Idea for small stone heads... Photodiodes

[ExpAnonColin]

I've been pondering small stones, as always.

What if you were to get the LFO's normal maximum rating (what was it, 1mA?) by a simple voltage and resistor circuit...  I=V/R so a 9v supply and a 10k resistor.  Then, wire it in series with a photodiode...  Photocouple it with an LED, make a CV to LED driver circuit, and wham, you've got a CV input.  As the LED shines, the photodiode lets more of the current flow... as it fades to black, no current flows.  With some tweaking to make sure the output of the photodiode is still proper for the OTA (and still below the maximum rating!) you could easily then adapt it with all of the various normal old sequencer, LFO, envelope controls, theremin controls whatever voltage control you could think of to use as an LFO instead.

Someone correct me before I get too excited  :|

-Colin

[gez]

It's just a lot simpler to use a constant current source.  A simple p-channel MOSFET (plus source resistor) works well, just vary voltage to its gate.  

There are all sorts of options that are a lot simpler than what you suggest.  Check out some schematics for OTA filters (voltage controlled).

[puretube]

I prefer a resistor...

[Vsat]

Remember that photodiodes only operate with one polarity! You will need to use some kind of back-to-back arrangement... at least if you want 50:50 tri-waves.
Regards, Mike

[Vsat]

The photodiode will make an excellent current source for driving a Smallstone LFO OTA Iabc pin if that is what you are after. Then only one photodiode will be needed (as compared to two for a normal tri-LFO). If you use "voltage-drive" for the illuminator LED then you can get an expo "light intensity" response. The photodiode itself is one of the most linear devices around.
Mike

[ExpAnonColin]

Gez and Puretube-Wouldn't it be much harder to tweak properly?  If you think about it, you'd need the resistance range to be perfectly in line so that V/R equalled the small stone's normal LFO, so it would have to go just about perfectly from 0-10k or whatever or else your depth would be too big, to small, or it would just fry the OTA.  With the photodiode you have the max you need, the only thing to worry about it slope.  And, once you start using a FET, it's just about the same thing... just different parts.

Vsat-Right, it's the Iabc pin so you only need one photodiode.

-Colin

[puretube]

ahh, you`re aiming at  VC depth  :?:

[ExpAnonColin]

ahh, you`re aiming at  VC depth  :?:

Just a CV in for the LFO...  Pin 5, Iabc, where the current LFO goes.  I guess you could call that depth?

-Colin

[gez]

Just a CV in for the LFO...  Pin 5, Iabc, where the current LFO goes.  I guess you could call that depth?

You're being cryptic again Colin, I don't understand what you mean!  
All you need is a pot in series with a stop resistor if you want to vary the current going into pin5(s).

[puretube]

does Colin want to voltage-control the speed of a SmallStone-LFO ("rate"),
or the depth ("intensity"/"amplitude") of a (given) LFO/envelope/whatever...?

[ExpAnonColin]

Shoot, I'm not to be cryptic trying this time

I want to voltage control the depth of the small stone, if you put it that way puretube.  Not the depth of a given envelope, the depth of the small stone.  The LFO of the small stone.

The reason I suggest a photodiode instead of a resistor is that the resistor would be harder to tweak.  LDRs are harder to get within range... in this case you'd make the existing peak current (1mA or whatever) and use the photodiode to let it pass.  It's another way of varying the current, except in this case it's easier to get it within range.

-Colin

[puretube]

free information block:

[ExpAnonColin]

OK, clearly I'm still not explaining myself.

I'm just making a way to use a different LFO in the small stone.  There's an LFO in the small stone, there just is.  I'm just making a way for a CV input for that LFO.  An LFO input if you will.  It's exactly as musical as the pedal itself.  You're just using something else for the Iabc LFO.

-Colin

[puretube]

free information block:

[ExpAnonColin]

It would be to control exactly what the existing LFO in the circuit controls.

-Colin

[puretube]

free information block:

[ExpAnonColin]

ahh, now I understand: you wanna throw out the old (hypertriangular) LFO!
Well - I still prefer the passive V/I resistive converter...
(cloned from the various app.-notes)

Right, make something more interesting...

Don't you agree, though, that perhaps it would be easier to tweak if you had the max current already and (at least, how I think of it) "amplitude modulated" that?  That way you could control the depth completely seperately and it would be easier to find the sweep.  Getting a LDR to go from 0-10k perfectly is no easy task.

-Colin

[StephenGiles]

......and perhaps include a bounce circuit and maybe even some fixed phase shift in the feedback path.......
Stephen

[gez]

I still don't understand why the hell you want to use a LDR Colin.  The whole point about OTAs is that you avoid things like FETs/LDRs etc.  The Iabc pin is a diode drop above ground and the voltage drop across it is fairly static so it's easy to set current with a resistor.  Vary this resistance (or voltage) and you get a fairly linear response.

Sorry mate, but you really need to spell things out (for me at least) as half the time I haven't a bloody clue what you're on about!  

[ExpAnonColin]

I still don't understand why the hell you want to use a LDR Colin.  The whole point about OTAs is that you avoid things like FETs/LDRs etc.  The Iabc pin is a diode drop above ground and the voltage drop across it is fairly static so it's easy to set current with a resistor.  Vary this resistance (or voltage) and you get a fairly linear response.

Sorry mate, but you really need to spell things out (for me at least) as half the time I haven't a bloody clue what you're on about!  

I don't want to use an LDR.  I want to use a photodiode.  The reason I suggest this is because you could set the current before the photodiode-let's say from .8mA to 1.2mA.  The photodiode would then let the current pass, or not let it pass, depending on the CV input driving the LED that is optocoupled with it.  You get easy control of the current on one end, and sure-thing current modulation with the photodiode.  Otherwise you'd be using an LDR anyways, or an LFO into a resistor... what I'm suggesting is that it would be easy to set the depth and easier to tweak than any resistance-based setup.

-Colin