IR Phototransistor as ON/OFF Switch

Discussion in 'General Electronics Chat' started by teekon, Feb 8, 2009.

  1. teekon

    Thread Starter New Member

    Feb 8, 2009
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    Hello,
    I was wondering if any one could help me on how to make an IR phototransistor act as a switch? I have a IR emitter that is in line with the IR phototransistor but it doesn't do anything when i block them. (http://www.radioshack.com/search/index.jsp?kwCatId=&kw=ir diode&origkw=IR Diode&sr=1) When i test the volts coming out of the phototransistor it changes from 5.2 volts to 5.0 volts and i thought it would actually change to about 1 or 0. Any help would be great thanks!

    Tyler
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Can you post a copy of the schematic showing how you connected up your phototransistor?

    hgmjr
     
  3. teekon

    Thread Starter New Member

    Feb 8, 2009
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    Last edited: Feb 10, 2009
  4. Audioguru

    New Member

    Dec 20, 2007
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    The circuit should work if the parts work properly.
    RadioShack might sell manufacturer's rejects at a very high price.
    The range might be very short, especially in daylight.

    R7 to R10 bias the input of the comparator to half the supply voltage so that when the photo-transistor conducts then the comparator can sense it.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    The voltage at the collector of the photo-transistors should be less than 1 volt when it is not blocked and near the 5V supply when blocked.

    What voltages are you measuring when the sensor is unblocked and when it is blocked?

    hgmjr
     
  6. teekon

    Thread Starter New Member

    Feb 8, 2009
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    Thanks Hgmjr and Audioguru for your posts so far i appreciate your time taken to help me out with this.

    When it is unblocked it reads 5.13 volts and when it is blocked it reads 5.17 volts. i also tried these schematics with my IR and the led light never turned on, it appeared there was never anything going through the IR.

    [​IMG]

    [​IMG]
     
    Last edited: Feb 11, 2009
  7. hgmjr

    Moderator

    Jan 28, 2005
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    You say that the IR led did not come on. Can I assume that you used a digital camera in a cell-phone to confirm this fact?

    hgmjr
     
  8. teekon

    Thread Starter New Member

    Feb 8, 2009
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  9. teekon

    Thread Starter New Member

    Feb 8, 2009
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  10. teekon

    Thread Starter New Member

    Feb 8, 2009
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    I found out the problem, i wired the IR Phototransistors backwards so it is working now. I was wondering if there is a way to speed up the phototransistors from reading the Ir emmitter? if i run my hand through them really fast it doesn't notice it, which isn't very effective for a pinewood derby finish line.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    Your schematic has an IR photo-diode feeding a transistor.
    If you used a photo-transistor then please show us how you connected it.

    A photo-transistor has a fairly slow response time.
    A photo-transistor used as an emitter-follower is slower than if it is common-emitter.
    A photo-diode driving an opamp is much faster.
     
  12. midnjerry

    New Member

    Feb 9, 2010
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    I just built my own circuit using the radioshack Infrared Phototransistor, and it turns the circuit on / off pretty fast with a laser pointer. You can get a laser from Wal-Mart for $3 in their "As Seen on TV" display, but if that's not there when you go, you can go to the pet section and buy a laser there for $4. Just look for cat toys.
     
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