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Comparator issues:newbie

Discussion in 'General Electronics Discussion' started by wannabegeek, Aug 17, 2011.

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  1. wannabegeek

    wannabegeek

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    Aug 17, 2011
    Hi All,

    I'm looking for an electronics forum to get help with my personal projects. I studied physics in school with some electronics, but I don't have a lot of experience and still make a lot of novice mistakes.

    I have a comparator made from a 741. I want this comp to output 0 volts when it has an non-inverting input of 2 V (normal mode no detection). So, I put 3 volts on the inverting input to off set this and keep the comparator off.

    However the output shows 1.27 V which will turn on the gate of the MOSFET instead of keeping it off.

    thanks in advance,
    wbg
     
  2. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    What power supplies have you got? Is there a negative supply?
    Give a rough sketch of your circuit.
     
  3. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
    hi Duke37

    oops forgot to mention the supplies....I have an old Lambda 10 A 0-20 volt bench supply
    which is my only one. So I don't have a formal negative supply at hand.

    For this project, since it is staying on for weeks at a time, I've decided to use D cell batteries. I suppose I could get more and make a negative supply. Or two 9 volt batteries.

    As for the circuit, I am only using positive 0-6 V. Not the recommended supply for the 741's I know. I am hoping to use what I have since money is short right now.

    Serial port pins 3 and 5 are my input. Pin 3 is normally at -10.83 V wrt pin 5, the signal ground. Pin 3 swings up to ~ 5 V when data is sent. I call a script to make it swing, then take a picture with a webcam. It's a time lapse project.

    The bleeder resistor in the schematic is not actually a discrete component. I forgot it originally, but because the 741 has low input impedance it worked out great anyways.

    Thanks !
    wbg

    EDIT:
    I have a 2N3904 in the schematic with the LED in the emitter, this deals with the 1.27 leak I have. I'd rather have a MOSFET run a 5 A lamp.
     

    Attached Files:

    Last edited: Aug 17, 2011
  4. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The 741 cannot pull its output all the way down to the negative supply (here the ground).
    You can get op-amps with fet output which can do this or you can provide a negative supply.
    You may get a low enough output voltage by putting a resistor to ground.
     
  5. wannabegeek

    wannabegeek

    133
    0
    Aug 17, 2011
    Thanks Duke now i now....I guessed as much from your post and put another battery wired in reverse. That did it.

    I put my first piece of stale bread on the target and eagerly await the pics of it molding away to nothing in my basement.

    Thank you,
    wbg
     
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