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Op Amp novice

  1. Jan 14, 2010 #1
    Dear experts,

    I am thinking of using an op amp to trigger a circuit.

    The +ve input to the op amp should raise '1' (high) if the voltage is 5v and not anything less.
    The -ve input to the op amp should always be '0' (low) and so i thin i short it to ground.

    I wonder what op amp should i use, assuming just to achieve the above?
    Am i right to say usually, op amps V+ raised to high when voltage is a few mV?
    So, if my source is 5v, does it mean that i can achieve the objective by using a resistor as voltage divider to lower the V+ such that it is low enough to trigger a '1'?

    Sincerely
    Ramone
     
  2. jcsd
  3. Jan 15, 2010 #2
    What do you mean by "The +ve input to the op amp should raise '1' (high) if the voltage is 5v and not anything less."? Which voltage do you refer to here as 5v(Input or Output)?
     
  4. Jan 15, 2010 #3

    vk6kro

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    A good opamp if you want to run it off a single 5 volt supply is the LM324. You get 4 opamps in one package.

    However if you do this, you need to establish a bypassed 2.5 volt reference point. This is just a voltage divider across the power supply with a large capacitor to ground from the junction point.

    If you take a circuit from some drawing using a dual power supply, there may be an "earth" symbol shown. To adapt these circuits to the LM324, these "earth" points have to be returned to the 2.5 volt reference.

    So, if you return the + (non inverting ) input to this 2.5 volt point, the input voltage on the - (inverting input) will control the output.
    If the input is less than 2.5 volts, the output will be 5 volts. If it is greater than 2.5 volts, the output will be zero volts (assuming a 5 volt supply).

    So, you could feed the 0 to 5 volt square wave input to this input and it would give a good inverted version of the square wave out.
    Note that this is not a normal square wave. It is a series of pulses that vary from 0 volts to +5 volts.

    There are much better ways of doing this, however. You could use a Schmitt trigger circuit with your opamp:
    400px-Opampschmitt_xcircuit.svg.png
    This will give a cleaner square wave. (Note that it assumes a dual power supply).

    Better still is to not use an opamp at all. You could get a 74HC14 logic chip which contains six Schmitt trigger inverters.
    These are very fast logic chips and would clean up a square wave that had poor rise and fall times into a very fast version.
     
  5. Jan 16, 2010 #4
    Dear vk6kro,

    Its great to hear from you again.

    Sorry, i really need to ask another very beginner question in order to understand op amps.
    Referring to the circuit diagram (based on your diagram) shown below, does Vin refer to the voltage taken using a volt-meter between the Blue or Green dot and ground?

    <br />
    <img src="http://www.flickr.com/photos/46348930@N03/4280551664/" [Broken] border="0" alt="" /><br />

    I know i sound like a fool but i could not visualize what is Vin.
    To me, Vin would be the voltage across the capacitor.

    It seems that voltage if taken at the blue dot (with reference to ground) and green dot (with reference to ground) should be different since there is a resistor R1 between the 2 dots.

    This is where i am confused what is Vin.


    Sincerely
    Ramone
     
    Last edited by a moderator: May 4, 2017
  6. Jan 16, 2010 #5

    dlgoff

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    Yes. The input voltage is applied to R1 referenced to ground.

    Here's your link: http://www.flickr.com/photos/46348930@N03/4280551664" [Broken]
     
    Last edited by a moderator: May 4, 2017
  7. Jan 16, 2010 #6

    vk6kro

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    4280551664_aceab72faa.jpg

    The input voltage is across the capacitor in your diagram, although putting a capacitor in that position would bypass some of the high frequency component of the AC input signal (or all the AC) to ground.
     
  8. Jan 17, 2010 #7
    Dear vk6kro and dlgoff,

    thank you very much for clarifying my doubts.
    I really appreciate your help.

    I wonder what is the general purpose of using an Op amp?
    I read that it will try to make the Vin and Vinverting to be the same?
    I wonder what application would this device be useful for?
    I am trying to use an op amp to learn about it.

    Have a nice day.

    Sincerely
    Ramone
     
  9. Jan 17, 2010 #8

    vk6kro

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    That is a pretty broad question.

    Opamps have many uses. Mainly they are a useful device when you need to design for a precise result.

    You might have a voltage coming from some sensor that gives a small DC voltage out. But you would like to use this after amplifying it for something else. You might do it with an opamp.

    You might want to produce a gain of exactly 23 or whatever other precise value.
    You could get a transistor to produce this gain, but transistors vary a lot in their internal characteristics, so you would have to make the amplifier and adjust it for the right gain.
    Opamps can be nearly as precise as the resistors you use to control the gain.

    I think I should refer you to books or Internet resources like Google or Wikipedia to read up on opamps.
     
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