choose me a capacitor

Discussion in 'General Electronics Chat' started by yabasoya, Oct 17, 2009.

  1. yabasoya

    Thread Starter New Member

    Oct 8, 2009
    8
    0
    This circuit flashes the LED about twice a second. I changed the capacitor to 100uF and now it flashes once every 5 seconds. What do I need to change it to so it flashes roughly once every 60 seconds? Thanks.

    [​IMG]
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Proportionality should work. You increased the capacitance by 10 and slowed the flash by a factor of 10. You want to alter the origianal by a factor of 120. Try 1200 uF.
     
  3. count_volta

    Active Member

    Feb 4, 2009
    435
    24
    Is that an astable multivibrator?

    How I wish someone taught me in my EE classes how one of these babies works. Its always boring circuits that do nothing or very little. Meh...

    It looks so simple, but its not right? ;)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, the trouble with increasing the size of the capacitor is that peak current through the LED will skyrocket.

    The cap charges up until the NPN transistor kicks on, which turns the PNP transistor on dumping current through the LED. The larger the cap, the more current gets dumped.

    Time to change over to something like a 555 timer.
    [eta]
    Have a look at Bill Marsden's blogs:
    LEDs, 555s, Flashers, and Light Chasers
     
    Last edited: Oct 17, 2009
  5. yabasoya

    Thread Starter New Member

    Oct 8, 2009
    8
    0
    The 555 is my next step if this doesnt work out. The reason I am trying to get this to work is that I have a 4.5v voltage that I need to stick to. The 555 needs around 9V.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Look at using CMOS 555's.
     
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