False trigger in a 555 based on off circuit

Discussion in 'General Electronics Chat' started by lubnaan90, Mar 4, 2012.

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  1. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
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    Hello Members,

    Here is this problem which i haven't been able to rectify, i hope with the help of some of the members in this forum i might be able to over come this small problem.

    I have recently built a Toggle on/off switch based on the NE555 chip, i have attached a schematic and a pcb design if someone else wishes to build it. The circuit works fine except i have some problem with "False triggering" in this circuit. False triggering in this circuit occurs in several different way, some are mentioned below...

    1) Scenario when the relay is disengaged (no activity) & at the same time if i start some other electrical appliance such as a vacuum cleaner, my Power amp or the air-conditioning, the relay engages automatically (with out me touching the switch).

    2) Same as above but the opposite, when the relay is engaged & at the same time if i start some other electrical appliance, the relay disengages automatically (with out me touching the switch).

    3) This one is Little strange, some time when I Unintentionally touch the transformer that is powering this circuit or the chassis in which this circuit is located, False Triggering occurs.

    Plz Note: The Above mentioned problem do not occur every time, just frequently.

    As Recommended by another member to stop false triggering crutschow I tried the below:
    1) Add a 0.1μF decoupling capacitor directly from pin 8 to pin 1------- I Did this
    2) Add 10μF capacitor across (Parallel) R3 to minimize false triggering.-------- I did this as well but after adding this cap the relay fails to Engage for some reason.

    Any Help from Kind members will be deeply appreciated, Thanks in Advance.

    Best Regards.
     
  2. NM2008

    Senior Member

    Feb 9, 2008
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    0
    Hi,
    A .1uf cap tyed between control pin 5 and ground might help.

    Regards NM
     
  3. k7elp60

    Senior Member

    Nov 4, 2008
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    I'm not sure what you are trying to do. You have pins 2&6 biased a 2.5V.
    The way the 555 operates is when pin 2 goes at or below 1/3Vcc the output pin 3 will go high. When pin 6 go to or near 2/3Vcc the output goes low. The only time that the relay will energize is when the output pin 3 is high. Since you have R4 connected to the output. When you depress S1 capacitor c1 may act as a short and cause the output to go high as pin 2 goes below 1/3Vcc(1.66V). I don't see how the output will ever go low(pin 6 at 2/3Vcc(3.33V) as R2 and R3 allow the charge of C1 to only charge to 2.5V.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is the source of your +5V?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    C1 gets charged to the output voltage through R4. When the output is low, C1 gets charged to ≈0V. Momentarily closing the switch puts ≈0V on pins 2 and 6, causing the output to go high. This charges C1 to ≈vcc (ideally). Pushing the button again causes pins 2 and 6 to go to ≈vcc, and the output goes low. Ad nauseum.
     
  6. mlog

    Member

    Feb 11, 2012
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    For power supply bypassing, National recommends a minimum of 0.1μF in parallel with 1μF electrolytic.
     
  7. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
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    Plz pay attention to the picture i have attached with this quote, on the right side of the board you can see a Transformer (Primary 220v - Secondary 12v), a Bridge rectifier, Filter Cap, Regulator 7805 and hence 5v Dc routed to the circuit.
     
  8. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Just Tried this, But the false trigger still persists.
     
    Last edited: Mar 5, 2012
  9. k7elp60

    Senior Member

    Nov 4, 2008
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    Once again Ron is correct. I forgot that when the push button is not depressed the capacitor will charge to ≈Vcc, because of the connection of R4 to the output.
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Without a schematic of your regulator circuit, it's impossible to say. However, from the photo, I think that you don't have sufficient filtering on the regulator circuit. Note that the attached regulator is for 12V, but C1, C2, C3, & C4 would be the same with a 7805 instead of a 7812.
     
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  11. Wendy

    Moderator

    Mar 24, 2008
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    The problem is quite simple, 5VDC is too low for this circuit to work. The reason is the output voltage is 3.6-3.8VDC when the 555 is high.

    The 2/3 trigger voltage is 3.3VDC. Basically it is too close to the power supply voltage.

    Raising the power supply voltage will solve the problem.

    I thought I invented this circuit when I came up with it. Fat chance, the 555 has been pretty much explored.

    555 Bistable Multivibrator
     
  12. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello,
    Thank you for writing, it's a pleasure to learn that you might be the inventor of this circuit.

    I found this circuit here http://www.electronics-lab.com/projects/misc/012/index.html and modified it as per my requirement.

    I will try raising the voltage to 12v Dc. Should i change the Relay to 12vDc aswell?

    Best Regards,
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    No, I just invented independently. Bill Bowden had it on his website in 2006, and it is probably older still.

    Yes, match the relay to your power supply.

    Assuming proper interfacing, the CMOS 555 will work with much lower voltages. It is specifically the Darlington drivers inside the 555 that is causeing the problem. A standard 555 is not rail to rail (goes to the power supply levels), a CMOS 555 does.
     
  14. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello,
    I changed the Relay to 12vdc, Raised the voltage to 12vdc (by changing the regulator from 7805 to 7812), The circuit now seems to be working fine, no false triggers now. Thank you for the Useful Advise.

    However i need to ask, earlier one of the members recommended me to solder a .1uf cap between control pin 5 and ground. I did this, which offered no change, so Should i remove this cap or let it be as it is now.
    Best Regards.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    I'd leave it. It does no harm, and might help.

    Specifically it makes the 555 very immune from noise. For a circuit such as this application, this is a very good thing indeed.

    When I am wiring an experiment I do not bother with it, but if I know the final item is going into a noisy circuit, such as digital, I add it.

    The advice about using power supply bypass caps is good for all of the same reasons, and I use the same criteria whether I add it or not.

    Decoupling or Bypass Capacitors, Why?
     
  16. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello,
    Ok I will let the Cap be as it is, Thank you for the help, i feel relieved that after trying so many unsuccessful methods i finally solved the problem with your help. Thanks again.
    Best Regards.
     
  17. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello Bill,
    The problem (False Trigger) started again, Some time when i touch the chassis, or switch on/off other appliance, the false trigger occurs, really stubborn problem this false triggering is.

    Should i try changing the power supply for this circuit?

    Best Regards,
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    It sounds like your power supply is very noisy. Caps can help with that, but only so much. I am willing to bet if you looked at your power supply the voltages (on the power supply line) are hitting very low voltages.

    At this point do everything you can to clean up the power supply. Do you know what a PI filter is?

    Another experiment is to use a 9V battery by itself only for the 555 circuit, it will be isolated from the rest of the circuits.
     
  19. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Hello, Tried the circuit with several different 12V power supplies, but the problem still persists. However, i have noticed one thing, the center pin of relay (As shown in the Picture attached) is really sensitive, for e.g if the relay is engaged and i touch that pin with a Multimeter lead then it reacts to it (Relay Disengages), as a matter of fact i am using this circuit as a 220V on/off switch, so this explains why the Relay reacts on other appliances. Also i have tried changing the relay aswell but no Luck.

    This problem is getting frustrating but i dont plan to give up on it. I Really appreciate your Help.
     
  20. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Have you tried adding a 10nF to 100nF cap from pins 2 & 6 to ground? That might desensitize it to transients.
     
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