Driving an LED with the guitar signal?

[ashcat_lt]

OR

* Add a heavy bleed resistor.

The heavy bleed resistor got the circuit up and running! Thanks! However, I'm going to explore an actual envelope detector as Kipper suggested I think. For some reason, the guitar signal from the 386 seems to somehow be making it's way back into my main circuit... the modulated rate is working, but the very distorted guitar signal is constantly in the background.

This is the same reason I am always suspicious of "asymmetrical" clipping arrangements.  It completely doesn't work to use assymetrical diode pairs in a circuit like the Rat, and this is why.  It might start asymetrical, but pretty quickly floats to be centered between the diode thresholds.

The bleed could be just proximity with other wires/components, but it could also be that you're pushing and pulling on the "ground" pretty hard.  It's not supposed to move, but when I lived in New Orleans, I could feel the streetcar coming sometimes before I heard it.

Edit - Actually, as proud as I am of that streetcar thing, you're really pulling just as hard on the positive rail.  It's quite possible that both are wiggling some.  That old thing of careful decoupling.  Put a BAC across the 386's power pins as physically close to the chip as possible.  Might help to do the same with other amp stages in the circuit.

[kforte318]

Oh man, that rail car analogy was so gooood. Well done. Forgive me, but what is a BAC?

[ashcat_lt]

Oh man, that rail car analogy was so gooood. Well done. Forgive me, but what is a BAC?

Big Ass Cap

[kforte318]

Big Ass Cap

lol o

[PRR]

> return to a working state after a minute

This suggests the LED back-leakage is extremely low. A 2uFd e-cap would likely self-bleed almost that fast.

As you see, 10 Ohms makes the bleed really quick. Most of one cycle's charge is bled-off before the next cycle.

> is what I am currently doing technically just a very crude envelope detector?

On its own, it reproduces half-cycles directly.

An "envelope follower" normally filters the violent rectified wave into a smooth outline of the short-term average of the wave. To do this it needs some notion of how "fast" the envelope can change. Plucked strings need a rapid attack. If left to ring, the decay is always slow. If damped (or muted), the decay can be abrupt, so an envelope follower should not be too slow. We usually compromise with a small tail after an abrupt mute.

The raw LED "does work" if used to modulate a photo-resistor, because foto-Rs have a somewhat slow response to light on and a long decay after lights out. Different chemicals and cookings give different time constants. Different circuit impedances give different effective times on the same foto-R, because as R goes up the action gets slower.

[kforte318]

@Kipper4

So I've been working with the Sweller circuit trying to derive the envelope detector from it.



I had it printed out to examine, and circled the bits I figured were important. Based on my ~intensive~ Wikipedia research, is the right-most circle the heart of the detector? What are the two left-most circles for (I assume input buffering and amplification of the signal before it gets to the detector)? Also, could I simply replace the Pad, Attack, and Decay pots with resistors if I didn't want controls for those parameters?

[Kipper4]

Good work.
Ic2a is as you say just a buffer for the coming signal.
You can leave out the pad and attack pots if you want and play with the decay outs value.
Then you can play with the values of the caps c4 and c5 to suit your needs.
Ic1b is the envelope detector and is driving the vactrol led too.
Hope this helps.

[kforte318]

Probably a dumb question, but with the attack and decay pots wired as they are in that schematic, wouldn't they just act as resistors as opposed to variable resistors?

[Kipper4]

No dumb questions here.
They're wired as variable resistors. Since two of the lugs are tied.

[kforte318]

Does that work that way?! This whole time I've been using one of the outside pins and the center pin, leaving the third disconnected.

[Kipper4]

Yer it works like that. Sure you can leave out the other but if one soldered lug fails you have no resistance at all, this way at least you have some resistance if not all.
Just a kinda protection wiring.
That make sense?

Edit.
I may have got my terminology wrong.
Here you go.....


http://www.allaboutcircuits.com/textbook/experiments/chpt-3/potentiometer-rheostat/

[kforte318]

Yes it does! Good to know. Thanks again.

[kforte318]

So I wired up the portion of the Sweller (@Kipper4), omitting the Pad and Attack pots entirely and replacing the Decay pot with a 1M resistor. The result seems to be the LED simply blinking along with the LFO and not reacting to the incoming guitar signal at all. Any ideas of what I may have done wrong?

EDIT: I replaced the 10n cap between the output of the first op-amp with a 100n cap, and it seems now to be responding slightly to the guitar (but still blinking along with the LFO when it's on instead of solely reacting to the guitar).

[Kipper4]

Take note it's an envelope detector ic1
Not an lfo .
Unless you have an lfo as well.
Note to I'm using TL072 for input buffer ic2


The envelope detector ic1is a lm358.
A lm741 ic will probably work well too for ic1
A fixed resistor instead of decay pot gives you no adjustment of how quick the led reacts.
Try a pot.
Change r5 to something much bigger 1M to 6M

[kforte318]

I have an LFO elsewhere. Ah! That might be part of it though. I was using a TLO72 for both op-amps. Didn't notice that the detector was a 358.

I'll try and pot and maybe mess around some with some different R5 values. Mostly I always just want the LED to react as quickly as possible so I wasn't too worried about not having control over the parameter.

[Kipper4]

Righty Ho.
Remind me what the "driving an led with the guitar signal" was for.

[kforte318]

I want to modulate the rate of an LFO with the guitar signal (strum the guitar, spike the rate). Basically just want to light up the LED in a vactrol.

[Kipper4]

I'm not even sure this is possible.
Show us where your up to ? Post a schematic please.

[kforte318]

I'll have to do it in the morning, but can do! It was actually working with the Uglyface fragment and the heavy bleed resistor suggested by PRR, but I could hear the distorted guitar signal from the 386 in the output (which I did not want at all).

[armdnrdy]

Google peak level indicator circuit.
You should be able to modify the circuit to do what you are trying to do.