Help with LED fade circuit

Discussion in 'The Projects Forum' started by shaolin_shadowboxing, Jul 16, 2011.

  1. shaolin_shadowboxing

    Thread Starter New Member

    Jul 16, 2011
    This is my first time building a circuit so I would appreciate any help.

    I want to build a circuit in which an LED lights up initially and then fades out.

    My idea was to put a capacitor in series with an LED so as the capacitor charges, the LED fades. Would this work?

    I have been trying to build this and am having some trouble. Here is my setup:

    With this setup, the LED doesn't turn on at all. Is there anything I may be doing wrong or overlooking?
    Thanks a lot.
  2. k7elp60

    Distinguished Member

    Nov 4, 2008
    I think you are using a AA battery. It is not enough voltage. If you use a 9V battery, a 1k ohm resistor and and a 470uf capacitor all in series it should work fine. The shorter lead of the LED is -. so connect the - of the capacitor to the - of the battery, the + of the capacitor to the - lead of the LED and the + of the LED to one lead of the resistor, the other lead of the resistor to the + terminal of the 9V battery.
    shaolin_shadowboxing likes this.
  3. Wendy


    Mar 24, 2008
    We have had similar requests, and I have written several article and responded to several threads to show how it is done. It isn't as simple as you think, the lack of a resistor for that LED and the low voltage feeding the resistor clearly marks you as a beginner. There is no crime in that, but resistors to limit current to LEDs is not optional.

    LEDs, 555s, Flashers, and Light Chasers
    Chapter 12

    I wrote this a tutorial for beginners who wanted to use LEDs, and it kind of took off on its own. One possible way is the Comet Trails in Chapter 12.

    Other ways are spelled out in these threads.

    LED fade in/ fade out

    Led fading circuit

    slow fading LED using a 555 timer.

    The Fading RGB LED

    Another key word is throbbing.

    If you want I can draw up a quick schematic using a 555 timer or a switch to do what I think you want. You want an LED to turn on quickly, then slowly fade out when the switch is turned off. Correct?

    Are you wanting this to oscillate? As in turn on/off continuously with no input from you?
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
    A couple of discrete component versions.

    R1 controls the fade in time. R2 controls the fade out time.

    Circuit 2 allows for better adjustment of fading times. The pot controls the
    initial LED on brightness as well as maintains a constant voltage on the capacitor.
    The PNP transistor helps control the cap discharge time and thus the fade out time.
    The fade in/Fade out times can be changed by R3/R4 respectively.
    Last edited: Jul 16, 2011