So THAT's where I left my nose!

[Mark Hammer]

Right in front of me.

Sometimes, you overlook the simplest and most obvious things.

I was coming home on the bus just now, and browsing through my tablet to find somethng to amuse myself with.  In a directory that I did not expect to find it in, I stumbled onto a schematic for the Morley Pro Series Volume, which you can see here: http://www.morleypedals.com/pvoes.pdf

It uses a simple, but brilliant, reciprocal LED/LDR to produce a virtual volume pot.  Basically, the foot treadle sweeps the pot.  As the resistance on one side of the wiper gets smaller (making the LED glow brighter and the LDR get smaller in resistance), the resistance on the other side of the wiper gets larger, dimming the LED it feeds, and increasing the resistance of that corresponding LDR.  The whole thing works as a scratch-proof pot.

The possibilities are endless for using this approach.  Imagine that you have one of those op-amp based wahs that use a symmetrical pair of caps in the feedback loop, with a resistance to ground at their junction, like this one: http://www.morleypedals.com/pwa.pdf

The led that gets brighter in the toe down position (resistance gets smaller) is used to sweep the wah filter upwards.  The led that gets dimmer in the toe down position can be used to increase the feedback resistance of the op-amp (R6 in the referred to schematic), so as to increase the gain and raise the volume a bit as the sound gets thinner.

If one had a phaser with variable speed and resonance/feedback, imagine that the toe-down position makes the resistance that sets the speed smaller (therefore faster speed), but the resistance that sets the resonance/feedback larger (therefore less feedback).  Pull the treadle back and you have a nice slow sweep with lots of resonance/emphasis.  Push it forward and the resonance is dialed back where it needs to be for bubbly sounds.

And so on.  Anti-wahs, and all sorts of mayhem.

The scheme also permits one to have more than one LED/LDR go in one direction while a different one goes in the other.  It does not require that identical LEDs or LDRs be used on each leg of the pot.  Moreover, one can adjust how much change in illumination (and corresponding change in resistance) is created via pot movement, by means of tailored parallel and/or series resistors.

[pickdropper]

That's a really fun idea.  Seems like there could be a bunch of fun had with that.

[Morocotopo]

Very very smart. The people who designed it and you for noticing...

I´ll keep the idea in my computer folder named "smart Mark Hammer tricks", right next to the SWTC.

[electrosonic]

Reminds me of a DIY Hifi attenuator - google "Lightspeed Attenuator"

Similar idea, but uses a double pot to control the leds.

Andrew.

[RickL]

I don't think the pedal works the way you've described. The pot marked "min vol" is not swept by the pedal. It is a user settable pot that sets the minimum volume that the pedal will output in the down position. I think the treadle changes the amount of light that falls on LDR1 from L1. LDR1 and LDR2 do form a virtual pot, set by the brightness of L2 and L3 but the treadle only changes the the resistance of the upper part of the vitual pot.

The idea you've described should still work, but I think you invented it, not Morley.

[jubal81]

Neat stuff. Concerned about getting the right LDRs for output impedance and how well matched they need to be.

[armdnrdy]

This is the same idea that is implemented in the Mutron Flanger to manually control the sweep. As the pedal moves toe down a parallel resistance changes in a voltage divider who's output is sent to the 4046.

It's no coincidence that we're using the Morley optical volume enclosure for the Mutron clone. The enclosure already has a slot for the shutter, and a pedal! That's half the battle.....well except for getting the flanger part going. 

I too have ideas for this technology......adjustable resistance expression pedal. One size fits all.....or maybe one size fits many?

[amptramp]

There was a patent in about 1966 (so it is now public domain) for a pot that had two concentric resistance rings with contacts brought out at opposite ends and a metal ring between them.  Between the reisistance rings and the metal were two photoresistive layers.  The pot shaft was hollow with a light on the rear end and a double prism mirror to reflect light on the photosensitive layer.  When you turned the pot, the changed the place where the light from the lamp hit and that connected the resistive layers and the metal ring (which acted as the slider) together via photoconductance.  You had a noiseless pot.  The problem was the need for an incandescent lamp that was a limited-life item.  Modern LED's would make this work.

