555 timer one shot question

Discussion in 'General Electronics Chat' started by hazim, Apr 10, 2009.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Hi

    How could I generate a single positive pulse of about 3 seconds width just after the timer is connected to a positive voltage terminal... and what is the amplitude of the pulse? is it the same as Vcc or what?
    I searched for a simple circuit for a single positive pulse and I didn't found any:confused:...
     
  2. Wendy

    Moderator

    Mar 24, 2008
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  3. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Thank you Bill. I tested that circuit and it worked fine on the two LEDs. I connected the output pin(3) to a relay as a voltage supply (I want the relay to switch for about 3 seconds), it worked but it didn't switched off as the green LED do when the relay is disconnected. I tried to connect pin 3 to a base of an npn transistor (BC548) and used this transistor as a switch to operate the relay but it did the same... What should I do??
    Thanks again.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Here is a common interfacing scheme. If it switches the wrong way, change the polarity of the transistor, and connect everything from R2 back upside down.

    [​IMG]

    This particular circuit will turn on when power is applied, basically a delayed power up. I'm not sure, but from your description it may be what you're asking for.
     
  5. italo

    New Member

    Nov 20, 2005
    205
    1
    there gabs of info on the 555 timer for a one shot add a cap to pin 2 series with a vcc pull up once fired this one shot cannot be reset except by pin 4. pin 2 needs 50 ua to go even your person can generate that. and addind a PNP OR NPN to drive a relay is really not necessary it can source or sink >200 ma.
     
  6. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    It worked fine.. Thank you very much Bill! and thaks italo too
     
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