HELP with a simple circuit to fade in a LED

Discussion in 'General Electronics Chat' started by lefam, Dec 2, 2010.

  1. lefam

    Thread Starter Member

    Nov 30, 2010
    33
    0
    Hi guys!

    I am pretty new to electronics. I have just designed a circuit that turns on a LED when you press a tactile switch and fades out the LED when you open the switch.

    Can you help me designing a circuit that fades in a LED when a switch is closed? Please something simple (in preference a RC circuit).

    Thank you
     
  2. tyblu

    Member

    Nov 29, 2010
    199
    16
    To fade at power-up you need to have a resistive source; at power-down a resistive ground, as you know; both with a cap across the diode. What minimum current do you need (minimum brightness), and what is the voltage drop at that point? The less current or brightness, the lower value cap you can use.
    This should work: [link] . Resistor values will have to be changed as they are made for a 0.6V silicon diode, not 1.7-2.5V LED.
     
    Last edited: Dec 2, 2010
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. elementalrage

    Active Member

    Jul 30, 2009
    59
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    I'm sure something like what I attached would work... and it's simple.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Actually not very well. The emitter is not connected to any DC source, the transistor is not conducting much at all, and the entire power supply (1.5VDC?) is going through 47KΩ.
     
  6. elementalrage

    Active Member

    Jul 30, 2009
    59
    3
    Although the schematic may seem inaccurate, the emitter, capacitor (negative side) and the 47k resistor are all connected to ground. There probably should have been a black dot at each connection.

    Using a 4.5-5 volt power supply, it works well.
     
    Last edited: Dec 2, 2010
  7. Wendy

    Moderator

    Mar 24, 2008
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    That is the convention. Schematics are not accurate otherwise.

    The circuit still has problems. You need to use a common emitter design for best results, a more linear fade. This is because a common emitter design has a fairly low resistance between the base emitter. One emitter resistance turns this around, and makes the base high resistance.
     
  8. lefam

    Thread Starter Member

    Nov 30, 2010
    33
    0
    Thank you guys. Remember I asked for simple solutions as I am pretty new to this.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Most tactile switches (I would say all) are not push on / push off. Is this what you mean to describe?
     
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