4047 monoshot(1ms pulse width)

Discussion in 'General Electronics Chat' started by vinodquilon, Mar 2, 2013.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    I have designed oneshot for 1ms Pulse width using 4047.

    R5- makes output current >= 10mA to ensure load regulation.
    C4- prevents bouncing of trigger switch.
    1N4002- prevents discharging of c1 through LM317T.

    1. Is it possible to see the 1ms 'short time' switching between RED & GREEN LEDs at output of 4047 ?

    2. Can I use C4 in parallel with C1 as both have different roles in the circuit ?

    3. 10K or 100K which one is better to pull down trigger switch to GND ?
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    The persistence of vision at least is 1/16 Sec=62.5mS.

    I don't get it, why you want to do that, if you want maybe you can increasing the value of C1 some more.

    10K is better.
     
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  3. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    1. That is I have to use oscilloscope in trigger acquisition mode with proper trigger level and trigger slope for detection of 4047 1ms output instead of LEDs:(
    The output source current of HCF4047 at 5V is 1mA. Is it enough to drive output LEDs ?

    2. C1 is used to improve transient response of LM317T and C4 to suppress switch bounces.:confused:
    As C1 and C4 comes in parallel the net result will be C1+C4. But I want 1uF tantalum to improve transient of LM317T and 0.1uF monolithic capacitor as switch debouncer.
     
    Last edited: Mar 2, 2013
  4. w2aew

    Member

    Jan 3, 2012
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    1mA is a bit small for LEDs, unless they are low current / high-efficiency types.

    To denounce the switch, C4 should be in parallel with R8. It is not doing any debouncing where it is.
     
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  5. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Can I use the following debounce arrangement ?

    During switch press C4 would be charged through 100K and 100R to 5V and this time is very high as compared to discharging interval. Due to instant release of switch, C4 will be in partially charged condition. When switch is released C4 will start discharging through the switch thus opposing bounces at the 8th pin of 4047.
     
    Last edited: Mar 2, 2013
  6. ScottWang

    Moderator

    Aug 23, 2012
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    If you want to do the debounce, you can do as below.

    [​IMG]
     
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  7. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    I have made another arrangement as shown in reply posted just above your post.
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    Sorry, the circuit that i posted is designed for the push sw put on Vcc.

    This one is designed for the push sw put on ground.
    [​IMG]

    You can compare your circuit and mine, and you can try both to choose one.
     
    Last edited: Mar 2, 2013
  9. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Can you pls explain how the above arrangement does remove the bounces at trigger input pin of 4047 during releasing of the switch after triggering ?
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    Sorry, I forgot to remove the line between trigger pin and Gnd.

    The below circuit is the cap charge when the sw is pressed and the cap discharge when the sw is released.
    [​IMG]
     
  11. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    But discharging direction is always in reverse of charging direction with opposite polarity at terminals of tantalum/electrolytic 1uF. Although your indicated direction for both charging and discharging same !!!

    Similarly, I want zero level at +trigger input at released condition and low-to-high edge at pressed state to trigger the monoshot. But your arrangement is not for that.
     
    Last edited: Mar 2, 2013
  12. ScottWang

    Moderator

    Aug 23, 2012
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    Because you changed the sw position on #5, it seems like that you need a low trigger signal, so I changed the direction of rc circuit, the circuit on #6 will suit for your need.
     
  13. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    For HCF4047BE, trigger high voltage should be greater than or equal to 3.5V and low level should be less than or equal to 1.5V.
    Your arrangement will take around 56.5mS to charge to 3.5V. It means I have to push the switch for minimum 56.5mS time to trigger the 4047.

    Can I reduce this time by changing 47K to low value and 1uF to 0.1uF ?
     
  14. ScottWang

    Moderator

    Aug 23, 2012
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    You can try it, if you have oscilloscope, then you can measure the trigger pin and pulse of output.
     
  15. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Here is another arrangement with output drive and debouncer. :)
     
    Last edited: Mar 3, 2013
  16. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    As per my friends opinion the debounce circuit has a slow rise time and fast fall time, so would best be used for triggering the -T rather than the +T pin of the mono.


    Hence I simply change the trigger line from 8th pin to 6th pin (no changes in other parts).

    OR

    Can I put two schmitt NOT gates in series between trigger line and debounce output so that sharp edges trigger signals can generate ?. How to take care Lower and Upper threshold of schmitt trigger NOT during the design of debouncer ?
     
  17. ScottWang

    Moderator

    Aug 23, 2012
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    Below is also has debounce function, you can refer this as you want, whatever you want to put schmitt before the debounce or after.

    [​IMG]
     
  18. ScottWang

    Moderator

    Aug 23, 2012
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    I have a new idea to replace the function of debounce, that is -- Lock for one clock trigger only.

    I haven't try it yet, so if you have any problem then post it.

    [​IMG]
     
    Last edited: Mar 3, 2013
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  19. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    As per my friends opinion the debounce circuit has a slow rise time and fast fall time, so would best be used for triggering the -T rather than the +T pin of the mono.


    Hence I simply change the trigger line from 8th pin to 6th pin (no changes in other parts).

    If I change the trigger line from 8th pin to 6th pin, pulse will generate only when the trigger is released.

    To compensate the anomaly I just reverse the connections on the switch as shown below.
    It has an added advantage that the minimum time required for keeping the switch pressed is also get reduced.:)
     
  20. ScottWang

    Moderator

    Aug 23, 2012
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    Have ever try your circuit, and measure the pulse of output?

    If using the debounce circuit that i drawed, it can be solve the bounce problem, but you don't want the debounce time two long, I'm afraid that it will lose the fuction of debounce.
     
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