change the state of a 4013 flip flop at power on

Discussion in 'General Electronics Chat' started by bytraper, Apr 21, 2013.

  1. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    Hi,
    does anybody have a small circuit that would change the state of a 4013 flip flop at power on?

    I need to supply +5v (from a 5v rail) for about half a second (after which it turns off) at initial power on to reset the state of the toggle at first power on.

    I would appreciate any help or advice!
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Connect a 10uf capacitor from 5V+ to the reset pin, and a 47k resistor from the reset pin to gnd.
     
  3. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    wont that hold it high indefinitely ?
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Initially with no power applied the capacitor will be discharged at zero volts.
    When 5 vdc is first applied, the capacitor will pull up the reset to 5 volts for a short period of time, then charge to zero vplts on the reset pin through the 47k resistor.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Better put another resistor across the cap in case you want this to work the second time you use it.

    With CMOS you need to make a path for the cap charge to go.

    There are also chips that do this for you, Microchip among many others make these. The one I used was a SOT, but I believe they have some leaded styles too.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Not necessarily. When the power goes to zero, the capacitor will discharge through the resistor and also through the CMOS input protection diodes which go to both ground and the V+.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605

    OK, point taken... can you predict how much current will flow thru the protection diode?

    I can't... so I'd put a resistor in series with the pin.

    On second thought... maybe not...
     
    Last edited: Apr 21, 2013
  8. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    Thanks guys, such a simple but elegant solution!
     
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