555 fading leds for pc case

Discussion in 'The Projects Forum' started by dude72, Jul 22, 2014.

  1. dude72

    Thread Starter New Member

    Jul 22, 2014
    Hello, i've found this projects (http://forum.allaboutcircuits.com/showthread.php?t=90721)

    i'd like use this circuit in a pc case mod with leds ropes (30blue led SMD 2.4W 12Vcc with a 170 ohm resistor each 3leds) but i'm going to use just 18 leds on npn and 18 on pnp(6segments*3leds).

    ... i have a few question and request (original circuit and part list http://www.simple-electronics.com/2011/09/fading-led.html):

    _the original circuit is for 9vdc but the ne555 power range is up to 16V dc; since the 12v should be stabilized from the pc supply unit, can i use the 555 on 12v ?
    _the npn and pnp transistor from the original project are general purpose (9012;9013) and im not sure if they can handle the current for 18 leds each side so i need some help picking the right transistor.
    _am i right if i replace the red circle parts with the leds rope since there are already 170 ohm resistor on it ?
    _the fading time should be around 3-4sec, can i change the 20k resistor and the 220uF to play with the fading time or i'll damage the 555 ?

    i did some learning from Bill_Marsden's post about LEDs, 555s, Flashers, and Light Chasers but for the moment i cant sort this out without help.

    Thanks a lot for your time!
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    yes a 555 will work from 3v upto 16v, 12v would be ideal. Use higher power transistors if your in doubt about the current, like TIP31, TIP32
    Last edited: Jul 22, 2014
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    MOSFET's would not load the timing cap., or at least darlingtons.
  4. wayneh


    Sep 9, 2010
    I'd look for MOSFETs as Bernard has mentioned. You'll need both an N-channel and a P-channel. They should be rated for at least ~4X the current draw of your LED ropes. By the way, the NPN and PNP locations are reversed in the diagram.

    [edit] I messed up and misunderstood the circuit. You need regular BJT transistors positioned as show. I thought they were switching, but they are cross fading. For that you need BJTs. Again, make sure they are rated for the current you anticipate. Look also at power dissipation, as they will be used in linear mode and will need to drop several volts at, say, half of the max current. Volts dropped times amps will tell you the watts that will be dissipated in the transistor.
    Yes, if they are meant for direct connection to 12V, then you don't need to add a resistor.
    No worries. Experiment at will.
    Last edited: Jul 22, 2014
    dude72 likes this.
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Using a MOSFET, IRF740, as an amplifier is a bit touchey. Dim at 2.5V full bright at 3.5V, not worth the effort of building a high Z V divider.
  6. Wendy


    Mar 24, 2008
  7. dude72

    Thread Starter New Member

    Jul 22, 2014
    Thanks all, ill update the post when something working.
    Going to burn some TO before swich to TM.

    P.S. any good info is still welcome