Turn on led at 50 ohms

Discussion in 'General Electronics Chat' started by asipple1, Jul 18, 2011.

  1. asipple1

    Thread Starter New Member

    Apr 13, 2010
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    0
    Hello everyone,

    I am trying to build a test jig that will turn on a green led on when the resistance is 50 ohms. Anything less or more the led would not light up.The power would be coming from two AA batteries
    Any input would be greatly appreciated.

    Thank you
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    This is a pretty easy project. You pump a constant current through the resistor (you did not state the wattage), and if the voltage exceeds a set value a simple window comparitor circuit turns on the LED.

    With 3V (which is variable) you will have a max of 60mw, which should be gravy.

    So, how much experience in electronics do you have?

    Picking some starting values, a 50Ω resistor will drop 1.5V at 30ma.

    This might work. R1 may need to be upped a little. Adjust R2 for 30ma, then adjust R4 to pass a good resistor. You don't have much choice in LED color, due to the low power supply voltage.

    [​IMG]

    Doh!, saw the less, but missed the more. Fortunately, the spare comparitor can be used for a window comparitor. Will be back with another schematic.
     
    Last edited: Jul 18, 2011
  3. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    OK, this will work.

    [​IMG]

    To calibrate this unit set the output to 30ma with R2. Adjust R4 for maximum voltage on the wiper, R8 for minimum voltage on the wiper. The LED should be lit.

    Use the maximum acceptible ohms on the test leads, then adjust R4 until the LED goes out, then back it off until the LED is lit.

    Use the minimum acceptible ohms on the test leads, then adjust R8 until LED goes out, then back it off until the LED is lit.

    You are good to go.
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    There are also a number of different circuits on the web for "continuity tester" that can set the resistance at which they indicate continuity. These could easily be adapted for what you want to do.
     
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