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Fading LEDs

Discussion in 'Electronic Basics' started by m7n, Dec 20, 2004.

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  1. m7n

    m7n Guest

    Hi Group,

    I'm trying to design a simple circuit that will fade a white LED in,
    hold it for a few seconds, then fade it back out. I thought LED's were
    either ON or OFF, so I'm not sure if it's possible. I do notice that my
    computer has a LED light that seems to 'breathe' while it's sleeping.

    Can someone tell me if this is possible? If so, are there any kits or
    schematics that you know of that I can start with?

    Thanks in advance,

    m7n
     
  2. Yes, this is possible. Here is a simple design using a cmos 555
    (view with fixed space font)


    .--------o---------------o---o--------------. 9V
    | | | | |
    .-. | | | |
    | | 10k | | .-. |
    | | | CN555 | | | 470k |
    '-' | __ | | | |
    | .--------o| |o----|' '-' |
    o--------------o| |o--------+ |
    | | | .--o| |o--------+ |
    | | '--|--o|__|o. | .---o
    | | | | | | |
    | | .-. | | b|/c | 2N4401
    | | | |1MEG | | .--| |
    | | | | | | | |>e |
    | | '-' | | | | |/c
    | | | | | | '-| 2N4401
    | | | | | | b|>e
    | | | 0--------|-------|-----' |
    -| | | | | |
    | | | | | | White V
    | | | | | LED -
    ^ | | | | | |
    | | | | | | |
    | | | | | | .-.
    | | | | | | 470 | |
    | | | --- 1uF ---.1uF --- 10uF | |
    | | | --- --- --- '-'
    | | | | | | |
    | '-----o-----o--------o-------o--------------' GND
    |
    |
    Normally Open Pushbutton

    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    Push the button, it triggers the 555, which stays on for
    about 3 seconds. The white light LED goes to full brightness in
    about 2 seconds. When the 555 output goes to GND, It takes
    another 2 seconds for it to fade.

    Increase the 470k resistor or the 10uF capacitor to make it
    stay lit for longer. Decrease the 1MEG resistor to make it brighten and
    fade more quickly.

    The output transistors can be any NPNs. A single NPN darlington would
    work too.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  3. LEDs produce light in rough proportion to the current through them.
    You can vary the brightness either by pulsing them off and on (faster
    than the eye can keep up with) with various proportions of on and off
    time or by operating them continuously while varying the current
    through them. How many do you want to run in synchronism? The
    continuous approach is simpler for one (low efficiency, perhaps, but
    it is such a small total power that it may not matter), the pulsed
    approach better for many (more complicated but possibly, higher
    efficiency).
     
  4. I put pin 1 in again - you left it out.

    I've never seen a 555 used in this kind of 'integrated square wave'
    circuit. I'll have to try it sometime.

    I would make one change, ground the cathode of the LED, so that if it
    gets shorted to ground, it won't have to tolerate the excessive current
    and 'let the smoke out'. Usually you put the circuit somewhere and run
    a wire pair to the LED on the panel. If you use a grounded cathode, you
    could just run a single wire if the panel is grounded.. You get the
    idea. ;-)

    God.
     
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