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Discussion in 'Electronic Design' started by [email protected], Mar 4, 2007.

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  1. Guest

    HELLO, we r in great trouble for my senior design project in school,
    I will ask for some advice. my project is to make a device to find
    polarization axis of a lens. we are passing laser light(using laser
    pointer) through the lens of which axis needs to find and a analyzer
    lens. if 2 lens's axis are perpendicular thn they block light. as we
    know the axis of the analyzer, we will rotate the lens with a stepper
    motor unless all the light is blocked. We r trying to detect the
    light by using a photodiode. built atransimpedence amplifier without
    any Cf using lm741. now we see theoutput is very noisy. gives very
    very different datas at differnettime . i took data from the TIA and
    tried to find fft of data andfound all signal is in DC and noises on
    higher freqs. but spec saysoutput will be 100 level. i dont know y i
    am getting output at DC. Ihave built a filter also, cutoff at 50 Hz.
    Is filtering with this willhelp?? we need to detect measurable voltage
    change in TIA with every 1degree rotation of the lens by the stepper.
    we gonna use ADC to changethis voltage to use in microcontroller. my
    question is whether i should use filter or need to use lock in amp.Is
    modulating the signal or demodulating is easy?? i need to finishthis
    part in 1 week. as i am a student dnt have tht much idea and timeat
    all. I still have to work on the rest of the project by april. So,
    what will be less time consuming ?? money is not a factor. if Ican buy
    good less noisy transimpedence amp, filter or lock in amp itwill work
    too. All i need less time consuming direction. If anybody give me some
    suggestion I will be obliged. will eagerlywait for reply.
  2. Jamie

    Jamie Guest

    you need to modulate the laser with a set frequency and use a decoder or
    pass filer in the op-amp stage.
    the photo detector should be set up to detect sudden changes of light
    and not the DC component.
    i guess you could do that a couple of ways.
    Capacitive decoupling or common mode input to both + & - of the op-amp
    having one input using a bypass cap to remove the DC. also you will need
    to use the Nulling inputs of the 741 since the Cmr (common mode
    rejection) isn't really that good and the offset is a little off.
  3. Rich Grise

    Rich Grise Guest

    Next time get an earlier start.

    Good Luck!
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