Phototransistor Setup Questions

Discussion in 'The Projects Forum' started by aves911, Jan 12, 2011.

  1. aves911

    Thread Starter New Member

    Jan 12, 2011
    16
    0
    Hi,

    I'm working on a phototransistor control for a project and haven't worked much with electronics in a few years (my recollection is a little shaky, consider me a newbie)

    I attached the datasheet for the photransistor I would like to use. It's a combo IR LED/Phototransistor. I'm trying to set it up so that when something passes in front of the sensor, the reflection triggers the transistor.

    I'm using an 18V power source and need to switch up to 2A on and off. I can step the voltage down for the LED but I am a little lost on the transistor side (trying to wade through the spec sheet).

    Can I feed 18V directly to this transistor? If not can I step it down with a resistor also?

    What would be the best way to connect the emitter to another transistor (capable of switching the 2A at 18V)? Any suggestions on a good transistor to use for this? Digikey, Radio Shack, etc.

    Lastly, I've heard that phototransistors can have some issues with ambient light causing false signals. I've seen some circuit diagrams that use a resistor to avoid this. However, I'm unsure how to size and place it.

    I apologize for the beginner questions and any help/pointers would be greatly appreciated.

    Thanks,
    -Jacob
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What you have is called a "reflective sensor". These have a very short focal length on the order of a half inch or so. You will probably be unable to detect the passing of an object that is more than an inch away from the sensor. Does this limitation cause you any problems?

    hgmjr
     
  3. aves911

    Thread Starter New Member

    Jan 12, 2011
    16
    0

    In this application the 0.5-1" range is perfect for me. Thanks for the tip :)
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The technique for optical sensors that will be operated in ambient light of various wavelengths and intensities is to drive the emitter with a pulse and then look for return pulse. By pulsing the emitter you can drive it harder than would be prudent if it were being powered with a constant voltage/current. The pulse also overcomes the ambient light problem.

    hgmjr
     
  5. aves911

    Thread Starter New Member

    Jan 12, 2011
    16
    0
    Thanks for the suggestion, unfortunately that seems like it may be a little bit out of my league. I attached a quick schematic that I am thinking may work, however I'm having a little trouble reading the component datasheets.

    From looking at the schematic, I'm using a resistor to step down the 18V to the IR LED (hopefully it is the proper size?) and in the data sheet for the phototransistor is says the breakdown VCE is 30V so I'm assuming it can pass through 18V?

    My other question is about the power transistor, I don't see anything on the datasheet that specifies what the switching voltage or current should be? If I pass an 18V signal from the phototransistor to the base of the power transistor will that "switch" it on properly?

    Lastly, I was thinking about using a pot to connect the power transistor base to ground. My thought was I could adjust the resistence here to play with the responsiveness of the "on/off" function of the circuit.

    Does this look like it would work?

    Thanks
     
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