LED powered by audio signal

Discussion in 'The Projects Forum' started by FSTOP, Jan 27, 2009.

  1. FSTOP

    Thread Starter New Member

    Jan 26, 2009
    HI. I am trying to find an LED that can be powered by just the audio power of a typical stereo amp's single-channel output. NO battery or other voltage input, just the audio signal from the amp to the speaker. Does not need to be particularly bright, blink, or do anything but light up with the audio power. I am assuming i need something that will run on 10mV or less? I cannot seem to find anything like this searching online. I am sure such an LED exists, as i have bought VU meters that run on just the audio signal with no power. But i am only needing a single LED to light up with audio power. ANY IDEAS or know where i can find such a product??
    In case you are wondering the purpose, i am trying to back-light an epoxy mold for a logo that will light up when in use on a custom speaker system...

  2. thingmaker3

    Retired Moderator

    May 16, 2005
    Have you measured the channel output? Shouldn't it be on the order of a few volts? :confused:
  3. SgtWookie


    Jul 17, 2007
    If you hooked up a peizo sensor to an LED, if the audio were loud enough, it might just light the LED.

    There was an article on EDN.com a few months ago on harvesting energy from sound; using a piezo sensor to charge up a supercapacitor.
    Link to article:
  4. FSTOP

    Thread Starter New Member

    Jan 26, 2009
    I don't have a way to test at the moment. Even cranking a high-power amp up wouldn't get actual voltage would it? I am generally using lower-wattage T-amps as well. Would like to get some amount of light without continuous high dB levels :). Inform me, if you can, as i am no electronics expert--That's why i am here. Is there a place to find low-level LED's?
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    LED's take a certain amount of voltage to push electrons across the internal structure. That energy is what lets those electrons emit the photons that we see as the characteristic light. You might find an LED with a forward voltage a bit less than 1.5 volts, but never as low as 10 mv.

    VU's are sensitive voltmeters. Low voltage and current.

    You may have to resort to a battery operated circuit to give the results you need.
  6. Externet

    AAC Fanatic!

    Nov 29, 2005
    Connect two red LEDs in counterparallel and to a speaker output from an amplifier.
    They will light up unless the amplifier is too weak as from a portable radio.

    Exceeding volume level can damage them.

    The newer the technology of the leds be (efficiency), the brighter they will shine compared with older ones; but all will light up at the same volume level.
  7. Audioguru


    Dec 20, 2007
    Sound from speech and music has very short duration peaks. An LED would instantly light then turn off on those peaks so quickly that your vision's slow response to brightness will not see the blips of light. The circuit needs a "peak detector" to hold the LED turned on for a moment that is long enough to be seen. The LM3915 bar-graph IC's datasheet has some peak detector circuits.

    If the max power to a speaker is 60W then the voltage at the speaker is 21.9V RMS if the speaker is 8 ohms. A low sound level from the speaker is caused by a signal that is 0.22V RMS.