With one, you either have poor cut-off or poor turn-on.

The sweep of an LDR is not quite right for a pot. I bet the additional LEDs bias the mid-point of the attenuation range.

[[WZ]]

Is there a reason that the Little Aligater uses a voltage divider composed of LDRs ( just like a potentiometer), instead of a single high ohm LDR directly to ground, such as various diy tremolos out there??

I've built a few DIY tremolos and the LDR in those builds is always buffered, maybe that makes the difference? Maybe since this is all passive it's more sensitive?

You need TWO variable resistors for good work. With one, you either have poor cut-off or poor turn-on.

The sweep of an LDR is not quite right for a pot. I bet the additional LEDs bias the mid-point of the attenuation range.

About the sweep, that makes sense I guess; the 3rd LED isn't affected by the pot, so it's always on, and prevents a choppy transition of On to Off?

But, intuitively I would say that a single LDR can provide a high enough R to easily kill the whole signal, and at brightest it should hardly reduce the signal...
So, could you explain more why that wouldn't work? (Sorry, bit of a noob still)

[Myampgoesto12]

You need TWO variable resistors for good work. With one, you either have poor cut-off or poor turn-on.

The sweep of an LDR is not quite right for a pot. I bet the additional LEDs bias the mid-point of the attenuation range.

About the sweep, that makes sense I guess; the 3rd LED isn't affected by the pot, so it's always on, and prevents a choppy transition of On to Off?

But, intuitively I would say that a single LDR can provide a high enough R to easily kill the whole signal, and at brightest it should hardly reduce the signal...
So, could you explain more why that wouldn't work? (Sorry, bit of a noob still)


[/quote]

[WZ], do you mean by using a single LDR in series with the signal In and Out?, I NEVER thought of that, I guest its because I'm new to using LDRs and accustomed to pots that the idea of using an LDR to ground is all that stuck out to me.. I may try this, but I'm not sure my two diffused LEDs would be bright enough to yield a low enough resistance for unity gain with a series LDR.

I did a little googling looking for other options, how about using the wah pot as a voltage divider , 9v to the wiper and two LEDs, one on the CW lug and the other on the CCW lug, and each LED be a part of a diy vactrol. The vactrols could be the two sides of a voltage divider (as in the Little Alligator), but the dimming would be from voltage reduction instead of a shudder. I'm just thinking of ways to eliminate light contamination since the wah enclosure has a gaping hole for the rack. I bet I'd have to reduce the pots value a bit to keep from the "off points" from being to soon in the treadles movement. I guess parallel trimmers on each end of the wiper meeting at the wiper...

Thoughts?

[PRR]

> I would say that a single LDR can provide a high enough R to easily kill the whole signal

Not if the load resistance is *infinite*.

Not well, if the load resistance is very large. (A common thing in guitar-chains.)

And if the load is small, the attenuation is much higher. "Same foot angle" is very different volume cut.

It is a simple voltage divider. Draw it out, assume some values, and figure the attenuations.

[[WZ]]

[WZ], do you mean by using a single LDR in series with the signal In and Out?, I NEVER thought of that, I guest its because I'm new to using LDRs and accustomed to pots that the idea of using an LDR to ground is all that stuck out to me.. I may try this, but I'm not sure my two diffused LEDs would be bright enough to yield a low enough resistance for unity gain with a series LDR.
Thoughts?

If you buffer it you can compensate for volume loss.
The Tremulus Lune (schematics are all over the DIY forums) is a good example of how an LDR in the audio path could work:
Input signal >> Buffer stage >> LDR >> Buffer stage (w/ trimpot for gain, to make up for the little loss in signal) >> output

Works like a charm. I've used that thing as basis for lots of other stuff too.
But there's not one 'right' or 'best' way to do the same thing, so try stuff, see what works for you (and listen to guys like PRR with +10K comments; they probably know more than me, hah!)

Not if the load resistance is *infinite*.

