What sort of optosensor?

Discussion in 'General Electronics Chat' started by ChrisR, Oct 23, 2011.

  1. ChrisR

    Thread Starter New Member

    Aug 17, 2009
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    0
    Hi
    I'm looking for advice on choice of a sensor. Requirements are physical, really. I've looked in the catalogs but maybe someone more up to date than me has an idea.

    I want to sense a small LED blink. It's on the back of a camera, only about 3mm high, and I want to trigger something off its on and off edges. It stays on something like 20ms. Not bright, something slightly brighter than the HD led on my desktop. In the old days I would have said it's about the brightness of a 3mm led with about 2 mA going through it, but I'm way out of date. I drew the red dot on the image, the "lens" is translucent colourless.

    So I'm looking for something with a risetime quick enough to not be an issue, maybe 100us. As you can see there's nothing much to attach it to. I was thinking of magnetic tape. I don't want to use the tripod screw, that'll be used, and I need to be able to remove it easily.

    I can botch a two-legged thing on to a thin flexy wire and hold it there with PVC tape, but that's a bit yuck.

    I'll need to trigger a 555, or something like that, and it would be nice to be able to "gate" it, or use a wired-AND, as there are no spare logic chips being used.

    So - photo resistor/diode/transistor? Fiberoptic?

    Any neat ideas?

    Now, will this image upload... :)
    I attached it - is that good?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    A photo-transistor has the best sensitivity and is fast enough for you requirement, although a photo-diode would also likely work. You could just tape it to the camera with black tape to block ambient light. You may have to play with the photo-transistor load resistor value to get optimum response.

    Don't understand you comment about "wired-AND" and no spare logic chips. What signals do you want to "AND".

    An exclusive-OR gate can be used to generate a pulse on both the leading and falling edge of the sensor signal.
     
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Of all the light sensing devices I come across, the LDR is the flattest, having the thinest height profile. You would have to experiment with it to see if it would work. A dual op-amp, one amp to amplify the signal, the other as a comparator could be used to trigger the 555.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    One possible problem with an LDR is that much of its track length needs to be illuminated in order to get much output (think of resistors in series). If the LED is small compared to the LDR area, it might not work too well.
     
  5. ChrisR

    Thread Starter New Member

    Aug 17, 2009
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    I'd quite like to avoid goo all over the camera, from tape..:eek:

    Is there a phototransistor or similar thing out there that's kinda like the "MSWord sketch" below, that anyone's seen? How are they normally "located"?
    Can you get fiber optics with right-angled ends?

    Point taken about the LDR -the last one I used was an ORP12, about as big as a thumbnail, in a darkroom enlarging meter. That would have been with a 741. I expect things have progressed since the 70's?

    Edit - there are some SMD phototransistors here, not sure how fast they are though, and whats MLE?
    http://uk.farnell.com/jsp/search/browse.jsp?N=204193+110144917&No=0&getResults=true&appliedparametrics=true&locale=en_UK&divisionLocale=en_UK&catalogId=&prevNValues=204193&filtersHidden=false&appliedHidden=false&mm=1002460||,&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D204193%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_UK%26divisionLocale%3Den_UK%26catalogId%3D%26prevNValues%3D204193
     
    Last edited: Oct 23, 2011
  6. iONic

    AAC Fanatic!

    Nov 16, 2007
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    There are some pretty small LDR's such as those found on hall dark sensing lights. I like the SMT choices.

    Is the camera ferromagnetic, aluminum, or carbon composite? What is the indicator light for?
     
  7. ChrisR

    Thread Starter New Member

    Aug 17, 2009
    3
    0
    Camera's some sort of plastic, but by the SD card and battery compartments as you can see - doors open in the pic.
    It's a current Canon.
    Low profile has to include the connections.

    It blinks when the shutter fires - reasons are a bit complex!
     
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