555 edge trigger question

Discussion in 'General Electronics Chat' started by mbohuntr, Jan 11, 2011.

  1. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    The timer circuit is still acting erratic. I went back to Bill Marsden's edge trigger using 10k's on either side of the .1uF cap, then put LED's on the IR detectors to visually watch the action. Powered on, it acts fine, when motion is detected, it will cycle normally. Occasionally, the first timer fails to completely reset. The trigger voltage is fluctuating as well as the PIR output on the other side of the .1uF cap. Here's the question... If I place my DMM probe on the trigger pin, the ground probe on the circuit ground, the circuit will stabilize. Am I adding to the pullup resistor, or taking away from it??? Will changing either resistor value improve the stability??? I don't think I can shorten the time on the first stage because the R/C decay on the second stage trigger is already slightly shorter than the time on the first stage. Any help would be greatly appreciated!! (sorry, I tried to save it as a .png)
     
  2. Ron H

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    Apr 14, 2005
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    Do you have a datasheet or schematic for the PIR detector you are using?

    Why do you have more than one thread for the same topic? It makes it really hard to see what you have done previously.
     
  3. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Here is the link, Other than trying timing changes, and triggering methods, And fixing the omitted connections for the inverter, nothing changed.

    http://www.sparkfun.com/products/8630

    I was hoping something would stand out as either an error, or needing change.
     
  4. Ron H

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    Your PIR detector is designed to run on 12V. Are you using 5V on it? If so, that might be part of your problem.
    Your pullup resistor is correctly connected to 5V.

    EDIT: This guy says it will work on 5V, but I would still try 12V if I was having problems.
     
    Last edited: Jan 11, 2011
  5. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks, I removed the regulator earlier. I will try to connect the output to my scope to see if that will reveal anything. Unfortunately, it's an older analog model. I wont be able to capture signals, but I might see a "flutter"
     
  6. Ron H

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    What is the second 555 supposed to do? I don't see how it can trigger, as shown on your schematic.
     
  7. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    The positive pulse of the first stage goes to the inverter and ,when inverted, cuts off Q1. This allows the R/C circuit to decay slowly enough to delay the triggering of the second stage. The Camera needs to receive a "wake-up" pulse, then delay for around a half a second before triggering the shutter. It works nicely on it's own, The problem starts when I connect the trigger from the PIR. I have a feeling the PIR trigger is being held low longer than the first stage pulse, But didn't correct when I lengthened the timer for the first stage. I wasn't using Bills trigger method when I did that test, so that might work as well...
     
  8. Ron H

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    When Q1 cuts off, there is no discharge path for C3.

    EDIT: Maybe your schematic is wrong. Is R2 in parallel with C3, instead of in series, as shown?
     
    Last edited: Jan 11, 2011
  9. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    You know...your right!!! I wonder if I have drawn it wrong, The circuit does indeed delay, and trigger. The LEDs clearly show it. Or perhaps I got lucky and pin 2 will bleed off the current....
     
  10. Ron H

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    Did you see my edit on my previous post?
    Pin 2 on a bipolar 555 will actually charge the cap, albeit very slowly. Pin 2 on a CMOS 555 is basically infinite resistance.
     
  11. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Yep...you were right, I built it parallel, drew it in series. Good thing this is a hobby....:p
     
  12. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Sorry, My lack of clarity makes things harder. Pin 2 of the 555.
     
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