Just make sure that your input signal is such that you keep the transistor in linear mode. A Tip31 has an hfe of ~100 at low currents, so to get 20mA out for your LED's you only need 200ľA of input current, so you'll need to measure the maximum output of your sound card (voltage wise) and select an appropriate resistor so that you don't exceed that.
A good project, and good ideas from the HWC community!
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Folding with rig under My System Specs, GTX460 in the kids' computer and whatever my dad can contribute.
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