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Help Needed with 555

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by James12345, Feb 19, 2018.

  1. James12345

    James12345 New Member

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    Hey folks,
    Just created an account here to ask some advice since i have a somewhat similar scenario..
    But really all i need to ask is in regards to 555 timers...
    I'm just trying to get the darn thing to put out a one shot trigger. I'm using it within a 3PDT switch to run the 4017 sequence. The other two pulls do other things. My concept is just to have a separate 555 dedicated to putting out a trigger from a button press, which hits the reset on the 4017. Via other circuitry the sequence starts at the same time, so this all serves as a nice musical way to have control over the stop and start of the whole cycle, and having it start exactly at the begging when hitting start.
    Problem is, I have a weird way of getting the 555 to make the necessary pulse. I'm basically using this.. http://555-timer-circuits.uk/operation/one-shot2.html?[​IMG]

    It's designed so that the pulse only lasts the specified length no matter how long the momentary button happens to be pressed. This is exactly what i need, rather than needing to press and release the button a nanosecond :) And i DO have it working, however i can only get it working when i simply switch on and off the actual +5 rail itself. This seems to do the trick.. on the scope, the pulse is exactly what i specify with the "Ra" and "C" above. Looks great. But, this is not how the circuit above is laid out. I've tried redoing and triple checking the above circuit, and never got it to send a pulse other than when i just powered the darn thing up. Nothing whatsoever showing up on scope. Spent hours just making sure it was perfect. So, i relocated the momentary switch to be the main power switch for the IC and just jump'd the spot where the momentary used to be in the schematic above.

    Any thoughts on using this method? My greatest concern is propagation time for when turning the whole 555 on... what will it be? In my final design, I need the pulse to last around 125 uSec so that it doesn't overlap past clock pulse #2's rising slope. I am designing to have a max frequency of the pulse clock coming out of the main 555 (not this one) as around 4K hz. As a reference, this means that instead of my pulse needing to last less than 1 millisecond (0.001) for a 1 kHz rate, i need it to first be 4 times shorter (0.000250) but as a safety just half that to be (0.000125). That is 125uS or 125,000 nanoseconds.

    I'm pretty sure that's a fine range of operation for a 555 making a pulse, but powering it up first too? And is there a delay first, and then the pulse occurs?

    Or, how can i get the above circuit working? I'm powering it with +5VDC. I've separated it completely from the rest of my circuit, and recreated it exactly as above, a few times over. I get nothing on the output with that circuit. As an easier test to measure, I've been using a .1uF cap and a 1M resistor. This gives a nice clear 100mS pulse. When i switch the power to this 555, the scope indeed shows a nice clean approx 100ms pulse. But i can't get that 100ms pulse with the above switch design.

    Thanks!!!
     
  2. Ian Rogers

    Ian Rogers User Extraordinaire Forum Supporter Most Helpful Member

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    I have given you your own thread... Thread hi-jacking isn't encouraged here..
     
  3. James12345

    James12345 New Member

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    Ok no prob.
    Would love some 555 advice if anyone can chime in.. been driving myself bonkers over here
    thanks!
     
  4. dave miyares

    Dave New Member

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  5. AnalogKid

    AnalogKid Well-Known Member

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    Add a 10K pull up resistor from the switch/capacitor node to V+. The trigger circuit works only if there is zero charge (both pins at V+) on the capacitor when the lower end is switched to GND.

    ak
     
  6. James12345

    James12345 New Member

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    YESSSSSSSS
    thank you. worked perfectly :)
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    This is from the original NE555 datasheet:
     

    Attached Files:

  8. dave miyares

    Dave New Member

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  9. James12345

    James12345 New Member

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    Hmmmm. It appears that my situation is a bit more complicated that i'd expected..
    This pulse is hitting a 4017's "reset" pin.
    It works great, to reset the 4017's sequence. Basically, along side of switching on the "running" of the 4017, i am also sending this pulse i've created to the "reset" on 4017, with the same 2PDT switch, so that the sequence starts from the beginning.
    It works now. The sequence starts up right on the money and it runs as expected.... except, i hadn't realized that the "reset" pin will also read the falling edge of the the pulse i'm sending in, and interpret the timing of proceeding to step 2 after this falling edge... I slowed down the sequence and created a long 1 second pulse and it's loud and clear what's going on... The 4017 receives the rising edge and thus kicks into step 1, and then when it receives the falling edge of the pulse, it then gives 50% of the step duty length until it proceeds to step 2. So, if my pulse is super long, like 10 seconds, the 1st step will hang for that whole 10 seconds of course, but then the 4017 will wait 50% of ITS OWN step rate until it goes to step 2.

    The consequences are two fold.. if my 555 pulse is shorter than 50% of the 4017's total step duration, the 4017 will proceed to step 2 faster than it goes from step to step for the rest, making the 1 to 2 motion too soon. And if the pulse longer than 50% of of the 4017's step duty cycle, the first step will hang too long in total because first it counts the pulse and then it counts 50% of its own total step duration.

    So, when one patches up the 4017 to make one of its steps to reset it and thus shorten the total number of steps, the reason that works is because the 4017's outputs are perfect 50% duty cycle.

    Is there a way for me to grab a single complete square cycle from the main 555 which is acting as the clock for the 4017, so that i can send that single square to the reset of 4017 and have it be the same rate/duration?

    Or, is there a way to sync two 555's together? If so, i could keep my trigger setup from the isolated 555 and have it's rate be controlled by the main 555 that's clocking the 4017. The total duration rising and falling would be the same as each single pulse from the main 555's rate, but this 555 would just run the rising and falling one time as a pulse?
     
  10. AnalogKid

    AnalogKid Well-Known Member

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    Way too many pins and combinations of connections to discuss with text. Full schematic?

    ak
     
  11. James12345

    James12345 New Member

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    was just about to make a clean version of schematic and upload, but thought of one last thing and fixed it. works great now.
    i had been combining a few sources into the Reset destination, and even while the others were idle i hadn't realized the impact a resistor to ground was having things at the Reset pin. It had been necessary for one particular source, but, i suppose it's throwing things off now. I removed it and the entire circuit functions just as i'd hoped now.
    I will likely be cleaning up the hand drawn circuit and post it soon here for general thoughts.
    thank you!
     

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