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Computer Audiophile


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About wanxiang

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  1. Thanks for your useful help
  2. 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,