Photodiode

Discussion in 'The Projects Forum' started by odm4286, Nov 17, 2013.

  1. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    [​IMG][​IMG]Uploaded with ImageShack.com

    Excuse the quality of that picture its the best I can do at the moment. Ok basically I'm trying to create a tachometer circuit that will ultimately interface with a PLC digital input. Only thing is for some reason I can never get 24vdc emitter of my npn transistor to ground, right now it stays at 18 volts then drops to 16 or so when I break the infrared beam between the infrared diode and infrared emitter:confused:. What I want to happen is switch the 24VDC off and on every time the infrared beam is broken. Any help would be GREATLY appreciated.

    Apologize if this isn't all that coherent, I've been troubleshooting this for a while and I'm getting tired. :p
     
  2. antonv

    Member

    Nov 27, 2012
    149
    27
    What's connected to your transistor's emitter?

    Is there any way for light to leak onto your photo diode when the LED is blocked?

    What are the part numbers for the photo diode and the transistor? In a circuit like this understanding their datasheets is crucial.
     
  3. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    Theres not too much info on the diode, blame radioshack http://www.radioshack.com/product/index.jsp?productId=2049723, and the transistor is a tip3055 npn.

    I did some thinking and figured instead of trying to switch 24vdc I could just use the change in current as my signal to the PLC. Hooked it up to an analog input and its working great so far.
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
    2,435
    315
    Can't see what emitter is connected to either.

    I would connect emitter to ground.
    Collector thru 20k or so to +24. (Enough to bring plc input high)
    +voltage thru suitable current limit resistance to P.D. then to base.
    Possible 10k or so from base to ground. May not need it.

    P.D. seeing light will conduct thru base emitter junction bringing collector to ground.
     
  5. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    My emitter was connected to the PLC input
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
    2,435
    315
    Most sensors designed for connection to plc use open collector.
    Are you copying a design or is there a reason to use emitter follower design?

    The open collector inverts logic.
    Use a pull up resistor from +24vdc on the plc input and let the transistor pull it down to ground or -24vdc.

    I think it will be easier for you to design and trouble shoot.
     
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