Sensitivity of comparator

Discussion in 'General Electronics Chat' started by wannaBinventor, Apr 28, 2010.

  1. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
    179
    4
    I've got the concept for a project that will work much like a solar tracker. I will have two photoresistors in series tapped in the center to form a voltage divider. I'm also going to have a negative rail by way of tapping from between two batteries. One input of the op amp (that I'll use as a comparator) will be connected to the tap at the two batteries, and the other input will be fed from the voltage divider. This way, with the negative rail, a DC motor will move forward or in reverse depending on the resistance levels of the two photoresistors.

    My problem:
    I don't want to eat up power by the motor constantly going one way or the other based on very minute changes in the resistance. If the reference voltage is 3v, I don't want it constantly trying to adjust back and forth if small fluctuations in the resistors cause the "adjusted input" to move from say 3.05 to 2.95 volts.

    How do I make the comparator wait until input A is ~ .1 volts away from the the reference voltage before it switches to the other rail votlage?

    Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Use hysteresis feedback to the noninverting input.
     
  3. kingdano

    Member

    Apr 14, 2010
    377
    19
    you could also use a uC...but that wouldnt help your low power needs at all
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you post your circuit as it exists now, we can help you more with it.

    Otherwise, it's pretty tough to make specific recommendations. That's why you received rather generic replies to begin with.

    If you don't have schematic capture software, Cadsoft Eagle is available in a freeware version. http://www.cadsoftusa.com

    You can use it to create schematics and make PCB's. The freeware version is limited to one sheet per schematic, two layers on a board, and max board size of approximately 3"x4"; but you can still do quite a bit with it.

    If you need software for SPICE simulation, try Linear Technology's LTSpice - it's good, and it's free. Googling "LTSpice download" will get you to it.

    There's an LTSpice support forum on Yahoo! Groups. Join it; you won't get spam E-mail, and they have lots of SPICE models available.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here's an example of how you could drive a motor using a dual power opamp (L272) and a dual comparator (LM2903/LM393) with hysteresis and a window.

    See the attached.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you're going to use two CDS cells in resistive dividers (a good idea, because then the solar cell array will know which way to turn) here's a better circuit.

    See the attached.
     
  7. wannaBinventor

    Thread Starter Member

    Apr 8, 2010
    179
    4
    Thanks Sgt Wookie. You've been most helpful with this.
     
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