Can someone tell me what the point of C1 is in this circuit....

Discussion in 'General Electronics Chat' started by critiera119, Nov 21, 2008.

  1. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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  2. mik3

    Senior Member

    Feb 4, 2008
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    Its 10nF not 100nF. C1 does not get involved in the determination of the output frequency value, thus why if you remove it you see no change in the operation of the circuit. Its purpose is to bypass the 2/3 of Vcc point (internal) to ground as to minimize noise variations on it and keep the frequency more stable.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Check this project out, it has the same capacitor.

    http://forum.allaboutcircuits.com/showthread.php?t=14018

    Basically it is something to help with noise, especially on the power supply. You're right, you probably don't need it unless you are using the battery for other devices. Techs and engineer throw it in almost as a habit. Occasionally it actually does something, but it is better not to need it and have it than visa versa.
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    They are commonly called decoupling capacitors. They are extremely crucial in R.F. circuitry, but might not be notices in most digital circuits.
     
  5. davebee

    Well-Known Member

    Oct 22, 2008
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    A National Semiconductor LM555 datasheet says that pin 5 can be used to modulate the position of the output pulse.

    So visual displays will probably look and work fine without the cap, but if you needed precision pulse generation for servo control or something you'd probably want it.
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
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    Just because logic is usually in the sub-gigahertz frequency range, doesn't mean that decoupling isn't crucial. It is the edge-rate of the signal that matters. In the past, I had some digital circuitry that ran at 48MHz and decoupling capacitors were crucial because I was using a fast-edge rate FPGA.

    Steve
     
  7. KL7AJ

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    With such sharp edges, you ARE working with R.F. circuitry! (Fourier is your friend!)

    eric
     
  8. hgmjr

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    Jan 28, 2005
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    To add another voice to previous replies and reiterate many of the comments already provided, C1 is often recommended for use in filtering the internal DC voltage reference to which the pin is connected. The purpose of the pin is to permit a degree of control over the frequency by the application of a voltage.

    hgmjr
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Looking at pin 5 with a scope was verry revealing; showed a strong damped oscillation on ever output transition. The cap went back in.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The circuit is missing an important supply bypass capacitor (try 0.1uF in parallel with 100uF). The 555 draws up to 400mA on each edge of its output waveform that a supply bypass capacitor will supply if the power supply or battery cannot supply. Maybe that is why pin 5 is bouncing up and down.
     
  11. KL7AJ

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    Nov 4, 2008
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    Yep...very very typical!
     
  12. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    Thank you all for commenting. Very interesting responses. As you can see, the circuit works beautifully without the aforementioned C1. Voltage to the ouputs for the flashing showed no change with or without it. I would agree with the response that it was probably included in the pdf for this specific circuit by way of habit.

    http://www.youtube.com/watch?v=xTtj1xR80Zk My vid.
     
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