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Advice for light detector/counter

Discussion in 'Electronic Basics' started by pstnly, Jan 13, 2006.

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  1. pstnly

    pstnly Guest

    For a school project, I need to create a circuit which registers
    whenever a red laser hits a photoresistor, and displays the number of
    counts in a LED bar graph.

    It seems that CMOS 4017 is the right choice for counting and driving
    the LEDs - that part is easy and obvious. The difficulty I'm
    anticipating is in producing a reliable input pulse for the 4017, since
    the amount of time the laser hits the LED may vary significantly from a
    quick pass over the sensor to hovering over the sensor for 1 second or

    I suspect the answer is to use a 555 monostable circuit on the input.
    I found the following link via the wonder that is Google:

    I'm thinking the 3rd to last circuit is basically what I want - trigger
    on the leading edge of the voltage spike coming from the photoresistor
    (wired as a voltage divider as input to this).

    Is this going to do it for me?
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