Regulating LED brightness using headphone output

[wanxiang]

Recently I'm trying to design a circuit to regulating LED brightness using headphone output. My PCs headphone jack (green) outputs a 4Vpp (2V amplitude) sinusoidal signal at 100% output level set in the sound preferences. What kind of circuit would I need to drive a red LED (2.2V forward voltage, 20mA forward current) based on the voltage level present at the headphone jack?

The LEDs luminous intensity behaves linearly in the range from 0 to 30mA. My idea would be to balance the LED current like follows:

Soundcard output:

+2V: driving the LED with 30mA

0V: driving the LED with 15mA

-2V: driving the LED with 0mA

I experimented with a circuit that puts the signal in parallel to a 9V battery that provided DC bias voltage. And I also tried to use a BC547C npn transistor in combination with the 9V battery. Unfortunately unsuccessful. The outcome was always a offset of the LED current: e.g. +2V: 28mA, 0V: 22mA, -2V: 16mA

I was not able to balance the current through the LED as described above. I guess to achieve this, a much more elaborate circuit is needed.

Edit:

I would like to modulate the LED with the frequency of the audio signal.

A receiver catches the signal using a phototransistor.

I achieved the transmission using a simple Common-Emitter-Amplifier, the outcome was OK. But I want to make use of the whole LED intensity spectrum (0-30mA).

One simple circuit to do what I want is a rectification circuit. The chain of sub-circuits would look like:

Decoupler
Rectifier
buffer + bias op-amp
Filter
LED bias.
The decoupler will remove any DC component from the waveform.
The rectifier will convert from a ground-centered 4Vpp sine wave to a 2Vpp half-sinewave.
The buffer will reduce load on the laptop and should be connected to shift up the signal to be biased such that a max amplitude will present 30mA to the LED, and a min amplitude will present 0 - for this you will need to know the forward voltage drop of the LED.
The filter will convert from the half-sinewave to a DC rail.
The LED bias will be a simple resistor.

I would really appreciate any advice. Thank you very much!

Best regards,

 

[sandyk]

See if you can get any ideas from the attached link.

Alex

 

http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/

[wanxiang]

5 minutes ago, sandyk said:

See if you can get any ideas from the attached link.

Alex

 

http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/

Thanks for your useful help

[mansr]

Are you trying to make the LED intensity follow the waveform of an actual audio signal, or are just using the headphone jack as a convenient signal generator? If you explained in a bit more detail what you're trying to achieve, we'll be able to offer better suggestions.