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Op amp unexpected results

Discussion in 'General Electronics Discussion' started by Jelleyzeefirst, Jun 7, 2017.

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  1. Jelleyzeefirst


    Jun 7, 2017
    Hi guys,

    I'm trying to build a circuit that i can measure the continuity of a L.E.D (red), while being able to turn that L.E.D on and off. I also don't want to set it off while testing if its there. I decided to do this using a voltage of 1volt through the red L.E.D while off and 5v when turned on.

    Then have that 1volt go into an op amp (741) with a feedback resistor and for the output to turn on a green L.E.D. So the green L.E.D should only go off when there's no red L.E.D in place.

    I only have a battery pack of 3AAs so I've used 2 voltage dividers to get 1volt as well as +4.5volts (close enough).

    The only problem is i cant get the circuit to work like this. I've tried 2 different op amp chips and i had the same problem, the green L.E.D stays on no matter what. With the out pin and inverting pin stuck on about 1.8volts.

    I don't know why my circuits behaving this way or if I've just screwed up the design badly, but any help would be much appreciated

    Attached Files:

  2. Bluejets


    Oct 5, 2014
    Draw your circuit. Fritzing is simply a waste of time.
  3. Audioguru


    Sep 24, 2016
    The design of a 741 opamp is 49 years old and it was designed to use a 30V power supply. Some 741 opamps will work when the power supply is as low as 10V but none will work with only 4.5V. Your 4.5V battery will soon drop to 3V or less.

    You show a transistor with no part number and its pins are not labelled. Its pins could be in an American (EBC) , European (CBE) or Oriental (ECB) order.

    I agree that your solderless breadboard wires all over the place is not a schematic. it looks like the battery negative does not connect to the larger breadboard.
  4. davenn

    davenn Moderator

    Sep 5, 2009
    that is the significant problem
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