Are Bypass Caps needed here ...

Discussion in 'General Electronics Chat' started by CoachKalk, Jun 20, 2012.

  1. CoachKalk

    Thread Starter Member

    Sep 20, 2011
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    So I know, based on the knowledge shared on this site, that bypass caps need to be included on all Power Supplies for IC's.

    But, if you will take a look at the circuits I have included, I am not sure if I need them (only referring to the 555's). The reason I am unsure is that all of the 555's are either getting voltage supplied through the arduino or the other 555.

    Thanks in advance for your input.
    [​IMG]
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I'll quote sgtwookie:
    Right about now I bet you're probably saying out load to me "sheesh dude, did you read WTF I just typed? all of the 555's are either getting voltage supplied through the arduino or the other 555!"
    Well, to that I say, I don't care. it's your circuit. If it were me, I would put them. The caps are to overcome inductance in the wiring/pcb trace. Unless your ICs are literally stacked on top of eachother and sharing a cap, they probably have traces or wires, which have inductance, which requires caps to overcome it.
    Take this with a grain of salt, I really don't know if I'm right or not. But I highly suspect that I am.
     
  3. elec_mech

    Senior Member

    Nov 12, 2008
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    Thanks for SgtWookie's post Strantor - that's the first thing I thought of but couldn't remember exactly what he had said regarding 555s.

    First I default to SgtWookie's knowledge, two caps per 555, one 0.1uF ceramic/poly and one 1uF electrolytic. Irregardless of where the power comes from, the caps help reduce noise and glitches, both going to and from an IC. This being my layman's explanation in my limited understanding.

    Take a look at this thread: http://forum.allaboutcircuits.com/showthread.php?t=45583

    And if you really want to get in depth: http://www.allaboutcircuits.com/vol_1/chpt_13/1.html
     
    Last edited: Jun 20, 2012
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Even though the voltage on a micro pin is a fairly sloppy voltage I would not put caps across a pin to ground even if using it as a power source. That way the pin is routinely driving an unlimited pulse of current into or out of the cap when you turn the 555 on or off. Such currents should always be avoided.

    The 555 does have a nice RESET pin you've tied off to power: it would be better if you run the 555 off the fixed power supply, with a nice close bypass cap and drive the RESET from your micro.

    However, the best thing is to realize anything the 555 is doing on the outside can be done by the micro on the inside. So for the siren drop the 555 completely and drive R2 directly from pin 47 for the 5 seconds. (I don't know what the "penalty circuit" drives.)

    Another thing: the way you have the 12 LDR's wired they see a resistor of 8.3K and not 100K, as every 100K is in parallel with the others. Grounding the LDR to select each in turn is a good way to lower the number of analog signals to measure, but just use a single 100K for all the LDR's.
     
  5. Brownout

    Well-Known Member

    Jan 10, 2012
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    Why power the 555's that way? I'd connect them to the power rails with bypass caps, and use their input pins the way they are meant to be used.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Don't power 555 from an I/O pin. That's simply bad design.
    Power 555 from the power rail with 0.1μ and 10μF decoupling caps.

    Also put 0.1μ and 10μF decoupling caps across the Arduino if they are not there already.

    Why do you need 555s in the first place? You can do this in the Arduino software.
     
  7. Sparky49

    Active Member

    Jul 16, 2011
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    "Don't power 555 from an I/O pin. That's simply bad design."

    While I appreciate that it doesn't seem very sensible - may I ask why it's bad design? Thanks.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    The output pin of an MCU or any logic gate makes for a poor power supply because of the relatively high impedance.
     
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  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    For starters, I/O pins aren't designed to power IC's. It's OK to turn things on and off with I/O's, but the correct way to do that is to use the I/O to control a device meant for connecting and disconnecting power, such as a realy. IC's should in most cases be powered up when the system is turned on and remain powered up. There is no need to power the 555's off and on like that. They are designed with input pins for triggering thier functionality. In this case, as already pointed out, the timer IC's arent' even necessary.
     
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  10. Sparky49

    Active Member

    Jul 16, 2011
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    Many thanks guys.
     
  11. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Plus the output pin of a microcontroller is typically only good for sourcing about 20 ma.
    That means the 20 ma must power the 555 and the load connected to the 555.
     
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  12. CoachKalk

    Thread Starter Member

    Sep 20, 2011
    139
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    Thanks for the feedback everyone.

    To be honest, I used the 555's mainly because that is what I first learned about when I jumped into this project, so I was most familiar with what they could do.

    I will reevaluate the way I power and input them, but I may still use them in the end just for the simple fact at least I understand them enough to actually have them do the things I wanted.

    I suppose I knew/know that the controller can do the same tasks, but I am at the very beginning of learning the programming side and because the use of the delay function seems to be frowned upon, it just seemed easier to use what I knew about the 555 timer. I know there must be other ways to set time limits for different I/O pins, but all of the examples I looked through seemed way over my ability.

    If, after I get more involved in the programming, I find that I can follow/understand the other options I may in fact use the arduino to its fullest potential.

    Until then, I have to go with what little I know ... make no mistake, I was happy to get the 555's working the way I calculated/planned, even if I was powering them wrong.[​IMG]
     
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  13. Sparky49

    Active Member

    Jul 16, 2011
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    Quite right. If it works, it works.
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Instead of switching power I would use the reset inputs to stop the 555s.
     
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