"soft start" audio fade

Discussion in 'The Projects Forum' started by timburly, Feb 1, 2011.

  1. timburly

    Thread Starter New Member

    Feb 1, 2011
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    Hello all,

    I am working on an audio project and am having a hard time finding any ideas on schematics. I am trying to build a simple circuit that will fade in music upon a switch closure. When the switch is open there is no audio. When the switch is closed the audio fades in from nothing to a preset limit. This fade in period needs to take about 1 minute.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Might be able to use analog opto-coupler as volume control. Need to then provide a variable 0 to 20 mA drive to input LED.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    After thought: Not tried or simulated but might work. Q1 is darlington transistor to give high impedance load on C1. Vary R2 & or C1 to give desired slope. LED & R4 is analog opto-coupler or home built. R3 to suit LED-around 470Ω for IR LED.
     
  5. wayneh

    Expert

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  6. bertus

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  7. timburly

    Thread Starter New Member

    Feb 1, 2011
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    Thanks for the reply guys...a Digikey order has been made and looks like I will be bread-boarding until I figure this out.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    One technique that could prove effective would be to design your audio amp and then on the input use a digital potentiometer in the volume control circuit. That way you could use a 555 oscillator and a bit of additional circuitry to increment or decrement your digital potentiometer. The rate of increase or decrease in volume would be determined by the frequency of the clock generated by the 555.

    hgmjr
     
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