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Simple sound reactive LED circuit dilemma

Discussion in 'LEDs and Optoelectronics' started by Solph, Jan 10, 2012.

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  1. Solph

    Solph

    3
    0
    Jan 10, 2012
    Hey. So I'm trying to make a super simple LED color organ, and am running into some interesting problems. I apologize in advance if this is kind of trivial for a forum like this, but it seemed like my best resource.

    So I've made a prototype of the circuit found here, and it works incredibly well, as seen in this video that uses the same circuit. I have a headphone splitter plugged into the audio out of my computer with one line going to this circuit and one to my stereo, then I'm powering the circuit through a USB cable plugged into my computer, and the responsiveness is amazing. The circuit operating in the configuration picks up basslines and kicks, as well as mids and highs, and works with any song I run through it.

    Here's the problem though. With it plugged into my computer through both the audio out AND the USB port, the audio quality coming through my speakers is seriously degraded. Because of this I replaced USB power with a 5vdc wall adapter (old phone charger). It powers the LEDs identically to the USB port BUT the responsiveness of the LEDs to the music is greatly reduced! when externally powered they only react to mid to high frequencies like voices, pianos, and some synths, vastly reducing the number of songs that look good with the setup.

    It's obvious that the USB connection is adding something extra to the circuit. Is it possible to use it as a power source so the lights are more responsive, but at the same time restore my audio quality? or is there a way to modify it when powered by a wall adapter to pick up more frequencies (maybe using different NPN transistors)? Is is just my computer (the one in the video doesn't seem to have the problem)? Or is this just one of the pitfalls of using such an simple circuit?

    It's pretty easy to prototype up if you want to test it yourself. I'm sorry I'm hardly knowledgeable in this sort of thing, and I REALLY appreciate any help. :confused:

    -Solph
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    There are a lot of things wrong with your circuit.

    1) you need resistors in series with each LED to limit current and to prevent 1 LED hogging the current, and also to protect both the transistor and the power supply.

    2) You need a resistor in series with the signal input to prevent this circuit from loading down your sound card output, and to protect the transistor.

    3) you might be better off also with an input capacitor and some DC biasing of the transistor (it should improve "responsiveness")
     
  3. Solph

    Solph

    3
    0
    Jan 10, 2012
    Thanks for your reply Steve. I did know about your first point and have those resistors in place.

    About points two and three though, do you know offhand what value of resistor I should put in series with the input, or how to calculate it, and also what type/value capacitor to use? Would the capacitor go in series with the power source, or bridge two of the transistor pins? Still pretty new to all this so I really appreciate the assistance.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    Initially, I'd place a resistor in series with the input lead that goes to the base of the transistor. Pick something between 100 and 1000 ohms.

    At this point the transistor is probably sufficiently protected.

    As to why it's not as "responsive" with the power supply as compared with the USB connection, it may have something to do with the voltage or the maximum current available.

    Can you tell us what the specs are for the external power supply?
     
  5. Solph

    Solph

    3
    0
    Jan 10, 2012
    My power supply is an old LG phone charger, which takes wall power and outputs 5.2vdc at 800mA, which I picked because it was the closest thing I had to a USB port's output.

    But that resistor between the signal input and the base of the transistor seems to have made all the difference. The audio is no longer degraded when plugged into the circuit, and it seems to be responding as well as it was when plugged into USB. I put a 510 ohm resistor for each the right and left channels going to the base. Seems to have fixed the problem! Thanks so much.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    Glad to help :)
     
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