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Sound activated LEDs

  1. Aug 10, 2011 #1
    So I was wondering how to create a sound activated LED circuit for each of my speakers. I want the red LEDs to light up when the subwoofer is activated and the blue LEDs to light up when the left and right floor speakers are activated. I had a thought about wiring the LEDs parellel to in input for the speakers like in the picture below but there is not enough voltage in the speaker wire to activate my LEDs. I was thinking a voltage regulator may work but I do not know much and could not find much information online. Thanks for the help. Below is a picture of the set up I was thinking about.

    Attached Files:

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  2. jcsd
  3. Aug 11, 2011 #2


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    Staff: Mentor

    Are different audio signals sent to the different speakers? Or does each different speaker contain a filter (crossover) network to select what frequencies to emphasize?

    If all the speakers are receiving the same audio drive signal, then you will need to do your own filter circuit to determine which speakers are driving the most sound out, before you have your LED drive circuits monitoring.

    And regardless, your LED monitoring circuits should be non-invasive for the audio signal, so they should have their own power supply, and they should use a transitor or opamp amplification stage that connects to the audio drive signals (so there is very small loading of the audio signals). You would use comparators after the initial buffer stages (possibly with filter circuits) to detect when the drive for a speaker is above some threshold, and use the outputs of the comparators to drive the LEDs...
  4. Aug 14, 2011 #3
    There is something crutial that is missing from what berkeman has described. You problably want the LED current to be related to something such as the average power of the audio signal for some period of time, and not simply have LED current flow whenever the signal exceeds a constant value.

    Here are some options:

    signal from speaker wire --> high pass filter --> differential amplifer --> multiply signal with itself ---> low pass filter --> comparator ---> (LED + resistor)

    signal from speaker wire --> high pass filter --> differential amplifer --> rectifier/absolute value --> low pass filter --> comparator ---> (LED + resistor)

    The high pass would be to remove any 0 frequency (also refered to as "DC") offset.

    One would have to do calculations to make sure you won't exceed the power supply voltages for the active stages, that the LED recieves the right current, that the highpass does not significantly attenuate audio signals of interest, and the lowpass filter will provide decent smoothing without sacrificing the responsiveness.

    Additionally it would not be a bad idea to verify the LED current is within the comparator's output current capability (if it isn't you can fix this with another stage).

    As berkeman posted, you will may have to add additional filtering if you are connecting before an internal filter contained within the speaker housing.
    Last edited: Aug 14, 2011
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