"reseting" 555 timer

Discussion in 'General Electronics Chat' started by itel933, Nov 4, 2010.

  1. itel933

    Thread Starter Member

    Aug 26, 2010
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    Green light turns on when power is supplied
    Red light when time expires

    I want to put a momentary/ slide switch so that when its pressed the unit is "reset". I was told I need to put a resistor "in series" with the switch to prevent short circuit. How much of a resistor do I put? Im using a 9 volt batttery. and what do I attach the switch to?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    If you want to re-trigger the timing cycle then this should work.

    If you just want to reset the timer without re-triggering the timing cycle, then put a 4k7 from the 9V rail to pin 4 - instead of a direct connection. Then connect your reset switch between pin 4 and zero volts.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    I think he means reset to idle, using pin 4. Use a NPN transistor, emitter to ground, collector to pin 4, base/resistor as an input.

    This is a bad example, but it was the quickest I could pull up.

    [​IMG]

    I needed to flip on the opposite polarity, so I had to use a PNP.
     
  4. itel933

    Thread Starter Member

    Aug 26, 2010
    38
    0
    Actually what I was told is, in order to reset the timer I had to discharge the timing cap. I was told to use a Double Pole Double Throw switch. In that way im not shorting the battery and possibly causing damage to other components.


     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    If ckt is to be reset & re-triggered with sw, remove battery from pin 4 & connect p 4 to pin 2. Place sw in parallel with .1 uF cap. After power is applied and first timing cycle is completed, the ckt will already be reset[ origional ckt ] to re trigger just short .1 uF cap with sw.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Sounds like homework, given the conditions.

    Pin 4 on the 555 is a digital input. It is the "reset" pin, and does exactly what it sounds like.

    If you want to discharge the cap this is doable, but it is not considered reset.

    Here are several retriggerable monostables.

    http://forum.allaboutcircuits.com/showpost.php?p=289260&postcount=3
     
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