problems with my 555 timer

Discussion in 'General Electronics Chat' started by itel933, Nov 6, 2010.

  1. itel933

    Thread Starter Member

    Aug 26, 2010
    38
    0
    Functions:
    Green led on when time is going
    red led on when time expires
    micro vibrator turns on when time expires
    both micro vibrator and red led are on when time expires

    I put a switch on my circuit to turn it on and off. I notice that when I switch it on sometimes, both the red led and micro vibrator turn on immediately (thats not supposed to happen, time is supposed to pass then at the expiration of time then its supposed to turn on)

    Whats the problem? Why is this happening? What can I do to fix it? I dont want to add another circuit or transistor though.
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
    2,515
    785
    Last edited: Nov 6, 2010
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    This is probably because the 10uF capacitor remains charged and it triggers the timer. You can use the reset pin as to hold the timer in reset and after you toggle the reset switch it will start the timing. Also, the 50M resistor seems to be too high to bias pin6. Use the datasheet to determine the maximum resistor value. Also, use a resistor between the 10uF capacitor and pin7 to limit the discharge current to acceptable levels.
     
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