audio driving an LED circuit

Discussion in 'General Electronics Chat' started by BobbyTheD, Mar 20, 2016.

  1. BobbyTheD

    Thread Starter New Member

    Mar 10, 2016
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    Hi there -
    I've been looking through the forum back catalog and the many, many websites out there...but I haven't figured out what the issue is yet.

    I'm using a simple LED NPN transistor driver - this circuit, identically: http://www.learningaboutelectronics.com/images/LED-driver-circuit.png

    But the audio signal is "line level", around 1v, and the LED is connect to +5v and ground. The LED stays illuminated at all times.

    My goal is to use an audio signal, with as little circuitry as possible, to fully drive an LED. Is there a solution?
     
  2. #12

    Expert

    Nov 30, 2010
    16,346
    6,833
    Audio frequencies turn the LED on and off so fast you can't see it go off. You need to change the parameter of the audio signal to get a useful response. This is usually done with a rectifier, a capacitor, and a resistor to form a time constant slow enough for you to see the amplitude change.
     
    cmartinez likes this.
  3. BobbyTheD

    Thread Starter New Member

    Mar 10, 2016
    12
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    The audio, in this particular case, is spoken English. I was thinking an envelope follower wouldn't be necessary, since there are plenty of silences... Is that not the case?
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    It depends on all of the stuff not in that schematic, the audio source, any coupling capacitors, any DC boas, etc. For example, if the audio is riding on 1 V DC, then the LED always is on. If the quiet passages aren't so quiet, then the LED always is on. If if if

    Separate from that, your circuit almost certainly will not differentiate between syllables.

    ak
     
  5. BobbyTheD

    Thread Starter New Member

    Mar 10, 2016
    12
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    Ah, well, that does help. I put a cap prior to the transistor and it seems to be a lot closer to working now. There must've been some serious DC coming in. However, the light doesn't get very bright with the audio signal now. Is this a common issue? I would've thought this is a problem that's been solved many times over, by now..
     
  6. #12

    Expert

    Nov 30, 2010
    16,346
    6,833
    It has, but not with 4 parts.
    The main problem here is your lack of parts to do proper time constants, peak detection, and sensitivity control.
     
  7. BobbyTheD

    Thread Starter New Member

    Mar 10, 2016
    12
    0
    Is there a way to tune these things while keeping the part count low? Maybe one more transistor?
     
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