Hi -
I'm currently using a sound to illuminate an LED in a split-supply circuit.
The sound feeds the anode and the cathode is connected to ground, through a 1k resistor.
When the sound gets loud, the LED lights up. When the sound is quiet, the LED shuts off.
I would like to bias the LED, so that it doesn't shut all the way off.
I tried attaching resistors from both + and - 9v, on either side of the LED. But neither seemed to help.
I also wonder if I am connecting a resistor to the anode from the voltage supply, if that would not interfere with the sound signal connected to the same point.

Thank you for the advice.

 

You need to do two things:

1) Isolate the audio signal with a capacitor, a large electrolytic will do.
The + side should face the LED.

2) Add a diode in reverse bias across the LED.
This provides a path for the negative peaks in the audio signal.

This assumes you have the LED Cathode grounded, and a resistor supplying a bias to the LED from Vcc.

 

Great..thank you for the help. I drew a schematic according to what I understood - is this accurate?

 

...LEDs, 555s, Flashers, and Light Chasers

 

Hi -
I'm currently using a sound to illuminate an LED in a split-supply circuit.
The sound feeds the anode and the cathode is connected to ground, through a 1k resistor.
When the sound gets loud, the LED lights up. When the sound is quiet, the LED shuts off.
I would like to bias the LED, so that it doesn't shut all the way off.
I tried attaching resistors from both + and - 9v, on either side of the LED. But neither seemed to help.
I also wonder if I am connecting a resistor to the anode from the voltage supply, if that would not interfere with the sound signal connected to the same point.

Thank you for the advice.

I'd build a simple 1 or 2 transistor constant current generator and adjust the current to the required midway brightness - then its easy to couple the audio into the base of the control transistor with a DC blocking capacitor.

There's no reason you couldn't use the simpler "operating point" series resistor feeding the LED - the DC blocking capacitor to couple in the audio is the key component.

 

Here's an idea:

 

Here's an idea:View attachment 126068

Modulating a constant current generator works better.

The single transistor type is adequate. LED as collector load. Emitter resistor develops a voltage related to current which offsets the Vref supplied to the base.

You can generate Vref with a simple voltage divider and modulate that - if you have loads of drive; you can use a Zener for Vref and inject the signal into the emitter.

 

if you have loads of drive;

I think an important part of this circuit is that there is a good supply of current from the audio generator. It might be possible to do this with no transistor. I would love to see your schematic.

 

Maybe like this:

 

I think an important part of this circuit is that there is a good supply of current from the audio generator. It might be possible to do this with no transistor. I would love to see your schematic.

Only just got the PC (more or less) working again after W10 imploded yet again.

Whatever schematic package is now gone and I can't remember what it was called.

Even if I get round to installing something - I'm probably too lazy to do anything with it.

The single transistor current generator can be found online - it really isn't that hard to figure out where you can inject modulation.

 

Whatever schematic package is now gone

I could post Wendy's package of symbols for you to use in MSPaint like I do.
You want it?

 

I could post Wendy's package of symbols for you to use in MSPaint like I do.
You want it?

Probably easier to sketch it and take a photo.

 

The single transistor current generator can be found online

I know how to use a transistor as a current generator. The problem is that I disagree about that being the best way to do this unless you can show me something I didn't think of with a schematic.

 

easier to sketch it and take a photo.

True. Easier and faster, but never as neat and clean as using pre-fab symbols.
I think my first 20 schematics on this site were had-made, but I am willing to do the extra work to achieve sharp looking drawings.