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Whats the difference between LDR´s and phototransistors?

Discussion in 'Electronic Basics' started by [email protected], Jul 11, 2005.

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  1. Guest

    I've looked at a couple of circuits that I´m considering to use in my
    The purpose of the circuit is to detect a white line at the edge of the
    SUMO-ring and use this detection to steer away from it.

    My bot will have two powered wheels, 1 motor per wheel.
    I´m thinking of using one detetctor at each front corner (my bot will
    only drive forward and turn so rear detetction will be unnesesary).
    I figure that if I hook it up in such a way that it will cut the power
    to the engine on the opposite side it will do the trick.

    But when I look at possible solutions I find that some use LDR´s
    (Light Dependant Resistors) and some use phototransistors.
    I have trouble finding any LDR´s to buy and by the info I´ve seen
    (not very much I´m afraid) about phototransistors they seem to be the
    better choise, but can I use them as a substitute for LDR´s or do they
    have fundemantal difrences?

    Would I have to use them in pair with LED´s to get enough light? (the
    detectors will be mounted under the robot, so its possible that not
    much light will get in)

    Can anyone think of a circuit that might help me do the thing I want?
    (a www-reference would be lovely, I can't seem to find just the thing
    on the web and my electronics books are from the 70´s so quite a lot
    of the components are outdated)

  2. Both LDRs and photo transistors are based on the principle that in
    semiconductors, photons can generate charge carriers. In the LDR, the
    conductivity of the semiconductor is used , directly. In a photo
    transistor, the charge carriers forward bias the base emitter junction
    of a transistor, and then the current gain of the transistor amplifies
    the small base current into a collector current. The response time of
    a photo transistor is generally much faster than that of an LDR.

    Either type might be usable for your purposes, but I think, since you
    cannot rely on ambient light, you should consider a modulated light
    source and a photo transistor that has its signal AC amplified
    (ignores all continuous light) and synchronously rectified (ignores
    all light variations at any other frequency than the modulation).
    This gives you a very sensitive and interference free signal. There
    are combination emitter and detector module prepackaged that are
    focused at some set distance that would make nice, compact additions
    to your robot. For example (from Digikey): Data/GP2S09, 24, 26, 27.pdf Data/gp2s28.pdf

    Here is a filtered (as described, above) detector that includes the
    driver for the pulsed LED:
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