LED with an attitude

[merc]

I tried adding an LED into my Ruby amp by setting it up between the + terminal of the battery and ground, and not surprisingly it messed stuff out by replacing my guitar signal with a loud buzz noise.  I then hooked it up to the - terminal and ground, and it mysteriously flashes along as I play.  If I silence the strings, the light goes dim.  When I slam a power chord, the light springs to life.  It's a super-bright blue LED, so the effect sort of looks like lightning, which I like.  I'm going to stick it behind the speaker and let it shine through the front of the box.

BUT...

I tried the same setup with a red LED, and it let loose a dying flash as it fried from the inside out.  I did have a resistor hooked up to it.  So what's the deal, is the blue one just insanely tough or is there a difference because of the color?

I did try cranking the voltage up to 12 like a mod I had read about, which resulted in the slow and painful death of my blue LED.  No worries though, I have about 10 I salvaged from a broken lightsaber-type toy I found.  Supposedly all the 12V does is give it more headroom, with no other modifications necessary.  Since my LED fried, I suspect I have the LED wired up weird or something.

[gez]

I tried adding an LED into my Ruby amp by setting it up between the + terminal of the battery and ground, and not surprisingly it messed stuff out by replacing my guitar signal with a loud buzz noise. 

Are you sure you meant the + terminal of the battery?  Why would it produce buzz??

Are these LEDs you're adding for the purpose of distortion (clipping diodes), or status indicators?  Are you including current-limiting resistors?

[merc]

It was supposed to be a status indicator LED, and it did have a resistor.  There is a chance that I put the resistor on the wrong end or something though.

[gez]

It was supposed to be a status indicator LED, and it did have a resistor.  There is a chance that I put the resistor on the wrong end or something though.

What value is the resistor and what are you using as a power source?  It's possible that you're drawing too much current and loading the supply, pulling it down in turn.  This would account for the distortion in the chip.

[gez]

I then hooked it up to the - terminal and ground, and it mysteriously flashes along as I play. 

Do you meant between the -ve input terminal of the amp and ground?

[merc]

I then hooked it up to the - terminal and ground, and it mysteriously flashes along as I play. 

Do you meant between the -ve input terminal of the amp and ground?

Yeah, that's what I meant.  Sorry for the confusion.

It was supposed to be a status indicator LED, and it did have a resistor.  There is a chance that I put the resistor on the wrong end or something though.

What value is the resistor and what are you using as a power source?  It's possible that you're drawing too much current and loading the supply, pulling it down in turn.  This would account for the distortion in the chip.

I tried a 1k and 100k resistor.  The 100k worked, but the light was so dim there was no use even having it on there.  1k led to a fried LED.  Voltage source is either a 9V battery or a 300 mA wall wart.  I've tried both.  I've also tried the wall wart on 12V.  More charred LEDs.

[gez]

Well, if you've wired the LED between the -ve input of the amp and ground and it's turning on, you're pulling it down/up past its threshold so that it turns on.  Presumably you're boosting the signal going into the amp, or before the buffer if you're followed the ROG schematic I'm now looking at.  Either way, the signal must be pretty hot to exceed the forward/reverse threshold of the LED and it sounds like you're exceeding the max current rating of your red LEDs; or breaking it over and damaging it that way (same principal applies: too much current flowing).

Not sure why you'd get buzz by sticking a status LED across the rails.  Possibly a layout thing??

Check the colour codes of your resistors.  You may have made a mistake and it's too small.

[merc]

Alright, I'm getting kind of confused, so I will elaborate with some ASCII art 

BATTERY            BLUE LED (works w/ blinking effect)
  ____                -  O  +   
|     -  |-------------/   \------------|GND
| __+ |-------| + VOLTAGE IN
                             
Ground goes to - terminal of the input cap, + goes to the positive terminal of the cap.  I have the layout I'm using at http://www.aronnelson.com/gallery/main.php/v/merc1/render.gif.html

All I did for ground was to take the two ground references on the board and the ones from the jacks and hook them up to the - lead from the battery clip.  I'm not sure if that's how you do it, but it's what I did and it (sort of) works.

[gez]

Sorry merc, but you've lost me.  Layouts mean nothing to me and I don't understand what you've done/are trying to do. 

All I can say is your 9V wall-wart is a little low on the current rating side of things.

[gez]

From your diagram it looks as though you're wiring the LED between the -ve (power) terminal of your amp and the -ve pin of your supply.  Yes?  Raising the ground of the amp (and rest of circuit) by the forward threshold of the LED?  Why would you want to do that??  The LED would be in series with whatever the amp is having for breakfast (current wise), which is going to far exceed the current rating of your LED.

[merc]

From your diagram it looks as though you're wiring the LED between the -ve (power) terminal of your amp and the -ve pin of your supply.  Yes?  Raising the ground of the amp (and rest of circuit) by the forward threshold of the LED?  Why would you want to do that??  The LED would be in series with whatever the amp is having for breakfast (current wise), which is going to far exceed the current rating of your LED.

Yeah, that's right.  That would explain why all the RS LEDs I tried to put in bit the dust.  These blue ones I salvaged out of a broken toy, they must be some tough cookies or something.  They won't die even at that current, they just sort of get bright when there is input from the guitar. So having the LED in that specific spot is going to throw off the rest of the amp?

And about the wall wart... it was 7 bucks at the local hardware store.  I couldn't pass it up 

[head_spaz]

In the diagram, if I'm reading it correctly, it looks like you've put the LED in series with your power supply and the load/ circuit.
If so, the LED becomes the current limiter for the entire circuit. That's the job of a fuse, but LEDs can blow just like fuses.

Try placing the LED ACROSS your power terminals, (+ and -) with a resistor in series with the LED. A resistor of 4.7K ought to suffice. As always, watch your polarity.

[brett]

Hi
the blue LEDs might have an inbuilt resistor.  220 to 330 ohms would save a blue LED from the current draw of an LM386.
cheers 

[gez]

merc, if you want the effect you mentioned then the following might work OK.  Connect your LED and current limiting resistor (don't make it too small) at the output of the amp (pin 5 for DC connection).  Don't change anything else (take the output from the cap as shown).  You probably won't notice anything due to the frequency that the LED will be modulated at (the frequency of your guitar note).  Who knows...

Doing things the way you were is a bad idea.  Not only are you frying LEDs that can't take heat (literally), but you're dropping valuable supply voltage across the LEDs that can.  This means less headroom in your amplifier (possibility of distortion - and not the type you want!)


You could do with more juice from your wall-wart.  You can just about get away with what you're using, but at least 500mA from a 9V supply would be ideal.

[frank_p]

LOL (this is not mean) what are you cooking.  A powerchord intensity indicator or standard ON indicator ?
You have to decide.  An intensity indicator would be more complicated.
What do you whant to show with that LED,  Power ON ?
You are just limitting your power supply with that method.

[merc]

Well that might explain why there was no "clean" setting, even with the gain all the way down. 

What do you whant to show with that LED,  Power ON ?

Yeah, that's what I was going for.

I think I'll try gez's suggestion, and then try again with the LED across the terminals.  Does it matter which side of the led the resistor is attached to?

[gez]

Does it matter which side of the led the resistor is attached to?

No.