Cmos 555

Discussion in 'General Electronics Chat' started by fila, Sep 11, 2012.

  1. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    THE PROBLEM

    LM555N is causing me some major problems in my project (PIR sensor project, The Projects Forum). It is re-triggering itself. I've been reading some datasheets and posts on forums and I've learned that it (and 555's in general) creates some huge current spikes during transitions. That spikes are totally ruining my circuit operation. I connected some bypass capacitors (22 μF electrolytic in parallel with 0.1 μF) across it's Vcc pin and GND pin but it didn't help me at all. I've reached the deadline with my project and the whole circuit is soldered (IC's have sockets).

    A SOLUTION?

    Would it help to replace the LM555N with CMOS 555, for example with ICM7555. Here is a datasheet.

    View attachment ICM7555.pdf

    On page 5 you can see a comparison of supply current transients between ICM7555 and other 555's. Huge difference! Is it really that good or is it exaggerated?

    A GENERAL QUESTION

    Are the CMOS 555's more effective than the standard (BJT) 555's when it comes to supply current spikes and to what extent? What are the other key advantages? Any disadvantages?

    Just for the information the project is not a complete failure. Circuit sometimes works correctly but to often it gets re-triggered because of the 555 supply current spike.
     
    Last edited: Sep 11, 2012
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    Show us your schematic and I'm sure you'll get some help.
     
  3. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    Here is the entire project -> PIR sensor project.

    Just to emphasize that I can't alter the schematic anymore. The deadline is near and I have some other stuff to do. The circuit sometimes works fine but it would be nice if it could work always. So only hope for me is the CMOS 555. I am searching around the web to find something about it and I did. Here is a link to a page which discusses about 555's.

    If you have anything interesting to a add about 555's (especially CMOS) please do. Any help is greatly appreciated. :)
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,972
    744
    What is the circuit not doing for you, is the 555 not getting triggered, or is it triggering its self over again? I would lower the op amps supply with two diodes in series from the 555supply, and put a 100uf cap on both sides of the diodes,this should prevent the 555from pulsing the op amps supply. Or better still use a 5volt regulator for the op amps and 12v for the 555.
     
    Last edited: Sep 11, 2012
  5. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    Sorry, I thought I wrote that but I didn't. It is triggering itself all over again. Because I have an amplifier (active filter) in my circuit the noise gets amplified also. That is enough to re-trigger it. At least that is my observation. I've seen it happening in regular intervals of 5 seconds which is the time constant of my monostable 555 circuit.
     
  6. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    Thanks for the suggestion but I can't modify my circuit anymore. It is too late for that. I was wondering if a CMOS 555 instead of a BJT 555 would solve the issue.
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,972
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    yes it should cure it, or just try the diodes.
     
  8. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    OK, I will try with a CMOS 555. I looked it up, it doesn't cost too much (ICM7555). But I will have to order it. After I try it I will update you about the result. :)
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,425
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    BJT 555 are notorious for generating power supply noise, i.e. they cause other timer and monostable circuits to trigger earlier than anticipated.

    Definitely replace NE555 with CMOS versions, TLC555, LMC555 etc.

    Also put 0.1μF across the power supply lines as well as 4.7μF or 10μF electrolytics.
     
  10. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    I will replace it. I put 47μF caps across Vcc pin of every IC.

    Can you please recommend me some links, or books where bypassing and decoupling is thoroughly explained.
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,425
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    47μF across every IC is not necessary. One for the entire circuit is enough. The inductance in the cap is too high for it to be of much use except as a reservoir capacitor.

    Put 0.1μF across every IC instead. Keep the leads short and as close to the IC Vcc and GND pins as possible.

    I don't have any references off the top of my head.
     
  12. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    I put 22μF electrolytic in parallel with 0.1μF ceramic across the Vcc pin of the LM555N. Why didn't that help? The datasheet clearly stated that you put 0.1μF in parallel with at least 1μ electrolytic. And I've read here that 22μF is a good choice. But nothing helped. The LM555N is one tough cookie.
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    Again, electrolytic caps do not have a good high frequency response. They do not make good noise filter caps because of this. There is a reason everyone keeps saying 0.1µF caps, it is important, and not arbitrary. As Mr. Chips said, a 0.1µF next to each noise generating component (such as a 555) is what is needed.

    As for references, there is one right under your nose. It is a sticky.

    Decoupling or Bypass Capacitors, Why?
     
  14. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Connect pin 5 to ground through a .01 or.1 UF ceramic capacitor to prevent false triggering.
     
  15. fila

    Thread Starter Member

    Feb 14, 2011
    64
    5
    I read that sticky a long time ago. SgtWookie said:

    ''The more experienced folks know that every IC needs at least one 0.1uF/100nF "supply bypass" cap, and many datasheets recommend more than one, such as the 555 timer; it needs at least one 0.1uF poly metal or ceramic and one 1uF aluminum electrolytic or larger in parallel.''

    I did that. Instead of a 1μ I put 22μ because thatoneguy said:

    ''...It also explains the reasoning behind the common usage of a 22uF Electrolytic in parallel with a 0.1uF Ceramic.''

    But nothing helped.

    I even did that.

    Thanks for the feedback, I am now just going to wait for the 555 CMOS version. I will update you about the results.
     
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