Time limit shorter than input signal

Discussion in 'The Projects Forum' started by PierreK, Apr 28, 2009.

  1. PierreK

    Thread Starter New Member

    Apr 28, 2009
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    0
    Hi everyone.
    I need to build a Trigger that will be switched on for a period of time that may be shorter than the length of the input pulse.
    I tried to build this with a RC time constraint but I have a problem discharging the cap once the input signal disappears. Also if the Input signal is shorter that the time I want the switch on my plan is doomed (but I do not think this should be the case). I also tried a 555 timer in a mono stable state but that only add the time delay to the time that the input signal is switched on.
    It is possible for me to change the design to Start my trigger when the Pulse is absent like in the second part of the image.
    I hope this is clear enough. But a picture is worth a thousand words so please look at the attached picture. Like I said the second pulse should not happen. Also the actual length of the output pulse must be adjustable with a variable resistor.
    Please help me to get this done. Thank you very mush.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    This should not be a problem in a conventional 555 monostable, as it is edge triggered with a input signal conditioner.

    Read this and see if it doesn't address your problem...

    555 Monostable
     
  3. PierreK

    Thread Starter New Member

    Apr 28, 2009
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    0
    Thank you Bill This is exactly what I am looking for. The Mono stable diagram that I had did not include the R1 R2 and C1 and connected the Input directly to Pin 2. So it is essentially the same as connecting to Point B in your Diagram. Using Point A instead will solve my Problem.
    Thank you so much I, will try This tonight but I already know It will solve my problem.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Signetics invented the 555. Then Philips bought Signetics.
    The coupling capacitor and resistor that feeds a short pulse to trigger pin2 is in their datasheets. It is not in the datasheets of National Semi nor Texas Instruments.
     
  5. PierreK

    Thread Starter New Member

    Apr 28, 2009
    8
    0
    I did try this modifications last night and now I am stuck with a new problem. It is still related to the original question and will help future readers of this thread so I do not Post a new question but rather continue with this thread.

    The timer do stop after the desired time delay when I give it short inputs as well as when I give it long inputs. but when I give a long input it triggers again when the input signal stops. So for an input signal that is shorter that the time period the time trigger once and is switched on for the duration of the time period. but when the input signal is longer than the time period the timer trigger twice, when the signal starts and again when the signal ends. I inspected the two resistors and the capacitor that cause the limiting time on the input signal and realized that this is very mush the same as my first attempt to generate a short pulse from a long input signal using an RC time constraint.

    Can anyone help me figure out how to allow the Capacitor to discharge when the input signal stop without triggering the 555 timer again when the output is already low?
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The capacitor coupled 555 monostable circuit is usually negative edge triggered, meaning a high to low input signal voltage will trigger the 555. There should be no triggering when the input voltage goes from LOW to HIGH.

    You said your input signal "STOP". What exactly does it means? There is no stop in voltage, only high, low and changing level from one into another.

    You'll need to post a schematic of the component values that you are using along with the input and output waveform you are getting right now.

    Most importantly, what waveform you wanted to get in the first place.

    Are you using a manual switch as the triggering signal source?
     
  7. PierreK

    Thread Starter New Member

    Apr 28, 2009
    8
    0
    Hi in the first response to my question Bill Marsden pointed me to a link with a circuit explaining the 555 timer connected in a way that should solve my problem. I build this circuit exactly like in the article mentioned with the exact same component values. for all resistors and capacitors. except the resistor and capacitor that is used for the time constraint. As for the Signal stopping the diagram in the article
    555 Monostable display a wire that is connected to Ground and is used as a switch to connect to Point A or Point B. This is exactly the way I build my circuit.
    I also attach an image of the expected and actual input and output. Note that the Expected is the same as what I stated in my first post.

    I hope this explain my problem good enough.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Your input signal is not of the correct polarity to trigger the 555.

    As such, you will need to add a NPN to reverse its polarity. The following circuit will do exactly what you want. Note the change of R1 resistor value from 10K to 270K marked by a "star".

    [​IMG]
     
  9. PierreK

    Thread Starter New Member

    Apr 28, 2009
    8
    0
    Thank you very mush I will try this tonight and provide you with feedback tomorrow morning.
     
  10. PierreK

    Thread Starter New Member

    Apr 28, 2009
    8
    0
    Hi eblc1388
    Thank you for the Information. I still do not know what I did wrong but last night I disassembled the hole circuit and rebuild it and it is working fine. I can trigger it from the 0V rail as in the original diagram or from the positive rail as in the modified diagram with the added transistor and it is functioning as expected. I must have had a short circuit somewhere. Thank you all for all the information and help.
     
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