Comparator Question

Discussion in 'The Projects Forum' started by Peglam, Feb 11, 2009.

  1. Peglam

    Thread Starter New Member

    Feb 11, 2009
    4
    0
    Alright I need some help. I took my Electronics class a couple years ago and forgot everything basically.

    So I need to compare two voltages A and B over time, if they are equal or similar within a certain degree, it turns off the circuit.

    If voltages are different, I want to turn on the circuit, and engage a motor, in one direction or the other, depending on whether voltage A or B is higher.

    Will a comparator do this? Or is there a better way?

    Thank you for any suggestions.
    Laura
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    A comparator will tell you which of two instantaneous voltages is higher. You might be able to use an array of comparators to do what you want, however.

    What are the ranges for the signals in question, and how long are the time intervals needed?
     
  3. silvrstring

    Active Member

    Mar 27, 2008
    159
    0
    Yeah, what thingmaker3 said. But here is an instantaneous comparator setup to get you started.
     
  4. Peglam

    Thread Starter New Member

    Feb 11, 2009
    4
    0
    Thanks, I have looked around at some schmatics with arrays of comparators, and it does seem possible. I don't think I've ever been introduced to comparators. I think I'm going to have to buy a cheap one and play with it, to fully understand.

    Is it basically some kind of amplifier?

    The voltages I will be measuring will be from two CdS photocells, however I haven't specific thier resistances yet, I'm actually testing out some I have around to see if they react as I wish. Its taking me a while.

    The power source is a 12V.

    Basically I want the motor to engage for a small increment somewhere around 1-5s I'm estimating, then to compare the voltages again, before further movement.

    Thanks
     
  5. Peglam

    Thread Starter New Member

    Feb 11, 2009
    4
    0
    Thanks for the drawing silvrstring that will help me get started.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Yes. Any op-amp can be configured as a comparator, but the comparator chips have been optimized for this use. http://www.allaboutcircuits.com/vol_6/chpt_6/2.html

    Sounds similar to a gizmo for tracking the sun to maximize photovoltaic output throughout the day. I'll dig around in my notes - I was playing with a similar concept a year or so ago.
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I never built this, so there are probably errors:

    suntrack.png
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I don't suggest you to use just a simple comparator to the job because even if the inputs are equal the output will oscillate due to noise. It would be better to use a schmitt trigger like this one:

    http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

    Use a schmitt trigger circuit for each input and use a logic circuit on its output to make the desired function.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Mik3 is referring to the one with the "positive feedback resistor.";)
     
  10. Peglam

    Thread Starter New Member

    Feb 11, 2009
    4
    0
    Thanks everyone, you are all very helpful!

    Yeah I'm thinking about making a solar tracker to open and shut a door or window.

    No real application except that I'm bored, and think it might be fun. Maybe I can play some tricks on my roommates.

    I'll get back here in a bit when I have it together. Or more likely when something goes wrong and I can't figure it out!. :)

    Thanks again!
     
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