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Fade in/fade out LED

  1. Jun 26, 2012 #1
    I would like to start this thread in simple terms, at least initially, just to get an idea of what is required for the following project and then get more in detail when necessary.

    We are working on a project where we are using a simple mic an amplifier, controller and a few LEDs. the Idea is that the leds will be activated by the mic once the dB level har reached a critical level. (You talk and the lights go on basically).

    What I would like to discuss and get some feedback on is how to make possible a circuit so that the LEDs fade in and out when we have sound. We are right now thinking of using a condensator for this purpose. But we are not sure how to implement it in the system to make the idea funtion. Can one just connect the right condensator between the microcontroller and LED as simple as that or does one have to take more into account? Does AC/DC play a factor here.

    Attached you will find a picture of the circuit schematics.
    Thanks for your time.

    Attached Files:

  2. jcsd
  3. Jun 27, 2012 #2


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    Science Advisor

    I wouldn't think a capacitor would work as you describe. (Condensator is the wrong term here. Capacitors used to be called condensers, but never condensators.)

    If your micro can do pulse width modulation, you could use that.
    Just have a little routine to gradually increase the width of the pulses going to the LED and then gradually reduce them.

    If you really want a column of LEDs so that more LEDs would light up if the input signal was stronger, there are specialist chips to do this.
    The LM3914 is a linear one and the LM3915 is a logarithmic one.
    These are quite old so there may be better ones available.
  4. Jun 27, 2012 #3


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    vk6kro, You date yourself by mentioning LM3914 and LM3915! They are as old as sliced bread! They are still used, however, and there are two new types as well!

    "Dot/Bar Display Drivers

    LM 3914 - Linear Version
    LM 3915 - Logarithmic 3 DB/step Version
    LP 3943 - Low Voltage, 16 LED's

    The LM 3916 is a monolithic integrated circuit that senses
    analog voltage levels and drives ten LEDs, LCDs or vacuum
    fluorescent displays, providing an electronic version of the
    popular VU meter. One pin changes the display from a bar
    graph to a moving dot display.

    The audio level meter most frequently encountered is the VU
    meter. Its characteristics are defined as the ANSI specification
    C165. The LM 3916’s outputs correspond to the meter
    indications specified with the omission of the −2 VU indication.
    The VU scale divisions differ slightly from a linear scale
    in order to obtain whole numbers in dB."

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