phototransistor help

Discussion in 'The Projects Forum' started by bignick270, Nov 18, 2010.

  1. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    I am trying to prototype with a phototransistor, BPV11(http://www.vishay.com/docs/81504/81504.pdf ). I have it connected as a common emitter setup with 5v to the collector and the emitter to ground through a resistor. I am trying to read a voltage proportional to the received light so the resistor has to be a lower value to set it in active mode. I have used about every resistor value between 500 ohms and 100 ohms.

    My problem is that as soon as I apply power to the circuit, the oscope shows the voltage jump to 4+ volts immediately even inside of a dark box. There is no voltage change no matter if light is introduced or if it is in a dark environment.

    I attached a jpeg image with a quick shot of the circuit, it is very basic.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  3. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    yeah I have read that, it is where I got most of my information from.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Try looking more closely at the suggested resistor value.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Your circuit is common-collector that has no voltage gain so it is not sensitive. (You said it is common-emitter but it is not).
    Use common-emitter if you want voltage gain then it will be sensitive.

    Since your photo-transistor turns on hard without IR light then either it is connected wrong or it is shorted.
     
  6. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    well darn is suppose it is common collector haha, i was reading from the sheet wrong...oops

    The circuit is simple, I dont think anything is connected wrong unless I have place the BPV11 in backwards but I am sure it is connected right
     
  7. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    Okay I switched the circuit around so it is in the real common emitter setup.

    The voltage output still does not change, although the voltage level is lower at ~820mV
     
  8. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I have not found a datasheet that specifies which pins are assigned to 1, 2, and 3. Pin 2 is most probably the base. as to what pin is the collector and emmiter is not known. flipping the transistor around in the common emmiter configuration might help. You can even go as low as 100 ohms on the resistor.
     
  9. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    the first link is the datasheet for the phototransistor and on page 4 is specifies the emitter as the flat side
     
  10. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    I tell you what guys, I flipped the transistor around, opposite of what I believe the datasheet was showing and now when I apply light from a IR led the voltage drops from high to low and when I place a piece of paper in between the voltage jumps back up.

    I had a similar issue with a datasheet for a laser diode, it seemed like their anode/cathode image was flipped but that is a separate issue lol
     
  11. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    oh and I upped the resistor to 100k
     
  12. iONic

    AAC Fanatic!

    Nov 16, 2007
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    So there's the datasheet. And the center pin is not the base, but rather the collector.
    Are you sure you are baising the Base(Pin opposite Flat side) and not the collector(center pin)??

    Glad something's happening.
     
  13. bignick270

    Thread Starter Active Member

    Mar 25, 2009
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    oh man i feel like an idiot, i must have dyslexia at times, i swore that the base was the center pin

    *hits head

    lol
     
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