Not well, if the load resistance is very large. (A common thing in guitar-chains.)

And if the load is small, the attenuation is much higher. "Same foot angle" is very different volume cut.

Yeah I didn't really get most of this, sorry.. I googled "Same Foot Angle" but got a bunch of weird x-ray results of feet...
I'll start by reading up on Load Resistance (not the same as Impedance then?).

[Myampgoesto12]

How about using a simple boost circuit like an LPB1 that's tuned for less boost, maybe with an additional volume pot at the output, to limit the boost to ~unity when the treadle is all the way to the toe position?

[Myampgoesto12]

Wow that totally worked. Just built an LPB1 with a 1m to ground at the input( I've seen schems without) and added a 100k trimmer as a variable resistor in series at the end to dial it back, ~50k, set the wah pot to max volume with the toe down and this thing rocks hah. The taper is weird as expected. Basically crunched at the heel down position. But is usable.

Gunna try and find a 100k audio taper wah pot now hah.

This is placed between my comp(which is set to tame volume a bit for the sake of my phaser) and my chorus. No clipping that I can hear, it may be clipping TECHNICALLY speaking but its a pleasing tone that pretty much matches the bypassed tone.

Not bad for a shot in the dark. May eventually try the whole tremolo in this thing to, but that'll wait until I get the kit.

(Edit)

Just added a 100k resistor to the wah pot to play with the taper a little bit. Went a long way towards making this thing feel better. Adjusted for the lower output by decreasing the trimmer I added. Seems like its in good shape!

[Myampgoesto12]

So I've been looking for a 100K pot with the right taper, even started another discussion directly about wah pot taper and I may have found a solution.

The 470K Hot Potz has an audio taper and functions properly in a wah shell(except for reaching 0 volume) - as far as the taper and the direction of rotation. So if I were to simply use this pot with this LPB1 i believe the output would be far too great. I started digging for parallel resistance solutions, though I know that simply adding one resistor to the CW and CCW will lower the total resistance of the pot, it would dramatically change the taper.

Going by this calculator




To lower a linear pot's value and still retain a linear taper you add two resistors according to the schem above the calculator.

Will this work for a log or audio pot as well?

If so i could simply use two 100K trimmers dialed to the values calculated above or even do further adjustment to decrease the total value as needed.

Thanks

[pinkjimiphoton]

that works by changing a pot from linear to log or anti log. sorta. in some cases it will work well, in others... not so much.
go to geofex.com also and read rg's most excellent treatise, "the secret life of pots"

glad ya found joe's calculators page, that thing continues to save my ass!

[Myampgoesto12]

Ok, so its worth a shot.  I guess I could just use the 470k and further tame the signal in other ways and see what happens

Edit:
I have read the geo article before, it helped a lot int understanding how to user tapering resistors, but I haven't seen 2 resistors used on a audio taper pot in schem form.

[pinkjimiphoton]

thats cuz audio taper is already tapered. thats why you use a 100k linear pot, and use the parallel resistor calculator to see what ya need to make that pot another taper and value.

a linear pot is a reasonably straight line from 0 to 11.

an audio taper has a big hump on it so that it corresponds well to the human ear. good for tone controls, ok for some fuzz pots.

a reverse log/audio taper is better for some purposes, and will have the "longest smoothest sweep" generally.

you can make a linear pot into a log or antilog, i don't know if you can make an anti log or log linear.

[Myampgoesto12]

Right on.

I wasn't trying to make log into linear, just making log into a smaller value log.

But if I can get a hold of a antilog 100k for this unit then I'll do that hah.

Thanks

[pinkjimiphoton]

well, ya gotta play with tapering resistors then, most likely.
say its a100k pot

a resistor of 100k across it will make it a 50k pot. kinda. but you can adjust the sweep. say ya use a 33k resistor between wiper and pin one, and a 27k between wiper and pin 3... maybe vice versa... you see where this is going.

check your pm's for mr gagan's email