Sound to Light Circuit Not Working

[Vlad7329]

Hello, Forum!

I tried breadboarding a sound to light circuit (Seems to be by DeadastronautFX originally), but can't make it work.

I am using the circuit below:


The circuit schematic was provided to my by Leetut (here's his amazing take on a sound to light circuit, which is only a part of his Nuclear Rat pedal concept, check it out: https://www.youtube.com/watch?v=kYKYz5_5564)


The issue is that all 3 LEDs just light up as soon as I connect power to board.

They don't seem to react to the output of my guitar at all.

What could be the problem?

Also, if I decide to hook this circuit up to a fuzz pedal in future, wouldn't the signal 'split' and part of the loudness/volume/frequencies of the signal would go to driving LM386, the result being the signal going to fuzz circuit would lose some of the qualities mentioned above?

[antonis]

I know Rob is a lazy guy but not so lazy to left pin 2 "open"..

P.S.
Connect pin 2 to GND..
(and a 10μF or bigger cap from pin 7 to GND wouldn't do any harm..)

[Vlad7329]

Do you mean it should be grounded?

If so, I tried it already but it didn't help

[antonis]

Could you plz post the complete circuit..??

[Rob Strand]

The issue is that all 3 LEDs just light up as soon as I connect power to board.

They don't seem to react to the output of my guitar at all.

What could be the problem?

The LM386 power's-up with the output about mid supply DC coupling the LEDs to the output will make them light-up to some degree.   Say the LEDs are red the voltage drop is about 1.7V so 3x1.7V = 5.1V.   At low currents the voltage drop could be less and LEDs will light dimly.

I think the original design is banking on 9V/2 = 4.5V being close to 5.1V.

I know Rob is a lazy guy but not so lazy to left pin 2 "open"..

P.S.
Connect pin 2 to GND..

I think he is relying on the internal 50k resitor to pull the input to ground.  Perhaps not the best way to prevent noise and oscillation.  That might fit it.

I'd measure the DC voltage at the output of the LM386.  If it's not about 4.5V there's a problem with the LM386 or the wiring.

Another possibility is the LM386 is oscillating.  Try is add a Zobel network (R in series with C) from the output to ground - see the datasheet.   Caps across the supply pins is another one.

[antonis]

The issue is that all 3 LEDs just light up as soon as I connect power to board.
They don't seem to react to the output of my guitar at all.

At a first glance, that calls for a permanently positively saturated LM386..
In such a degree that even negative amplified (X200) signal can't lower its output..

[iainpunk]

I know Rob is a lazy guy but not so lazy to left pin 2 "open"..

P.S.
Connect pin 2 to GND..
(and a 10μF or bigger cap from pin 7 to GND wouldn't do any harm..)

theres good reason for not grounding that pin. there's an internal 50k resistance specifically so that you can leave that pin open. if you're going to connect anything, use a 100n cap to ground, to shunt the noise.

both pins internally have the same resistance to bias the circuit, because of input leakage current is kind of high, it will bias both inputs to the same offset. also aides in the internal biassing of the output.

cheers

[antonis]

there's an internal 50k resistance specifically so that you can leave that pin open.

Open to whatever wishes to enter inside amp..

P.S.
50k to GND internal resistance is the p-n-p input transistor Base resistance so anything entering there affects differential pair current..

[PRR]

...I am using the circuit below:...

I don't think that will work as expected without a huge dose of luck. The DC level of the idle LM386 versus the voltages drops of the LEDs.

Also, your question about loading the guitar signal is on-point. The 50K into a '386 will shave highs from a passive e-guitar pickup, even cause an audible drop of bass-mids.

This will work (done the same many times for other purposes).


The four diodes may conveniently be a full wave bridge rectifier module. It is 50X beefier than we need but may be as cheap and 4 legs easier.

Use a >1Meg buffer in front so as not to load your guitar signal.

Whether to ground or cap-bypass the '386 inputs is well covered in the application (sales) notes. Has Google lost those? But the DC offset is rarely limiting. Don't fret.

Yes, supersonic oscillations will foul-up results. Layout must be tight and neat. The 10r+0.05u on the output is normally "required". Yes, you may get lucky, especially to speaker. Or you may not. And pedal-builders are not trained to explore for radio oscillations. Just do it.

[stallik]

Fwiw, I played with Robs circuit a long time ago. I went down the rabbit hole of seeing how many LED's I could light. Turned out to be many hundreds using those RG's led strips. This was only possible because the strips were organised into parallel groups of series LED's. He quantity of LED's in series depended on the LED's chosen.

Found this on the way back - Rob shows 4 LED's, I'm still using groups of 3 high power 3mm for pedalboard bling. Try adding another led and make sure all LED's are the same type




One thing about this circuit is that when the lighted LED's are just going out, a little bit of fizzle can be heard in the audio. Buffers, splitters etc don't help me. Its only just perceptible but once you've heard it...

[anotherjim]

Don't misread the circuit drawing. C14 negative does not connect to the output pin5, it goes direct to pin8.

[antonis]

One thing about this circuit is that when the lighted LED's are just going out, a little bit of fizzle can be heard in the audio. Buffers, splitters etc don't help me.

I'd try to decouple audio input signal via a 10k + 10μF series string from pin 7 to GND..

[stallik]

Thanks Antonis - I'll give that a try

[antonis]

Thanks Antonis - I'll give that a try

Try a single 10μF cap firstly and then proceed to series resistor addition..
(pin 7 is connected to the junction of 2 X 15k resistor from Vcc to Q1 Emitter so maybe a single cap could be adequate..)

[deadastronaut]

yes i am a lazy so and so....

my router is on its way out , keeps cutting off, which is a royal PITA so just a quickie..

before it cu..............


yes there is a little fizzle on the dying light. and yes try 4 leds, all the same.

i go before bstd router pisses me of again...    back later...

[Vlad7329]

Don't misread the circuit drawing. C14 negative does not connect to the output pin5, it goes direct to pin8.

Thank you! It solved the problem. Now it works!

One thing about this circuit is that when the lighted LED's are just going out, a little bit of fizzle can be heard in the audio. Buffers, splitters etc don't help me.

I'd try to decouple audio input signal via a 10k + 10μF series string from pin 7 to GND..

Thank you, i'll try that also, if I hear the fizzle

[stallik]

Old thread I know but I've been playing with wood rather than electrons so only just got round to trying Antonis suggestion re the cap and resistor.

For posterity, 10uf cap helped (I think, difficult to audition without making the cap switchable) but adding the 10k resistor in series changed the behaviour entirely. Fizzle is present all the time the LED's are lit and rather than fade gently, the LED's pulse slowly as they go out. I had the resistor after the cap

[stallik]

I Know this is a really old thread but I've just found a simple solution for the fizzle as the LED's go out. Run this circuit from a completely different power supply to the pedals. Quiet as a mouse.

[deadastronaut]

ooh thats good to know....

so i wonder if it went though a 9v volt reg from the same supply it would behave.....hmmmmm

[stallik]

Ah, just realised that I have an AMZ jfet splitter in the signal path before the sound to light circuit. The splitter didn't help. Will have to check without the splitter before confirming that this is a solution.

Edit: just checked without the splitter. Still quiet