Unstable Monostable

Discussion in 'General Electronics Chat' started by mbohuntr, Sep 9, 2010.

  1. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Time for some back up, I'm stumped. :confused:
    I am building a two stage timer to trigger a camera refresh and shutter. I used two 555's cause my simulater doesn't offer a 556. The output of the first stage will trigger a opto isolator for the camera wake up pulse, while the high output charges the second stage delay cap/resistor(C6,R10). When the output on the first stage goes low, The second stage trigger voltage decays to <1/3 vcc, the second stage pulse is triggered to another opto isolater driving the shutter. Here's the rub; When either stage is low, a 460mV ripple appears on the outputs. This might not happen in real life, or it might. I tried tying the resets to ground with 100pf caps, no good. I tried the same w/ the outputs, no good. I lowered the cap/ resister values down to shorten the times so that my oscilloscope will not time out. The real values will reflect the delays needed. The ripple is killing my simulators ability to display all the delays. (The ripple is at 500Khz in case that helps...) I also tried tying the output ot the first stage to the inputs of the second, but that was worse w/ no triggering at all. Any suggestions?? Thanks in advance!! Both stages work separate, just don't play well together!!!
     
  2. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    After a couple hours of internet research, it seems that 555's are inherently "noisy" with some applications. They sometimes replace them with Schmidt trigger circuits. I'm going to try the build anyway since I don't think the noise will interfere with the operation. "Kinda proud of myself for stumping you guys!"
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Looking at your design you have no filtering caps on the power supply at all. For all digital type circuits you need something in the 0.1µF area. I would also use the same values for pin 5 (you don't have pin numbers on the 555's, or if you do they are incorrect).
     
  4. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks Bill, I will do that. Some of the circuit info on the net had the filtering caps as well. I didn't see any other 555's on my simulater other than virtual. I think the numbers are for nodes. The circuits work separately, Multisim is very "oversensitive" sometimes and requires modifications I have no desire to learn. There are circuits that won't run because of error msgs on the program that require parameter mods.
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
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    Why are you placing 1KΩ resistor in the Vcc supply to 555?

    The trigger pin of 555 should not be left floating in any circumstances. A floating pin will mean troubles.

    For the 555 monostable to work correctly, the trigger pin voltage must be high. Then a short pulse to LOW will trigger the monostable. The LOW on the trigger pin must be removed before timeout of the monostable or else the monostable will not time out.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    To the OP:

    This is correct. If you look at my 555 Monostable article you will see examples of how to handle this.
     
  7. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Your right!, I never noticed that. I was concentrating in charging the trigger delay circuit, I missed keeping it high initially. I put in the 1k to keep the opto at a safe current. The max input current at low state is 6mA according to the datasheet, so I figured i'd cover both issues at once.
    I was getting a trigger on the second stage at startup, The trigger not being high initially is probably the culprit. Thank-you!!!
    More thinkin ...
     
  8. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I borrowed a circuit from Bill Bowden (Thanks, Bill) to cascade to output from one timer to the other. While pinning the trigger high did solve the instability problem, it eliminates the delay between the outputs unless I add ANOTHER 555. :rolleyes: Thanks guys, I'm learning!!!

    (Bill M. , I wasn't able to see the schematics for your monostable from the link)
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Just checked, it is still there. Did you scroll down?
     
  10. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    I did see those... I thought your blank pics were for monostable... my bad... DUUHHH. I just rechecked, Now they load???
     
  11. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Are you inferring in your illustration that R1 and C3 act as a trigger voltage delay holding the trigger high until the first high pulse from the first stage takes over, then allowing for a discharge time delay once the trigger pulse ends?? If so, That's exactly what I'm looking for!!!
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The output of a 555 shorts the power supply with a 400mA current spike each time the output switches high or low. Therefore it is crutial that a pretty big (10uF?) supply bypass capacitor keeps the supply voltage from crashing. A 0.1uf ceramic capacitor shold be in parallel to supress high frequency transients.
     
  13. Wendy

    Moderator

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    The shoot through isn't that long. I measured 50 ns max rise time for a 555 on a protoboard (AKA breadboard), arguably the absolute worst high frequency environment you can have for a high speed chip (which a 555 is probably of the lowest class). The fall time was 10 ns. Datasheets tend to show absolute worst case.

    With simple circuits it doesn't matter, but if you have more than one digital chip each chip should have a bypass across the power supply pins.
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    R1 and C3 only let the leading edge of the trigger through, which is all you need. The rest of the trigger pulse is discarded. If the values of R1 and C3 are too large you can have the signal last longer than wanted, exceeding time out of the 555, so keep them small as practical.
     
  15. mbohuntr

    Thread Starter Active Member

    Apr 6, 2009
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    Thanks Bill, in my origional schematic, stage 2 trigger is delayed by the discharge of C10.(I was charging and discharging it w/ the opto.) I was wondering if there was a way to shunt the high level of the trigger long enough to allow C10 to drop the trigger voltage long enough to trigger the second stage pulse. Right now, the pulses happen sequentially, the camera needs time after the refresh pulse to enable the shutter circuitry.

    The only reliable method I can think of is to use the falling edge of the first stage pulse to temporarily shunt the high level of the stage two trigger ...

    Some wise people on the camera board decided to go w/ microcontrollers instead of all the fuss of timing circuits. I need the expierence with component level work, and I'm just plain obstinate!!! :mad:
     
  16. Wendy

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    Mar 24, 2008
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    Works for me, I still have yet to learn the PIC or Ardino, though the writing is on the wall.
     
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