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LM324 - IR Sensor

Discussion in 'Electronic Basics' started by Manimozhi Baskaran, Feb 7, 2006.

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  1. I built the circuit below with the IR sensor on the bread board

    A large bowl of snow was brought inside the house to the sense the snow

    Varied the 50 K pot (20 turns on the pot)

    No buzz output from sensor



    Any suggestions on what is going wrong?

    Thanks


    ---+-------+----------+----------- +5 --------VoutLM785 (NTE equivalent one) (Vin) +9V- --------- |
     
  2. ehsjr

    ehsjr Guest

    I tried to clean up your diagram, which was not clear, and came up
    with this:
    ------
    ---+-----+----------+--------- +5 ------Vout|LM7805|Vin--- +9V
    | | | ------
    | | | |
    | | 100K |
    | | | |
    | | +-----[22K]-----+ |
    pot 50K | | 1/4 | |
    | |\| | |\ 324 | piezo |
    ref(A)+---|+\ +------|+\ | 5V-9V |
    IR in | | >--. | | >-----+-+--|| |
    -------|---|-/ `-|<--|---+--|-/ | | |
    | |/| 1N914 | | |/ | +| |
    | | 100K +----[100K]-' | |
    | | | | | |
    100K | | === 0.2uF (B) | |
    | | | | | |
    -----+-----+----------+---+----------------+------+--- GND

    Assuming the diagram is correct, your piezo is installed
    backwards - you show the + going to ground.

    Ed
     
  3. John Fields

    John Fields Guest


    ---
    With the 50k pot cranked to zero ohms the reference input to the
    opamp will be 5V, and since:

    E1R2
    E2 = ------- ,
    R1+R2

    with the pot cranked to 50k the reference input will be:


    E1R2 5V * 100kR
    E2 = ------- = -------------- = 3.33V,
    R1+R2 50kR * 100kR

    So there's no way you can get to 0.8V, no matter how much you crank
    the pot.

    What you need to do is change this:

    +----------+----+5V
    | |
    [50k]<-+ |
    | | |
    +----+----|+\
    | | >--[<1N914]...
    Vin>--|---------|-/
    | |
    [100k] |
    | |
    +----------+----GND


    to this:


    +----------+---+5V
    | |
    [100k] |
    | |
    +---------|+\
    | | >--[<1N914]...
    Vin>--|---------|-/
    | |
    [50k]<-+ |
    | | |
    +----+-----+---GND

    which will get you your 0.8V point with the pot at adjusted to
    about 19k ohms


    Also, since you've got a slowly varying input signal it wouldn't
    hurt to add a little hysteresis, like this:

    +----------+---+5V
    | |
    [100k] +---|--[Rx]--+
    | | | |
    +------+--|+\ |
    | | >------+--[<1N914]...
    Vin>--|---------|-/
    | |
    [50k]<-+ |
    | | |
    +----+-----+---GND

    if you can ever figure out what kind of accuracy you're looking for.
     
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