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555 monostable

Discussion in 'Homework Help' started by 50RIn, Sep 12, 2017.

  1. 50RIn

    50RIn New Member

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    I try to make an edge triggered circuit and i am at the beginning of electronic so i need your help.
    My signal is a OUT Signal from an comparator, so it'a kind of sqare wave. I have to obtain an signal like in the picture but i have to use 555.
    sad.JPG
     
  2. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Welcome to ETO, 50RIn!

    Your question is a bit confusing.

    For one, the output of the comparator is a series of positive going pulses, not a "kind of sqare wave".

    Second, when you say that "i have to use 555", do you mean as the source signal?

    If so, have you googled "555 Timer Circuits"?

    Lastly, is this a homework assignment?
     
  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    1) Is "B" going to be a fixed frequency that you know in advance? (I assume answer is "no.")
    2) Can you use active components in addition to the 555?

    John
     
  4. dave

    Dave New Member

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  5. 50RIn

    50RIn New Member

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    Thanks, Bob!
    I know is's confusing, i don't speak english very good, i'm sory.
    It's a home project, and yes i tried to google, i mounted many schematics i found and i thought could work but it didn't.
    In the photo i make you can see i am using two op amps to rectify the input signal, next i am using a comparator to know where Vin cross zero.
    My problem is how to do a 555 monostable for rising and falling edge detector, with an output signal like in the OUT Expected signal i draw.



    IMG_2067.JPG
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Your "out" signal is a square-wave.
    That can be generated by using the signal from the XOR gate to clock a flip-flop.
    Why do you "have to use a 555"?
     
  7. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    I googled the above (from your post) and found this 555 circuit.

    Using the Circuit Lab example, it produced this graph:
    upload_2017-9-12_17-8-0.png
    Note that VOUT is a negative pulse (with a little bit of overshoot). To change that to a positive pulse, add an NPN emitter follower circuit to the output.
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An emitter follower is not an inverter. Use a common emitter transistor instead.
     
  9. 50RIn

    50RIn New Member

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    Thank's guys!
    I will use two circuits like this, one for the falling edge and one for the rising edge. For the falling edge i will use common emitter for inverting signal,
    hopefully it will work.
     

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  10. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Beg to differ, ag:
    upload_2017-9-13_8-10-44.png
     
  11. crutschow

    crutschow Well-Known Member Most Helpful Member

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    ??
    You are putting a negative pulse into the base of a NPN transistor which reverse biases the base-emitter junction and keeps the transistor off.
    Don't know what transistor model you are using but you do not get the output you show with a normal transistor model (below).

    A good example of blindly accepting a simulation result that makes no sense.

    upload_2017-9-13_9-49-4.png
     
  12. alec_t

    alec_t Well-Known Member Most Helpful Member

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    cowboybob
    Your voltage source on the transistor base is 'upside down', which is causing the inversion you show.
     
  13. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Thank you both. And I mean it.

    I did fall down the rabbit hole on this one, and I apologize, especially to 50Rin.

    Hopefully, lesson learned...
     
  14. JoeJester

    JoeJester Active Member

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    What happens in LT spice if the positive terminal of the Voltage generator source is connected to ground and the negative feeds the signal to the base?

    That is how a magnatron power source would be connected.

    In that case, Vo is the opposite of Vi source or one could say it inverts the source.

    It's not blindly following what the simulator says. The signal is still positive at the base of the transistor with respect to ground.

    neg pulse.png

    Vo is inverted with respect to the voltage generator.
     
    Last edited: Sep 13, 2017
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  15. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Thanks, Joe.

    That's how I wired the negative sig source and, as you saw, how the SIM (TI TINA) responded. Intuitively it should not have worked, but it's what the circuit produced.

    I should have bread-boarded the circuit, but I've come to trust my SIM after beau-coup proofs with actual circuits. The only exception I can recall is the 555 latch circuit that the SIM allowed to work consistently without a cap on the Reset pin.

    When the heavy hitters came at me, I caved...
     
  16. JoeJester

    JoeJester Active Member

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    You have to remember that all the electronics systems in modern fighter jets have been simulated extensively as has the air frame. Then someone gets to fly the jet to see if all that simulation was correct.

    Is the simulation 100 percent correct every time? No. It depends on the models used. The models have to be good enough to use with the caveat that you could find a case where it's not correct.

    Also bear in mind that even a $20,000 piece of test equipment could mislead someone, just like the simulator can. Case in point, if you swept a 60 db notch filter tuned to 77 kHz from 1 Hz to 1 MHz, don't expect to see the nadir. Hell, don't expect to see the notch. You'd have to change the sweep to a narrower band to see the nadir. I set my sweep samples to 9999 the maximum allowed in TINA. So using this example Tina would sample about every 100 Hz. I'd see a small dip in the signal, the same I would if I used an expensive spectrum analyzer. I first observed that sweeping from 50 kHz to 150 kHz while looking at a notch filter bank.

    It's just another case of knowing your test equipment, and the simulator is just another tool in your toolbox.

    There was s "paper" out on the internet that recommended a couple of changes to the 2n3904 transistor pspice model. There are a number of 2904 models out there, with some changes to each. I can find that link again if necessary.
     
  17. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Again, thanks, Joe.

    And here I thought I was the only ETO member that's a TINA user.

    ETO seems to favor LTSpice, which I find cumbersome and without any "real" time feedback for transient events. For all I know it does, but the learning curve put me off. TINA seems to be designed for the bread boarder type, like me: I like seeing the relay actually close/open, or the LED blink :rolleyes:.

    However, for someone like myself, trained in the use, calibration and repair of all manner of high end precision measurement equipment, you'd think I'd know to always be a tad suspicious of a readout (circuit output, in this case) that doesn't "smell' right.

    Lesson (re)learned.

    I'll look for the 2n3904 pspice model changes.
     

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