a new kind of optical volume pedal?

[comfortably_numb]

My EBMM VPJr has recently gotten scratchy as hell.  I was looking at doing the Anderton Descratcher and it seems the consensus on that is it's too noisy with the 3080 chip. 

I then thought to hell with it, I'll just buy an optical and be done with it - BUT...

What if I used the pot in my VP Jr to control an LED/photoresistor combo?  Could this still be a passive audio circuit?  The photoresistor would serve the same function as the pot (variably shunting signal to ground), but be controlled by the dimming LED. 

Would this work?  Is it way too easy to be true?

[armdnrdy]

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

[comfortably_numb]

Wow!  Thank you.  So it is a LITTLE more complicated, but not much.  The three pole should work nicely - I'll just have to fill in some values.



Above is the schematic for a three pole Rock N' Control (Figure 5). Notice that the circuit still only needs two connections to the rocker pedal, ground and the control signal. You have to do the usually watch of Hfe, bias, etc. with the transistors. A description for each of the parts is as follows:

R1, R2: 250k each?  Equal to VPJr pot valueSets the maximum resistance of the optocoupler. It should be the same value as the orignal control.
R3, R4: 1k?Current limiting resistor for the LED.
R5: 10k?Minimum bias value. Trim 1, Trim 2: 10k trim here?Trim pot to set the maximum brightness of the LED. Used to balance LEDs brightness with each other.
Trim 3: 100k trim?Trim pot to set the bias point (center) for Q1 and Q2 so Pot 1 can sweep the full brightness of the LEDs.
Q1: PNP transistor, 2N3906. Any PNP transistor should work.
Q2: NPN transistor, 2N3904. Any NPN transistor should work.
Pot 1: Rocker pedal pot used to control the LED's brightness.

Of course the parameter controls go to audio circuit IN, OUT, and Ground

[armdnrdy]

Let us know how it works out.

[comfortably_numb]

I was wondering, if this can be done with a series (2 pole) vactrol by just varying the resistance between 9v and Ground, could it not be done with a 3pole by sending 9v through the wiper of the pot with each side going through a minimum current resistor and then the LEDs to ground on each side of the pot from the VPJr? 

Like so -

ground-----LED-----1k------
                                      |
                                     250k pot------ +9v
                                      |
ground-----LED-----1k------

The audio circuit would still be arranged like the 3 pole in the link above.  Is there really a need for the transistors?  Would a 1k on/10M off vactrol work for this?

[comfortably_numb]

Anyone?

[Mark Hammer]

In fact, I think Morley does this.  I started a thread on this last year or two years ago, I think.

[Quackzed]

i'd think that with just a 250k pot and 2 leds/max resistors you wont get any light from the leds till just about full on and just about full off...
by the time you get one to dimly light up the other will be off off. maybee with a 10k pot you'd get some interplay, but not sure if  that is doable...

[joegagan]

hotpotz2 from dunlop will not wear out in your lifetime. they come in 20k , 100k wah taper, 250k and 470k.

why hasn't ernie ball switched to plastic element pots? dunno.

[comfortably_numb]

So Quackzed is right.  With the 250k pot there is no middle ground.  It's on bright, then off.  It's like the pedal is a three position switch.

I tried paralleling smaller value resistors with the two halves of the pot - 250k all the way down to 10k without too much improvement, but it seems like it's the LEDs themselves causing the trouble.  With 10k in parallel with each half of the pot, the LEDs stay lit the entire time, but it's like they have discrete levels.  They light brightly at the extreme of the rotation but then dim by half or so and stay that way.  So it works, and the max volume is the same level as the bypassed volume, but the sweep isn't really useful.

These are cheap radioshack LEDs though.  Would a water clear high bright LED from smallbear have more of a gradual fade in/out over this sweep?

I may be looking at those dunlop pots, though it makes me sad to say so.  I was really excited about this.  Not giving up yet though.

[PRR]

> With the 250k pot there is no middle ground.

Why not a 10K pot?

[Quackzed]

i'd try a 10k pot, even a 5k maybee, the parallel resistor thing alters the taper alot as you really lower the resistance, so a 250k with 2 10k's will be like 10k 10k 10k 10k 5k 1k ish... not gradual. a 10k pot will be more evenly gradual.
i dont think led difference will matter that much as far as more/less gradual... it might, not positive.
but using a 250k with low r tapering resistors will prevent a gradual resistance change, so i'd start there.
hotpots 2 sounds like an easier fix, but less experimental and fun!   
 

[comfortably_numb]

For some reason I was avoiding having to change the pot.  Not like it's terribly difficult I can't imagine.  Morley uses 100k.  I'll have to see if a standard pot will fit, but I doubt it.  I'll try it with what I have before ordering anything specific to the volume pedal. 

Thanks for the "duh."