Help with PLC to Micro controller interface circuit

Discussion in 'The Projects Forum' started by cstrutton, Sep 28, 2010.

  1. cstrutton

    Thread Starter New Member

    Sep 25, 2010
    I am working on a data logging device that will record production data from various CNC machines and robots on our production floor. The PLC's and robot controllers will pulse a 24V output when an event happens. My device needs to recognize that event and post it to a database log on the network.

    I have created a circuit with a resistor divider connected to an optocoupler. The optocoupler is connected to the micro with a pull-up resistor. (see attached schematic)

    This schematic is the result of about 10 hours of searching the on the web. I have breadboarded this circuit and it seems to work. My question is 3 fold:

    • have I missed anything here
    • how do I protect this from noise as the plc will also be driving relays and valves
    • how do I add a led on the micro side to provide visual indication of a signal
    Any help would be greatly appreciated
  2. bertus


    Apr 5, 2008

    Remove R2, a lot of current will go through that resistor.
    To have a indication reduce R3 from 1K to 560 and put a led in series.
    When the output is low the led will be lit.

    cstrutton likes this.