Powering LED's on the output of stompboxes why doesn't it work ?

[otter2]

Hi I like to chain stomp boxes to get some nice feedback noises. I decided to go a step further tied to replace the potmeter resistances of a stomp boxs with a variable resistance controlled by the output of an other stomp box.  My first idea was to just to add a led to the output of one and use a photo resistor in parallel with the original potmeter in the other stomp box.

So I hooked up the led to the output but it did not light up in combination with a resistor to avoid it from burning out. I tried other leds and switching the polarity, noting worked. I tried it again with the raw output. Still no light. Does anyone know why ?

[Kevin Mitchell]

The output isn't going to power anything.

Whatever you're trying to do makes no sense to me. I hope you're following a schematic or guide and not just poking an LED at your guitar signal.

[GibsonGM]

Hi Otter,

I think I see what you want to do. Like Kevin suggests, don't ruin your stuff to get there! 

There are a couple of things going on in most circuits that we build.  There is an AUDIO path, which is of course your sound signal.  It typically comes in from the guitar in the 1v range...and is an AC signal.   This is typically amplified to make it less susceptible to picking up noise, and to make it easier to work with.   So then, we do our thing to it...phase it, distort it, and so on.   When we are done, the level can be higher than the input (of course!), but not by a whole lot...usually a few volts...the power supply (often 9V) limits how much we can amplify the audio signal.

There is a DC COMPONENT here, too.  This is the power we add to active devices like opamps and transistors to give us the power, so to speak, to amplify the small AC signal.  This is kind of the 'work force' of our circuits.   It drives the  LEDs we use, gives us our voltage references, and all that stuff.  Sometimes we do 'superimpose' the AC onto DC, to do work with it, but in the interest of getting the sound we want we don't 'tack in' too many other things, which can destroy the nice audio tone. 

If you want to drive an LED with the audio signal, you (again, generally) must in some way make the audio signal big enough to provide the power to do so, in the right way (conditioning), without ruining your original signal.    Mostly, you will only get a small flutter in an LED from the typical gain circuit, since we're not amplifying it much and it shines when we are cutting off peaks of the signal.  There's no 'fine control' here.

A circuit called an 'envelope follower' is usually used to do what you want, better. It conditions the signal and amplifies it enough to drive the LED the way we want.  There are simple ones using an LM386, and ones that are very complicated for 'talk boxes' and things like that.   If you do some research on here, and the internet, you can see how this works.   It will take some learning to understand, I think   that's ok, work at it.   The 386 stuff will get you started!

A big concept, up-front, is that of "buffering", which is a way to 'grab a sample' of the audio and not disturb the other processes going on.

And also, if you're trying to do this with a potentiometer....the LDR isn't a pot, it's a variable resistance...you'd need TWO of them, working in opposition, to act like a pot...so you see, this is actually kind of complicated!  But has already been invented, if you search around...

  Welcome to the forum!

[duck_arse]

also, as kevin says, welcome to the forum.

you can try adding a power amp (and probably a buffer), like an lm386 circuit, as used in an uglyface to drive a led "from the signal". the output of most pedals is for driving an audio chain - a led is driven in/with/by a power chain.

[ ..... typing slower than gibson, again.]