Trigger Pulse for IC555 from an inverted square wave

Discussion in 'The Projects Forum' started by varundua, Jul 8, 2015.

  1. varundua

    Thread Starter New Member

    Jun 15, 2015
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    Hey,
    I have this circuit idea of which I'm somewhat confused about its logic on how I should on to implement it.
    I have a voltage sensing section that gives me a square wave of 12V DC. This is applied to a buffer stage which produces 2 12V DC Square Waves on parallel lines. One line goes to a mosfet A which is connected to a coil load and the second one goes to an inverting circuit.

    My doubt is how should I create a trigger pulse for IC555 from this inverted square wave such that the trigger is formed right at the rising edge of inverting square wave. And how would the buffer stage look like?

    Furthermore, the output of IC555 will go to a mosfet B which will go on to the coil load mentioned above. Basically alternate switching from the 2 mosfets depending on the voltage sensing section.

    Thanks :)
     
  2. MikeML

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    Post a schematic of what you have now, with the areas that need improvement circled...
     
  3. varundua

    Thread Starter New Member

    Jun 15, 2015
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    Oops I forgot to encircle but yeah, I need the buffer stage and the trigger pulse generator creating the trigger for IC555 at the rising edge of Inverted Square Wave

    Also, the IC555 works in a monostable mode with the option of having an option of varying the delay it can give.. 0ms, 1ms,2ms and 5ms IMG_2031.JPG
     
    Last edited: Jul 8, 2015
  4. GopherT

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    Nov 23, 2012
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    You don't need a trigger "pulse generator", you just need a 10nF cap and a 33k resistor pulling the input to 555 to 5v.

    The bottom trace is a 1 kHz square wave and top is output of the 555 timer.

    image.jpg

    image.jpg
     
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  5. varundua

    Thread Starter New Member

    Jun 15, 2015
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    Thanks Gopher but there's a slight change and so I would like to describe the whole circuit which would help in explaining the circuit.

    The first stage is a voltage sensing stage. It's output is such that if the Input voltage is above 60% of 230V.. we get a positive 12V DC as output but if lesser than 60% then we get a 0V DC Output thus creating a Square Wave.

    The buffer stage splits the output of Voltage Sensing Stage into 2 Equal lines of Square Waves of 12V DC each. The 1st once drives a MOSFET A (as a switch) such that when there's a postive peak of the Square Wave, it will switch ON the MOSFET and thus charge a coil connected to the MOSFET.

    The other 12V DC Square Wave line goes to an IC555 (in monostable mode) which is connected to a MOSFET B (as a switch). The MOSFET B should switch ON when the Square Wave comes down to 0V. The IC555 gives a delay of 5 seconds which is connected to the Gate of the MOSFET B. The MOSFET B is connected to the same coil mentioned above for charging purposes.

    There is a capacitor bank connected to MOSFET B which provides charging of the coil when there is a LOW on square wave which starts after MOSFET A closes. The duration of thecharging by the Capacitor Bank is decided by the output of IC555 which is 5 second

    So this is the logic that I want to implement. Voltage Sensing part is designed. So that's not much to worry about. My only doubt is how will the buffer stage exactly look like and how will the trigger pulse be generated in accordance to the logic I mentioned above.
     
  6. Alec_t

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    Sep 17, 2013
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    Is the 230V input (which should be mains isolated for safety reasons) at mains frequency, such that the 12V pulse from the sensor repeats at 50Hz (or 60Hz)?
     
  7. varundua

    Thread Starter New Member

    Jun 15, 2015
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    No not 50Hz, It is dependent on the Voltage Sensing section.. as in its variable on the input that is provided to the sensing section. If it is greater than 60% of 230V AC then we get a positive in the square wave otherwise if lesser than 60% of 230V AC then 0V.
     
    Last edited: Jul 9, 2015
  8. Alec_t

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    If the voltage sensor simply gives 12V out whenever a 230V RMS AC input exceeds 60% of the positive peak amplitude then surely the 12V pulse will occur in each AC cycle and have a frequency equal to the AC frequency? :confused:. If not, how does the sensor work to give a single 12V pulse over many AC cycles? I ask, because retriggering of a 555 monostable (depending how its used) can prevent it timing out.
     
  9. varundua

    Thread Starter New Member

    Jun 15, 2015
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    Slight changes to the requirement. The 230V AC Signal is applied to a bridge rectifier which converts it to 326V DC. The whole system is isolated.

    Now if due to voltage fluctuations if the applied voltage goes below 60% of 326V, then a LOW signal is created otherwise it's high if above the 60% marks. No such particular frequency of the rectangular wave being created.

    What I basically need is how do I create the falling edge triggered monostable multivibrator.

    And how do I produce 2 similar rectangular waves of amplitude 12V from 1 single 12V Rectangular wave that comes out of the voltage sensing section.. basically the buffer section I'm talking about
     
  10. Alec_t

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    Is there a smoothing cap to maintain this voltage? If so, why would the voltage drop to below 60% of 326V ? If no cap, then the voltage will drop to zero every AC cycle.
    It would help if you could post a schematic of your 'voltage sensor'.
     
  11. GopherT

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    Yes, how do you determine a 60% duty cycle vs 40% duty cycle of a rectified AC power if measured on the DC side of the bridge?
     
  12. varundua

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    Jun 15, 2015
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    The voltage sensor section is such that there's a voltage comparator. The 230V AC Supply is converted to a 326V AC using a totally isolated bridge rectifier. The voltage comparator then measure's the line voltage with 60% of 326V to check if its above the 60% or below. Thus we get a HIGH for above 60% and LOW for below 60%. No such particular ON and OFF times for the square waves since the variations in the line voltage is random..
     
  13. Alec_t

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    Please answer the questions in post #10 and post a schematic. Until we know the starting point there's no clear path for us to follow to help you.
     
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