Best Way To Cancel 1 Shot 555 Timer?

Discussion in 'The Projects Forum' started by NFA Fabrication, Apr 22, 2014.

  1. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    What is the best way to cancel a 1 shot monostable 555 timer circuit? I have a timer built that goes on for about 2 minutes when triggered, but needs to be cancelled by a voltage (Basically the same voltage as the positive rail).

    I have never needed to do this before, so I just connected to the + on the timing capacitor, which worked for the most part, but altered the time of the 555 because the trigger source to cancel the timer was a source that turned on some lights, but the lights were manipulating the capacitor because of the resistance to ground when the bulbs weren't lit. Any ideas?
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    There are two ways, discharge the cap with a transistor, or use the reset pin (4).

    [​IMG]
     
  3. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    OK. Because I was originally pulling the capacitor high to "Run out the timer". And I was thinking about the reset pin, but wasn't sure how I could do anything with it since it was already connected to the positive rail.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    It's only connected to the positive rail when you don't use it. You pull it to ground for the Reset function (you can use a resistor to the positive rail to pull it high otherwise).
     
  5. #12

    Expert

    Nov 30, 2010
    16,298
    6,811
    You can also pull pin 5 to ground to collapse the voltage on the internal resistor network and the timer will think it is finished.
     
  6. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3

    Ahhhh, Thank you!
     
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