Using LED/LDR instead of Potentiometer for Expression Pedal

[PKo]

I'm trying to build a volume pedal in an old Crybaby chassis.  I've heared a lot about these optical wahs that use an LED / LDR combo rather than a potentiometer.  Sounds pretty cool.  BUT...  how do they vary the voltage going to the LED to change the resistance of the LDR?  does it still use a potentiometer before the LED?  any help would be most appreciated.

Thanks!

[NaBo]

hmm... yeah...  there would have to be some type of variable resistor controlling the brightness of the LED.  IINM, the reasoning behind using an optoisolator is mainly to get the pot out of the signal path, so theres never gonna be any "scratchiness", not replace it outright.

[Alex C]

In some Morley volume pedals (I'm not sure how many) an LED or even an incandesant bulb is lit at a constant brightness, and the foot treadle moves a small "shutter" inside, which blocks light going to the LDR.  This was the case in my old chrome series volume, and I think even some of the more modern volumes and even the Power Wah use this setup.


Alex

[Paul Perry (Frostwave)]

hmm... yeah...  there would have to be some type of variable resistor controlling the brightness of the LED.  IINM, the reasoning behind using an optoisolator is mainly to get the pot out of the signal path, so theres never gonna be any "scratchiness", not replace it outright.

That isn't quite right.
The idea is, have a led going at a constant rate, and then (as Alex C says) you block some of the light going to a LDR. And the Light Dependant Resistor replaces the variable pot. Always takes some dicking around to get the physical relationship of led/shutter/ldr right, but worth it! and you can have ANY TAPER YOU WANT by shapinbg the shutter! Hell you could do TWO of these for an optical 'talking wah'!!

True, Craig Anderton did do a foot operated pot that varied a led to give a scratchless volume control, which is why the confusion arises.
Oh, while making an optical wah, be sure to shield it from the light!!

**BONUS POINTS: instead of a plain led, have an LFO (or an envelope follower!) drive it, then you have a combined autowah, envelope follower, pedal wah!! W0000aaoo00ooauuoo000ooahowhaao time for the tablets :oops:

[KB]

PKo

Try this link

http://www.morleypedals.com/downloads.html

download the pro series volume PVO.  I have no idea what make of LDR that Morley use.  As I have no Morley pedels, I cannot say about shutters and such.     But looking at the schematic L1 is on constantly and the brightness of L2 and L3 would vary.

Hope this helps

KB

[NaBo]

ahhhhh, i see.  i was thinking just a simple vactrol unit when he said "LED/LDR combo"!  that shutter thing sounds more DIY friendly than the average wah... i bet you could rig it up without using a wah shell at all.  hooray for thrift!

[gak]

i've cloned one of those a some days ago...
here i posted the report: http://www.diystompboxes.com/sboxforum/viewtopic.php?t=28558

the case is also diy, it moves a piece of metal blocking or allowing the light to affect the ldr
i've also built a volume pedal using the same system

the only problem i'm having with them both is that the LDRs i have need a very strong light to achieve a low resistance... i measured about 50k while my original morley is about 10k...

i have to buy a few ones and see if any works better...