[kaycee]

Sounds somewhat like RG's Wah Pot descratcher idea? Takes the audio off of the pot and uses an LDR combo to control the sweep. I implemented it on a wah I had a while back, worked well IIRC. It's on Geofx somewhere he added helpfully...

[mac]

You can get rid off pots by varying the distance between a LED and a LDR with the wah pedal.

Or by blocking with your foot the light of an external fixed lamp (or room light) hitting a LDR.
I've done this one while experimenting with Tim Escobedo's photon filter,



mac

[Mark Hammer]

I don't think the pedal works the way you've described. The pot marked "min vol" is not swept by the pedal. It is a user settable pot that sets the minimum volume that the pedal will output in the down position. I think the treadle changes the amount of light that falls on LDR1 from L1. LDR1 and LDR2 do form a virtual pot, set by the brightness of L2 and L3 but the treadle only changes the the resistance of the upper part of the vitual pot.

The idea you've described should still work, but I think you invented it, not Morley.

Gadzooks!!  I think you're right.  I suppose that's what happens when you read a schematic rotated 90 degrees! 

I have a bunch of things to attend to before we head out of town this weekend, but I will implement this on our return.

[deadastronaut]

^ cool looking forward to see what you come up with...


i love an led/ldr experiment..

http://www.youtube.com/watch?v=CPhrmxm999Y

http://www.aronnelson.com/gallery/main.php/v/chickpea/darklight2.jpg.html

[tubegeek]

One question from the schematic: the parts spec says "LDR1, LDR2, LDR3" but I only see two LDR's on the drawing.

What do you suppose is up with THAT?

My reading of what's going on is similar to RickL's - so I was wondering if there were a mechanical shutter, thanks for the details, armdnrdy!

Otherwise it's VERY unclear what the purpose of LED L1 is. The parts spec makes it clear (I think) that L4 is just a power indicator.

I have another question, though, for armdnrdy and digi2t.

What was the setup with the shutter on your pedal when it was a stock volume pedal? Is there a reversed pair of slots for L1 & LDR1, versus L2 and LDR2? So one slot goes narrow to wide while the other slot goes wide to narrow? That would make sense based on the schematic, I think. Then the adjustment of P1 would handle the minimum volume when LDR2 was low resistance (when LED L2 was shining brightly through the widest part of the slot.) At that moment I think there would be a bit of light shining on the other (dark) LDR1 from LED L3 at the back of the shutter (?) even when LED L1 is at its darkest, set by adjustment of pot P1. The drawing that shows the positions of the L1 and L3 seems to show that happening if I am reading it as it is meant to be read.

No question this is a little confusing without seeing a gut shot!

[armdnrdy]

Here's some images to help you better understand what the circuit is doing.


Stock shutter:




Gut shot of the board:



Pedal toe down:

[tubegeek]

Here's some images to help you better understand what the circuit is doing.

Thank you!

So - at the very least, it confirms that the 1's and the 2's are meant to go opposite to each other, OK, good.

Any idea where L3 is?

[armdnrdy]

Any idea where L3 is?

Look at the second image....all the way to the left. L3 is the indicator "light".

[~arph]

This seems like a relevant article here:

http://www.thetonegod.com/tech/rockncontrol/rock.html

[armdnrdy]

This seems like a relevant article here:

http://www.thetonegod.com/tech/rockncontrol/rock.html

Not really relevant.......these circuits use pots as a control instead of a shutter. I have bread boarded the Tone God's circuits...they don't work as advertised!

While testing for the Mutron Flanger build.....I have seen great promise in the shutter/LED/LDR combo.

[~arph]

Not really relevant.......these circuits use pots as a control instead of a shutter. I have bread boarded the Tone God's circuits...they don't work as advertised!

Well, I was going by the applications Mark posted.

[armdnrdy]

Everything on this page, including the links that Mark posted are utilizing a "virtual pot" made up of a shutter/LED/LDR.

No physical pots.