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Changing photo resistor resistance range.

Discussion in 'Electronic Basics' started by Chris W, Apr 22, 2005.

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  1. Chris W

    Chris W Guest

    I have a photo resistor that varies from about 400 ohms to about 15
    million ohms in the desired light conditions. However I need something
    that varies in resistance from 1k to 5k. Is there a way to do this with
    a transistor? The circuit will always be supplied with a 100mV supply
    which I suspect will effect the design of what I am trying to do.

    Chris W

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  2. What's wrong with the following?

    | |
    | |
    | 6.2K | |
    .-. '-'400 to >1M
    | | |
    | | |
    '-' .-.
    | | |
    | | | 620
    | '-'
    | |
  3. John Fields

    John Fields Guest


    R1 = 6.2k
    R2 = 400 -> 15M
    R3 = 620

    R1 (R2 + R3)
    Rt = --------------
    R1 + R2 + R3

    6200 (400 + 620)
    Rt1 = ------------------ = 875.9 ohms
    6200 + 400 + 620

    6200 (15e6 + 620)
    Rt2 = ------------------- = 6197.4 ohms
    6200 + 15e6 + 620

    875.9 ohms to 6197.4 ohms isn't 1k to 5k.
  4. John  Larkin

    John Larkin Guest

    Note the op's use of the word "about."

  5. John Fields

    John Fields Guest

    What makes you think I didn't?

    I don't know about you, but for me, "about" is about 5 to 10% off of

    -12.4% on the low end and +23.9% on the high end is a little more than

    But thanks anyway...
  6. Rich Grise

    Rich Grise Guest

    Well, what if you did a little algebra:

    R1 (R2 + 400)
    Rt1 = ----------------- = 1000
    R1 + R2 + 400

    R1 (R2 + 15e6)
    Rt2 = ------------------ = 10000
    R1 + R2 + 15e6

    and solve for R1 and R2?

  7. John Fields

    John Fields Guest

  8. John Fields

    John Fields Guest

    PS: If you're bitching about the ordering, go look up "associative
  9. Endoscope

    Endoscope Guest

    To which Barry should reply.

    Whoops! Sorry, Brane Fart...... Try 5K1 and 820R.....

    Don't hold your breath though.

  10. Endoscope

    Endoscope Guest

    Oh ****, Barry might be Trolling/Teaching. His question was......

    'What's wrong with the following?'

    Oh shit..... I've just given him the opportunity to write 300 lines of
    explanatory dribble.


    Kill Me NOW.


  11. Despite occasional intimations to the contrary, I can
    do arithmetic. What is not clear to me, and behind
    my question to the OP, is whether your result (and
    mine) is wrong for his purposes. I took the OP's
    "1k to 5k" as a given to be achieved with what is
    likely a cadmium sulphide cell illuminated with a
    somewhat uncertain intensity, (hence the OP's
    "about" qualifiers on its resistance range). Due to
    more uncertainties stemming from the cell's TC and
    its replacements, I elected to provide a range that
    extended a little beyond what the OP stated. The
    reason for more margin at the high end is that the
    cell's high resistance extreme is likely to be more
    uncertain than its low extreme. And if it is what
    occurs due to night illumination, the moon might
    cause considerably less than the stated MOhms.

    Anther potential "wrong" aspect is that the mapping
    from cell resistance to output resistance may not be
    what the OP needs. Again, I rely on the OP to help
    solve his own problem by stating such a deficiency.

    Perhaps I should rephrase the question:
    In what way will the OP's application misbehave if the
    above circuit is used for the light-affected resistance?
  12. Ban

    Ban Guest

    But why did you choose non E12 or E24 values, I can understand when you go
    off ideal values, and choose the closest standard ones, but you just write
    non standard values just more than 20% off.
    It might be in some old bin you found only those values, ok it will work,
    but to deliberatly choose some random values is IMHO an unnecessary source
    of error.
    Or maybe it is that you are not familiar with standard E12? That would
    qualify you as a bloody beginner.
  13. I suggest you study this table:
    Then come back and explain your puzzlement.
    I deliberately chose 5% standard values to avoid
    unnecessary confusion and low SNR posts here.
    Please elaborate on this concept after your review
    of the E24 table at the link I provided.
  14. John Fields

    John Fields Guest

    I had no "result", other than pointing out that yours was quite a bit
    away from what the OP asked for. Of course you could argue, ad
    nauseam, that "about" means different things to different people and
    put forth the conjecture that your results were "good enough", but who
  15. Genome

    Genome Guest

    Oh Shit.......

    Leave it John.

    Barry should have corrected himself properly.

    Whoops!! 5K1 and 820R. Job done.

    No not Barry.

    And before he goes further I would just like to take this opportunity to
    point out that his original answer did not include the 'caveats' he is now
    giving and you are responding to.

  16. ....
    Your "properly" is a private concept.
    Your "Barry" shtick is silly.
    My original question applies to your "perfect" values
    as much as to the ones I posted. Going on and on
    about "caveats" is pretty pointless when we have no
    idea what the application requires.

    I should point out that by misconstruing my added
    'potential "wrong"' as a "caveat", you are missing
    the point and making that obvious.
  17. John Larkin

    John Larkin Guest

    Should we make an official verb out of that?

  18. John Fields

    John Fields Guest

    Perhaps, since this _is_ seb, a more satisfactory approach would have
    been to supply the OP with the tools which would have allowed him to
    figure out his own solution(s)?

    Y'know, "Build a man a fire and he'll be warm for a day, but set a man
    on fire and he'll be warm for the rest of his life."
  19. That's a good point, on both the forum and approach.

    For the simple resistance calculation, your depiction
    of the arithmetic involved ought to suffice. Regarding
    the problem of what exactness makes sense, that is
    a much harder problem. I think some discussion of
    the relevant issues, (preferably informed by some
    knowledge of the application), is the best course.
  20. Genome

    Genome Guest

    Hmmmmm, if I can remember it right.

    I Barry
    You Barry
    We Barry
    He Barries
    She Barries
    We Barry
    They Barry

    Looks good and regular to me.

    I'll add,

    Stop Barrying about.

    Don't know the proper term for that one.....

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