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Need help with Infrared Detector circuit

Discussion in 'Electronic Basics' started by [email protected], Mar 11, 2007.

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  1. Guest

    Hi -

    I'm a digital electronics guy with what is probably a simple analog
    electronics problem. Please help educate the clueless (me).

    I'm designing a simple circuit that uses two infrared detector/emitter
    pairs as triggers. I've got the basics working - I've got +5v going
    into the two IR detectors which pass the voltage through as long as
    the light path from the emitters is uninterupted. So far so good.

    I envision the two IR detectors as simple switches: when IR is present
    they turn on (a digital 1 bit), when it is interupted they turn off (a
    digital zero bit). I've taken the output of these two detectors and
    connected them to a NAND gate. My goal is to have the NAND output go
    high whenever EITHER IR detector is shielded from the IR source. Like
    I said - I'm confident of the digital part of this circuit however the
    IR detector LEDs don't behave as I expected.

    First I tried using a CMOS 4011 gate. This caused the IR detector
    output to be held high even when the IR source was interupted.

    Just for grins I tried using a TTL 7400 gate. This worked a little
    better in the sense that the IR detectors work properly - they turn
    off when the IR source is removed (as they should) but the NAND gate
    does not trigger.

    My hunch is that I am not understanding something about the
    relationship between the output of the IR detector (a diode) and the
    digital gate (transistors). Obviously the output of the IR LED is more
    complex than a simple on/off voltage.

    So far this is the worlds simplest circuit - there is only one
    resistor to trim the voltage down to an appropriate level for the IR
    transmitter. Other than that everything else is digital gates.

    Any suggestions how I can get the output of the IR detectors to act as
    proper inputs to the digital side of the circuit?

    For extra credit - where did I go wrong? Why isn't the +5 I see on the
    output side of the IR detector sufficient to trigger the gate.
    Remember, I'm a digital kindof guy so use small words (pun intended).

    Thanks in advance!
  2. I'd start with a voltmeter. Pull-up resistor?
  3. me

    me Guest

    wrote in

    without knowing what you are using for detectors, they are commonly photo transitors with
    leads for collector and emitter. you should ground the emitter and connect the collector
    (assuming NPN) to a 4.7 k resistor that is connected to +5 V. Take the output from the
    collector giving a "low" when the trasnsitor is illuminated and a "high" when not. You
    might try that to start if you have not destroyed the detectors by connecing them
    directly accross the 5 volt supply.
  4. Chris

    Chris Guest

    Hi, Dave. I'm really sorry you didn't include the part numbers on
    your IR detectors and emitters. That would have made an answer easy.

    There are many different possibilities here. One is that you've got a
    simple IR photodiode and a phototransistor which is made to be
    sensitive to light at the emitter's wavelength. If that's the case,
    it's simply a matter of two resistors, like this (view in fixed font
    or M$ Notepad):

    | VCC
    | +
    | VCC |
    | + |
    | | .-.
    | | | |10K 1/4
    | .-. | | CD4001
    |180| | '-' __
    | | | | .--| \
    | '-' o-------o | )o-
    | | | '--|__/
    | | |/
    | V ~ |
    | - ~ |>
    | | |
    | | |
    | === ===
    | GND GND
    (created by AACircuit v1.28.6 beta 04/19/05

    With a 5V supply, the resistor values shown should work fairly well.

    The second option is that you've got a hybrid remote control emitter
    and detector pair. In those, there's circuitry to modulate the light
    output at the emitter to 38.1KHz (or some similar frequency). The
    detector is then optimized to pick up that frequency, which gives it a
    lot of immunity from false triggering by ambient light (like a TV
    remote control). For these, there usually isn't a resistor for the
    emitter -- just a power supply hookup. The detector usually has an
    open collector type output which requires a pullup resistor at the
    output (usually 10K or so).

    But there are many other different possibilities. Rather than have us
    guess which one you're using, how about posting again with the part
    numbers, and where you got them? Any links you might have to
    manufacturer info on the parts would also be helpful.

    Good luck
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