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Problem the 555 get triggered when i power up circuit

  1. Jun 9, 2009 #1
    I am connecting a Not and And gate to an activation switch (S1 or S2) that trigger a 555 to turn on a motor using a relay , i am using a main power up toogle switch that power the 555(vcc of 555 is sensor1) , so when it is off the 555 cant be triggered ,my problem is that everytime i turn on the toogle switch ( connecting 5 V to Sensor1) , the 555 get triggered alone and one cycle is run , attached is the circuit , and gate and not gate are connected to 5 V before the toogle switch is in position of sensor1......

    Any ideas of how to fix it ??????


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    Last edited: Jun 9, 2009
  2. jcsd
  3. Jun 9, 2009 #2


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    Are you using the 556 as a monostable? If so, I think I know what causes this.

    Try removing the capacitor C3 on pin 3. This is not necessary and it causes a delay on the lower two 5K resistors and you get a false triggering, just on turn-on.
    On a 555 this is pin 5 and you do the same thing.
  4. Jun 9, 2009 #3
    Yes i am using the 556 as monostable, i will try what you said , i let you know , thank you for your quick response......

  5. Jun 9, 2009 #4
    unfortunatelly i just tried that and it did not work , i took off the C3 and keep auto triggering when i put the toogle switch in position of sensor 1 :frown:

    any other idea ?

    Thank you
  6. Jun 9, 2009 #5
    Pull down resistor on TRIGGER_555

    edit: currently you have a pull up on trigger_555, thus during power up it will turn HIGH first.
  7. Jun 9, 2009 #6


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    Looks to me like you are triggering the 555 when applying the 5volt Vcc. (which is what ShadowPho1 suggested) Ususally one would have a power-on reset circuit of some sort. In this case, I whould hold the trigger low for a time while the power supply comes up. Not quite sure why you are using the switch for test/sensor.
  8. Jun 9, 2009 #7


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    Your monostable circuit is fairly standard.

    You could try removing the trigger input to the 556 (temporarily) to see if it still triggers on startup. It may be getting a pulse from earlier in the circuit when you turn on.

    Leave C3 disconnected, though. It does cause this effect.
    I had a camera being used for aerial photography which was being driven by a 555 monostable and it was taking a very expensive photo of the runway every time it was turned on. Removing this capacitor, which is always shown in circuit diagrams, fixed the problem.
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