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Not getting the ideal 555 output

Discussion in 'General Electronics Discussion' started by Varun Dua, Jun 18, 2015.

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  1. Varun Dua

    Varun Dua

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    Jun 18, 2015
    Hey,
    I am providing a 230v ac supply to the attached circuit which gets rectified and the output that I get from the zener diode is a constant 12v dc supply.
    The requirement is that there should be just 1 negative trigger pulse that should be applied when the power supply is switched on and the output of the IC555 timer should be there for 500ms. I'm somehow not getting an output which specifies the above 2 criteria's. The output that I get is a positive trigger pulse and the output which remains low till infinity time..

    Please help..
     

    Attached Files:

  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello
    I am really struggling to see the picture clearly. Would you be able to take a closer screen grab. Separate them into more than one section if you need to. Just make sure you overlap the section a little so we can see where they connect to on the other section.
    Adam
     
  3. Martaine2005

    Martaine2005

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    May 12, 2015
    Is this any better?
    [​IMG]
     
    Arouse1973 likes this.
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Oh wonderful Martin, thanks.
    Adam
     
  5. Martaine2005

    Martaine2005

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    May 12, 2015
    Oh dear.. Me making things bigger for you will have people talking...:rolleyes:
     
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yes that I need to wear my glasses:eek:
     
  7. Martaine2005

    Martaine2005

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    May 12, 2015
    Nope...Haha..This is me.

    [​IMG]
     
  8. Varun Dua

    Varun Dua

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    Jun 18, 2015
    Haha my bad.. tried this circuit for the first time..
     
  9. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi Varun,
    Just out of curiosity, have you tried the reset to high instead of low?
     
  10. Varun Dua

    Varun Dua

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    Jun 18, 2015
    Reset pin is connected to high :)
     
  11. Martaine2005

    Martaine2005

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    May 12, 2015
    Ah yes, missed the link above U3.
     
  12. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I think you have two problems. First is that you need to provide a small amount of settling time for the circuit. The second is that I think if you hold down the trigger terminal of the timer the output will stay high. The trigger pulse needs to be shorter than the output pulse. You could have a small delay circuit and using a small capacitor in between two resistor pulled to supply produce a negative going small pulse that triggers the timer. You will need a diode across the second resistor to protect the 555 timer. let me see if I can doodle a drawing for you.
    Adam
     
  13. Varun Dua

    Varun Dua

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    Jun 18, 2015
    Yeah if you could draw something like that.. It would be of a great help.. I'm unable to understand what you meant :p
     
  14. Martaine2005

    Martaine2005

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    May 12, 2015
    hey Varun. just found this on the 555.
    I don't know if it applies to you but it does switch the high to a low.
    • Pin 6: Threshold – also the negative input to the second internal comparator; it resets the internal flip-flop when the voltage applied to it exceeds 2/3 of VCC, causing the Output to switch from high to low; it also connects directly to the RC timing circuit.
    Probably of no use but hey!!!
    Martin
     
  15. davenn

    davenn Moderator

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    Sep 5, 2009
    and you have a dreadfully small smoothing cap in the PSU section 22uF is nowhere near enough
    the AC ripple is likely to be causing all sorts of weird problems

    And the zener isn't going to give you anything close to a stable 12V supply

    Dave
     
  16. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Here is a modified circuit that shows the principle. See if you can include this in your design. R3 and C3 provide a small power up delay which then turns on the transistors T1 and T2. This then produces a small active low pulse which triggers the 555 timer. The output then goes high for approx. 500 ms. This is only one way of many to do this.
    Adam

    555_PON_TRIG1.PNG 555_PON_TRIG2.PNG
     
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  17. Martaine2005

    Martaine2005

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    May 12, 2015
    Can you make it larger please? Can'y see a darn thing..:D
     
  18. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Sorry I got very long arms and have to sit 6 feet away from the monitor. Looked the right size to me :)
     
  19. Martaine2005

    Martaine2005

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    May 12, 2015
    So small then?:eek:
     
  20. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    The problem is this:
    Your resistor-fed power supply will deliver about 25mA and this is all being "eaten up" by the LED and other components. You have nothing left for the 555.
    Of course the circuit will not work.
     
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