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String of LEDs in parallel w/correct resistors and a TIP31 Transistor.

  1. Jul 9, 2011 #1
    I have a string of leds hooked up together with the correct resistors in parallel, going out to a TIP31 transistor which then goes to the power cord and to a 3mm jack. This 3mm jack goes into my computer and my leds "dance" to the music, I assume is the change in voltage. Anyways, I have hooked this up to one of my computers and it works flawlessley, but when hooked up to my other computer, the LEDs light up very bright and do nothing else. I am not sure how to begin debugging or how to fix it.

    If it makes any difference, I am using a sound card in the second PC. Any nudges in the right direction or solutions are appreciated.
  2. jcsd
  3. Jul 9, 2011 #2
    It sounds like you are using the transistor as a switch. Think about how much current it would need to "turn on", and what the current requirements likely are of the sound cards. They may be intended to drive different things.

    What do you think the input impedance is of your LED circuit? Have you thought of measuring it across different frequencies?

    Good job tinkering, this was one of my first projects!
    Last edited: Jul 9, 2011
  4. Jul 9, 2011 #3
    Sounds like a problem of DC offset to me. Can you show us the schematic diagram?
  5. Jul 9, 2011 #4


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    If you are connecting the base of the transistor directly to the computer sound card speaker output, then this could be risky.

    It would be better to bias the transistor properly and then connect to the computer via a series capacitor. Maybe 10 uF would be a good starting value.

    Measure the current your LEDs use. Divide this by the current gain of the transistor to get the base current.

    If you don't know either of these, assume 2 mA for the base current. These transistors have a gain of up to 50, so this would allow up to 100 mA for the LEDs.

    Then calculate the size of the base bias resistor.
    It will be (supply voltage minus 0.6 volts) divided by the base current (in Amps).

    So, for example, if the supply voltage was 12 volts and the base current was 2 mA, the base resistor would be (12 - 0.6) / 0.002 or 5700 ohms, so you would use a 5.6 K resistor from the +ve supply to the base.

    You may have to increase this resistor value until the LEDs just light up with no sound and then light up to full brightness with loud sound.
  6. Jul 10, 2011 #5
    Thank you for the responses, I have to travel for work the rest of the week, but I will start testing next weekend. I believe I need different resistors b/c this is not an onboard sound card. By the way I'm using 100 ohm resistors. and a 5v psu with 700ma.
  7. Jul 10, 2011 #6
    Got it working, I just pulled the other card out and used the onboard sound. Thanks for your help!
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