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Phototransistor Light Sensor — Parallax Forums

Phototransistor Light Sensor

NoBo780NoBo780 Posts: 94
edited 2008-01-13 03:39 in BASIC Stamp
I recently read in a robotics book that I can get better sensitivity with a phototransistor than I can with a CdS cell. Is this true? What kind of circuit would I need to concoct for it to be utilized properly? Thanks!

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Giggly Googley!

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,569
    edited 2008-01-12 21:49
    NoBo780,

    It really depends on what your application is to warrant which·has better sensitivity.· The question should be completed with "... better sensitivity to what?" Speed? Light Level? Light color?

    CdS cells are generally slower to respond than phototransistors, but this·can vary depending on the size of the CdS... larger CdS cells are slower than smaller ones but are more sensitive to lower levels of light because of their larger surface area in general compared to a phototransistor.· If you are comparing characteristics of the human eye, CdS cells are a closer match than a phototransistor.· Phototransistors on the other hand can respond much quicker (orders of magnitude) than CdS, and are most sensitive (usually) to·IR light.




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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • FranklinFranklin Posts: 4,747
    edited 2008-01-13 01:01
    If you hurry you can go back and delete your duplicate message.

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    - Stephen
  • NoBo780NoBo780 Posts: 94
    edited 2008-01-13 02:22
    I simply need it to function as an indoor light sensor; this will be mounted on a robot that scoots around on the floor. I tried a CdS circuit from the Boe-Bot text for use with my previous robot, but it slowed it down WAY too much, as you can see in the video (I tried nearly eliminating the delay time in the RCTIME function, but it was still slow).

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    Giggly Googley!
  • UnsoundcodeUnsoundcode Posts: 1,532
    edited 2008-01-13 03:20
    I dont know if you have checked out these little devices I·bought a couple when I spotted them at Radio Shack.

    http://www.parallax.com/Store/Components/IntegratedCircuits/tabid/154/CategoryID/31/List/0/SortField/0/catpageindex/4/Level/a/ProductID/88/Default.aspx

    They not only detect light but also detect the light level.

    Jeff T.
  • NoBo780NoBo780 Posts: 94
    edited 2008-01-13 03:37
    Oh! I forgot about those! Yes, I could definitely use that. Do you live in CA, because I don't think that the RadioShacks here in NC will get those for a while... Do you know how to use it? How would you convert the frequency to a workable digital value??

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    Giggly Googley!

    Post Edited (NoBo780) : 1/13/2008 3:42:46 AM GMT
  • UnsoundcodeUnsoundcode Posts: 1,532
    edited 2008-01-13 03:39
    I used the sample code on the product page without any problems, worked straight away which was a pleasant surprise.

    Jeff T.

    EDIT: I modified the sample code to count for 10mS and adjusted the output divisor so that I was getting a reading of between 110 and 180 in a room with soft lighting. Passing my hand between the light source and the sensor would drop the reading below 50.

    BTW I'm·one state over·from NC in TN.


    Post Edited (Unsoundcode) : 1/13/2008 5:20:22 AM GMT
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