Why can't I ever get simple 555 timer circuits to work?

Discussion in 'General Electronics Chat' started by hewad, Aug 18, 2008.

  1. hewad

    Thread Starter Member

    Sep 26, 2006
    12
    1
    Could somebody please help me. Whenever I try to use a 555 timer chip, the pulse duration is never even close to the calculated value. Oftentimes it is locked at a duration that will not change no matter what values of R or C I use. For example, I just made a monostable multivibrator. I dead bugged it and used a .1uf cap with a 62K resistor. The trigger uses a negative pulse from a shorted out .1uf cap. Pretty standard stuff. I was shooting for about a 7ms pulse to turn on a sensitive gate SCR. The pulse came out at about 12ms. Since the SCR had a turn on time of only 1us, I switched out the resistor to a .1k and left the cap in place. That should have given me about a 10us output pulse but the damn thing is still stuck at 12ms. This is not the first time it has happened to me. I have used different 555's as well as 556's and continue to have these types of problems. What could I possibly be doing to almost never be able to get a simple 555 circuit to work? Is it grounding? Ripple on the power supply? Bypass capacitors? Should I be hooking something to the reset and control pins? I'm at a loss and I could use some guidance.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    How are you triggering the 555 ?
    This must be done by giving a pulse on pin 2.
    It must be a negativ going pulse from V+ to less then 1/3 V+.

    See this page for more information.
    http://www.uoguelph.ca/~antoon/gadgets/555/555.html

    Greetings,
    Bertus
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Posting the schematic would be a help...
     
  4. hewad

    Thread Starter Member

    Sep 26, 2006
    12
    1
    http://home.cogeco.ca/~rpaisley4/LM555Basics.GIF

    It's the bottom One Second Oneshot Monostable Oscillator.

    This is as simple as it gets. Why the heck won't the thing work? Like I've said before, I can rarely get a 555 to work. What kinds of basic things could I be doing to cause this?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sorry, all I can see are chicken scratches.

    Do you have the LM555 data sheet? It covers every aspect of 555 operation.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    This circuit is not the best because pin 4 is left floating. This is not good because the chip does not know it has logic 0 or logic 1 on this pin. This problem arises with CMOS technology chips (which are the most). If the chip is with TTL technology then if the pin is left floating the chip will take it as logic 1.

    Can you provide your circuit to check it?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Did you click on the picture?
    When you view large pics using Microsoft Internet Explorer, it downsizes them to fit within the window. If you click on the picture, it will then be displayed at normal size. You'll then have to scroll around to see the rest of the picture.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    It's because you're changing the values of the wrong part of the circuit.

    It's the 1 Meg resistor and the 1uF cap that are setting the ON-time of the circuit. According to the formula presented in the schematic, your 12mS is very close to the calculated result.

    Change the 0.1k resistor back to 62k-100k so that you get proper debounce on the switch.

    Try decreasing the 1uF cap. The discharge pin (pin 7) has a current capacity of 15mA @ 15V, and about 4.5mA @ 5v, so it's rather limited.

    I think that 830pF (or 0.83nF) will get you in the ballpark. However, this is at the bleeding edge of what a 555 timer can do. You will need to have bypass capacitors across the Vcc and ground pins of the 555; I suggest 0.1uF ceramic/tantalum, 10uF tantalum and 220uF low-ESR electrolytic with the leads as short as possible.

    Pin 5 should have a 10nF (0.01uF) cap to ground.
     
  9. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    One other thing may be the tolerance of your timer cap. Check the capacitance with a meter, and make sure it's near the nominal value.

    --Rich
     
    Last edited: Aug 19, 2008
  10. Wendy

    Moderator

    Mar 24, 2008
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    I'm in the process of doing experiments with 555's, that will likely be my next addition to the AAC eBook. One of the thoughts that occurs is what mode you're using. I have to verify this, but I believe some configurations of the 555 timer monostable don't actually start timing until the signal is release. The was to get around this is a resistor / capacitor on the input (pin 2).
     
  11. hewad

    Thread Starter Member

    Sep 26, 2006
    12
    1
    I did some further testing and it seems to me that the output pulse is locked because it has a shorter duration than the capacitor discharge event. As long as pin 2 is low the output will be high, independent of the timing capacitor or resistor. Is my thinking correct? If it is, why is this not on every web site which claims to explain how to use this IC? This little tidbit of information could have saved me countless hours of frustration if I'm right.
     
  12. hewad

    Thread Starter Member

    Sep 26, 2006
    12
    1
    Oh, by the capacitor discharge event I mean the discharge of the capacitively coupled negative pulse going into pin 2 thus dropping it under 1/3Vcc. I suspect that if I make this event happen faster the output pulse width will decrease to the point where the timing capacitor and resistor have control.
     
  13. dogstail

    Member

    May 8, 2008
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  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You are correct. The information is buried in the fine print of Tony van Roon's 555 tutorial. I don't know about the sites you have visited.
     
  16. shj

    New Member

    Aug 19, 2008
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    This is due to tolerence in capator and resistors.
     
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It doesn't need to be. All you have to do is make the time constant of the triggering differentiator much less than the desired pulse width. If you do this, tolerances will be irrelevant.
     
  18. kammenos

    Active Member

    Aug 3, 2008
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  19. hewad

    Thread Starter Member

    Sep 26, 2006
    12
    1
    I did verify that this is exactly what has been happening. Too long of a negative going pulse to pin 2. I thought I was crazy. This information is left off of many sites that deal with 555's. If you don't know this, all the calculators in the world will not make a bit of difference. You'll only run into it when you start trying to get short pulses out of the IC. To get a small enough pulse I just left the .1K resistor and .1uf cap in place and decreased the size of the trigger capacitor to .022uf. This allows the cap to discharge very rapidly and I now have the output pulse down to 1.3ms. This should be fine for the SCR gate drive.
     
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