Turn LED into digital input

Discussion in 'The Projects Forum' started by vmcacador, May 19, 2009.

  1. vmcacador

    Thread Starter New Member

    May 17, 2009
    2
    0
    Hi!

    I'm requesting help on the following subject:

    I have an electronic board with output LEDs (2 V DC), the LEDs flash signaling any equipment malfunction.

    What I want to do, is to use these LEDs as digital inputs for a controller (Siemens Logo!). To have a logic value of 1 the controller needs a voltage bigger than 9 V DC, so I would have to amplify my 2 V DC up to 10 or more V DC.

    I was thinking of using an operational amplifier, but I don´t know wich one to use since I only have 24 VDC supply voltage...

    Any suggestion?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    How is the output of your board connected?
    Do you have a schematic of this board?
    I think you can use an optocoupler as interface.

    Greetings,
    Bertus
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A simple transistor with a few resistors can do the work.

    Will you remove the LEDs from the circuit?

    From where the LEDs are controlled?
     
  4. vmcacador

    Thread Starter New Member

    May 17, 2009
    2
    0
    Hi!

    Thank you so much for your quick reply!

    I've attached the board schematic, I do not have any other schematic with the wiring diagram.

    I will search some more information on this optocoupler.

    The LEDs are controlled by the microprocessor on the equipment's main board.

    I think I'll leave the LEDs on the board since they are used by servive technicians.

    Thank you again...
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Connect the output of the microcontroller which controls the LED to the base of a transistor via a 1K resistor. Then connect a 10K resistor between the collector of the transistor and 9V. Take the output from the collector and connect it to the Siemens controller. Note that the output from the transistor will be an inverted version of the output of the LED's microcontroller.
     
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