555 Astable with diode "Delay On"

Discussion in 'General Electronics Chat' started by mikehains, Aug 26, 2009.

  1. mikehains

    Thread Starter New Member

    Aug 11, 2009
    1
    0
    To obtain a "Delay On" the most frequent recommendation is to use a PNP transistor. Easy. But if controlling a higher voltage circuit, the 555 does not cannot raise the base of the PNP transistor high enough to turn it off.

    Looking for solutions, the simplest I've discovered is here: http://clarkson-uk.com/555-timer/operation/frames3.html.
    [​IMG]
    It works perfectly on a breadboard.

    What I don't understand, though, is what is the purpose of the diode ?

    Hoping for help (and understanding),
    Mike Hains
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I think it is to discharge the capacitor when the power is turned off. Other than that I don't think it is needed.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I think you're right. It allows for rapid off-and-back-on cycling.
     
Loading...