windows comparator

Discussion in 'The Projects Forum' started by sharpice, Jul 4, 2013.

  1. sharpice

    Thread Starter New Member

    Jul 4, 2013
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    im having this problem with my windows comparator circuit which is feeding a 555 timer. my 555 timer is activated even there is no inputs to the opamp (741) except the power supplies which are connected at both polarities. how do i remove this voltage output from the opamp
     
  2. sheldons

    Active Member

    Oct 26, 2011
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    post schematic- my crystal ball is being polished and serviced at the moment
     
  3. sharpice

    Thread Starter New Member

    Jul 4, 2013
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    unable to insert image. it says insert url. how do i get around that?
     
  4. LDC3

    Active Member

    Apr 27, 2013
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    Under the "Quick Reply Message Box" is a button that says "Go Advanced" - click there. Above the "Message Box" is a paper clip - click there and in the pop-up windows, browse to the file to upload, click upload and then close the window. When you post your message, the file is attached.
     
    sharpice likes this.
  5. sharpice

    Thread Starter New Member

    Jul 4, 2013
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    image attached.thanks
     
  6. LDC3

    Active Member

    Apr 27, 2013
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    First, I would connect pins 4 & 8 to the battery supply so the 555 is always powered. And add a capacitor from pin 5 to ground. Your circuit will power the 555 as long as the 741 has a high output. The moment the 741 output goes low, the 555 runs out of power quickly.

    Now the question is, do you want the 555 to trigger
    1) immediately when the 741 go high or
    2) after a delay defined by the RC network?
     
  7. LDC3

    Active Member

    Apr 27, 2013
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    If I read the resistance correctly, you have a 1K resistor and a 1K potentiometer with a 100μF capacitor. If this is correct, the frequency is about 4.8Hz with a 66% duty cycle.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That doesn't make sense to me. Now you are the one who needs to post a schematic.:D
     
  9. LDC3

    Active Member

    Apr 27, 2013
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    It will have to wait until I get home in about 5 hours.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Where does the 555 get power when the op-amp goes low? Maybe you were referring to something else not making sense.
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Why not connect 555 pin 8 to V+, add the cap to pin 5, but connect 555 pin 4 to the 741 output for on/off control of the 555 ?

    Happy 4th everybody :)
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    LCD said
    If the 555 is connected directly to the battery, how does the 741 affect anything?
     
  13. LDC3

    Active Member

    Apr 27, 2013
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    In the OP's schematic, the output of the 741 is connected to the end of the RC circuit. In this case the 555 doesn't run unless the output of the 741 is high.
    In many instances, it is connected to pin 2 of the 555. In this case, the 555 provides 1 pulse on pin 3.
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    OK, I get it.:(
    I must have been half asleep when I looked at the schematic.
     
  15. LDC3

    Active Member

    Apr 27, 2013
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    Here it is. I have only made the changes in the first paragraph of the message.

    Now, how do you want the 555 to respond to the signal from the 741?
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The timing calculation equations won't be accurate, because the 741 output won't go to the positive rail.
     
  17. LDC3

    Active Member

    Apr 27, 2013
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    True, but we still don't know how the OP wants to connect them together. Maybe he just wants a single buzz and flash instead of a series of buzzes and flashes.
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Agreed. I was just throwing out a caveat.
     
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