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555 output question

Discussion in 'General Electronics Discussion' started by FractalCat, May 11, 2021.

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  1. FractalCat

    FractalCat

    5
    0
    May 11, 2021
    I have a schematic of the 555 IC where the output pin is connected to an LED and also to a resistor and capacitor and then to ground, which to me looks like an RC circuit or a low pass filter. The LED is not in series with the resistor and capacitor they have different connections. What's going on here? What is the purpose of the resistor and capacitor in this circuit?

    Thanks

    Fractal Cat
     
  2. bertus

    bertus Moderator

    2,164
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    Nov 8, 2019
    Hello,

    Could you post a schematic?
    You can upload picture files upto 300 kB.

    Bertus
     
  3. FractalCat

    FractalCat

    5
    0
    May 11, 2021
    Hello,

    Here's the schematic. What are the resistor and capacitor for in this circuit?

    Thanks

    FC
     

    Attached Files:

    • 555.jpg
      555.jpg
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      27.3 KB
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      18
  4. Audioguru

    Audioguru

    3,440
    738
    Sep 24, 2016
    The maximum allowed output current of an ordinary old 555 is 200mA that will quickly burn out an ordinary LED that has a maximum allowed continuous current of 30mA. Therefore the LED needs a series current-limiting resistor.
    The 555 might be a modern Cmos LMC555, TLC555 or ICM7555 that has much less output current.

    EDIT: Your ordinary old NE555 has a resistor R1 to limit current to the LED.
    I do not know why there is R2 and C1 at the output of the 555 to ground.
     
  5. FractalCat

    FractalCat

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    0
    May 11, 2021
    I'm aware of this but it doesn't explain the purpose of the capacitor in series with the other resistor.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    11,826
    2,757
    Nov 17, 2011
    R2+C1 make no sense. Where did you get that schematic from? An explanation may be found on that website.
     
  7. crutschow

    crutschow

    162
    39
    May 7, 2021
    I also can think of no useful purpose for R2-C2.
     
  8. FractalCat

    FractalCat

    5
    0
    May 11, 2021
    This schematic was taken from Electronics for Dummies as a hobbyist project. So what are the series resistor and capacitor doing
    in this circuit?
     

    Attached Files:

  9. bertus

    bertus Moderator

    2,164
    811
    Nov 8, 2019
    Hello,

    Strange, the book I got shows a different circuit:
    dummies 555 timer.png
    There is no resistor / capacitor combination on the output.

    Bertus
     
  10. FractalCat

    FractalCat

    5
    0
    May 11, 2021
    Ahh, now I understand. This makes more sense. So the book has a mistake. Well, I've seen that before with
    other texts.

    Thanks Bertus
     
  11. bertus

    bertus Moderator

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    Nov 8, 2019
  12. Kevin713H

    Kevin713H

    10
    1
    Feb 28, 2019
    Although I don't know what the function of your schematic diagram is, here I have some 555 IC Circuits Analyses, hoping to be useful to you :)
     
  13. AnalogKid

    AnalogKid

    2,543
    725
    Jun 10, 2015
    In the first image, C3 is supposed to be the LED. The clue is the resistance value of R3, which is correct for a normal small LED and VCC values up to +12 V.

    ak
     
    hevans1944 likes this.
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