modify 555 one shot monostable multivibrator circuit

Discussion in 'The Projects Forum' started by steven334, Aug 23, 2011.

  1. steven334

    Thread Starter Member

    Aug 9, 2011
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    I would like to modify this 555 one shot monostable multivibrator.
    The circuit I built will buzz for about 5 second when the switch is pressed and released, and it will buzz continually if the switch is not released.


    I want to modify the circuit so that it will not buzz for more than 5 seconds.
    I can change my switch to a normal closed switch if that will help.


    My project is to sound a short alert when a door is opened up and then to be silent until the door is closed and reopened.

    Thank you
    steven
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Connect a resistor (a few tens of kilohms, not critical) from the trigger pin to the positive supply rail, and connect a capacitor of 0.1uF from the trigger pin to the output of your timer.

    When the timer goes low, the negative-going edge will appear at the other side of the capacitor, which will then charge back up to the value of the supply rail.

    It will stay at that potential until another negative transition arrives at your timer side of the Cap.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Use the circuit in the attached picture. the door switch has to be open when the door is closed.
     
  4. steven334

    Thread Starter Member

    Aug 9, 2011
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    I'm not sure if i understand values on C1 and C3 capacitors. Is nF nano Farad. I just see uF micro Farad in my radio shack store.
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Yes, "nF" means nano Farad. uF would not be appropriate. The most common capacitor values go from pF to uF, so they should have those.
     
  6. steven334

    Thread Starter Member

    Aug 9, 2011
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    ok so .001uF = 1nF
     
  7. praondevou

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    Jul 9, 2011
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    That's it. :)
     
  8. steven334

    Thread Starter Member

    Aug 9, 2011
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    It runs erratic , a small shake can trigger the buzzer
     
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    What do you mean with "a small shake". The normal switch bouncing is no problem, since the bouncing time is much smaller than the monostable time, so even if it retriggers the 555 it would still give you 5 seconds.

    Do you mean, that the door switch might close (through shaking) even if the door is closed? What switch is this?

    edit:Unless you mean the moment when the door was open and closes again... in that situation the 555 should not trigger. . so I guess you need to debounce the switch.
     
  10. steven334

    Thread Starter Member

    Aug 9, 2011
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    its not the debounce. its not working. I'm not sure what it's doing.
     
  11. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    do u have a bypass capacitor on the 555? from + to gnd?

    since you said "shaking" there is probably a bad contact somewhere.
     
  12. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    There is missing a wire from the cathode of the diode to "+" on your breadboard.
     
    Last edited: Aug 24, 2011
  13. steven334

    Thread Starter Member

    Aug 9, 2011
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    thank you i put that missing wire in.
    now i think i'ts working.
     
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