555 astable starting low?

Discussion in 'General Electronics Chat' started by rjjenkins, Sep 30, 2011.

  1. rjjenkins

    Thread Starter Member

    Apr 16, 2011
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    Hello
    How do you get a 555 astable to start the cycle low, please?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
    804
    invert the output?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    That would do it, but why would you want to start the output low?

    You can make an inverter with a second 555, or use a dual 555 in the form of a 556.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Connect the timing cap from Vcc to pins 2 & 6 rather than from 2 & 6 to ground.

    On power-up, pins 2 & 6 will be pulled to Vcc instead of ground, which will start the output off low.

    This trick won't work if you're using the RESET pin to restart the timer.

    If you wish to eliminate the extra time delay on start-up, then use 2/3 of the capacitance from Vcc to 2 & 6, and then 1/3 from 2 & 6 to ground.
     
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  5. Wendy

    Moderator

    Mar 24, 2008
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    Most of the astable functions of the 555 use the Discharge pin 7, so that solution has problems of its own. You could use a transistor to invert the discharge, or use a transistor to invert the signal.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, nothing needs to be inverted. Have a look at the attached pair of schematics and simulations.

    There may be a small "glitch" on the output until Vcc comes up to within the operating Vcc range of the 555. Vcc is taking 20uS to come up to 12v, and for about the 1st 6uS the output is following Vcc.

    Whether this glitch would occur with real-world parts is beyond me at the moment; don't have a 555 and a scope to test it with.
     
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