555 timer as a missing pulse detector

Discussion in 'The Projects Forum' started by Reshma, Mar 13, 2007.

  1. Reshma

    Thread Starter Active Member

    Mar 11, 2007
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    Hi Folks!

    I have undertaken a project which is part of my curriculum and I have chosen to use a 555 timer as a missing pulse detector. I am trying to keep my circuit as simple as possible.

    At the outset, the input for the missing pulse detector will be a signal will varying pulse widths. This can be generated using a 555 timer in the astable mode with a modulating signal applied at the Control pin 5. I will get a pulse-width modulated signal at the output pin 3. (This Circuit is functioning well).

    I gave this modulated signal as the input to the missing pulse detector circuit operated in the monostable mode. But I am unable to interpret the output waveform of this circuit. I know that this circuit will detect and missing pulse according to its R/C combination i.e. T = 1.1RC.

    Any flaws in my circuit design? Any suggestions you can make to improvise?

    I have attached the circuit diagrams that I have used.

    [​IMG]
    [​IMG]
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Attach a schematic of your circuit so that we can see how you have it connected up.

    hgmjr
     
  3. Reshma

    Thread Starter Active Member

    Mar 11, 2007
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    This is the schematic diagram with the exact values of the components I have used.

    [​IMG]
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Could you make the picture larger?
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
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    Pssst... Joe... click on the picture...
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Psssst ... thingmaker ... I did, and it didn't work.

    None of the properties of those pics exceeded 200 pixels.
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
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  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    It must have been a MSIE 7 problem. The latest links worked better. Thanks.
     
  9. Reshma

    Thread Starter Active Member

    Mar 11, 2007
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    Ok, now can someone help me out here? :)
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
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    As noted by Tony Van Roon himself on the website where you got your missing pulse detector circuit from, the o/p of your monostable is a function of resistance and capacitance, or 11mSec in your case. (You did even note this in your first post.) The frequency of your input pulses is too low, so your missing pulse detector is detecting oodles and scads of missing pulses. Either bump up your input frequency or change your timing component values.

    I suggest re-reading the Tony Van Roon website from which your missing pulse detector circuit was copied.
     
  11. Reshma

    Thread Starter Active Member

    Mar 11, 2007
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    Thanks, :) Also, can you explain the role of the pnp-transistor used in the missing pulse detector circuit. I have used BC557 in my circuit.
     
  12. hgmjr

    Moderator

    Jan 28, 2005
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    I believe its sole mission is to discharge the cap in preparation for the next pulse cycle. This in turn sets up the remaining internal circuitry for the next cycle.

    hgmjr
     
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