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Phototransistor help

Discussion in 'General Electronics Discussion' started by lindenrulz, Jun 23, 2013.

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  1. lindenrulz

    lindenrulz

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    Jun 23, 2013
    I am trying to make a Pinewood derby car race timer using super bright LEDs and phototransistors. I am also using the Arduino Uno to prosses it. I found plans for it online at http://www.miscjunk.org/mj/pg_pdt.html. I am having trouble getting the phototransistor to detect a car passing over it. I ran a modified debug program that tells me what state the transistors are in and it says they detect light but I can't get them to detect darkness, even when I have them totally covered. What would be wrong with it?

    Phototransistor specs:
    Lens: Transparent
    Peak Emission wavelength 940nm
    Collector-Emitter Max Voltage 30V
    Collector-Emitter Saturation V 0.4V
    Rise/Fall Time (5vce/IF 1ma/1K Load) 15us, 15us
    Collector Dark Current (10vce/no IR in) 100nA
    Operating Temperature -20° C to 85° C
    Collector Identification Short Lead
     
    Last edited: Jun 23, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You most likely have it wired incorrectly.

    You haven't told us how you have it connected, but a typical "beginner" error would be to connect the phototransistor between the pin and Vcc (typically +5V).

    If this is all you've done, what you need to do is to connect a resistor between the pin and ground. Start with 10k and see how you go.

    These pages have examples.
     
  3. lindenrulz

    lindenrulz

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    Jun 23, 2013
    Crisis may be averted

    I found out I made a rookie mistake but not the one you are talking about. With the use of a digital camera I was able to determine that I had gotten the emitter and detector mixed up as I had gotten them in a package. I had gotten a Super bright LED as well because I had heard they perform better. So, I have an IR emitter and a Super bright LED facing each other :D :eek:
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Surprisingly enough, that could be made to work, however a phototransistor would be a lot more sensitive.
     
  5. lindenrulz

    lindenrulz

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    Jun 23, 2013
    Thanks

    Thanks for your replies. Even though you couldn't tell I had them mixed up I stumbled upon it using your suggestions.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    This is where photos and circuit diagrams come in handy.

    The circuit diagrams tell us what you think you're doing and the photos show us what you're actually doing.

    Sometimes there are problems with your ideas, sometimes with the implementation.

    In this case it was the latter. We might have asked "Why hasn't that phototransistor got 3 legs?" (even though some only have 2).

    But yes, a phototransistor can look like a LED -- but it doesn't read like one to a multimeter and it doesn't glow if you pass a current through it. (Unless you pas a really big current through it :D)
     
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