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Need help on very basic stuff

Discussion in 'Electronic Basics' started by [email protected], Oct 16, 2005.

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  1. Guest

    Hi all

    I am really new to electronics and might have never joined, but I
    absolutely want to create this light barrier that I have seen done
    before, just not by me.

    I went out to radio-shack and with the help of the internet, was able
    to actually create a barrier that works with IR over a few inches.

    I am using all radio-shack parts, so here are my circuits:

    9V 9v
    10kR ---- 10kR
    ----100R---- GND ---collector


    I only have some Resistors,so I did what I could.

    The parts are:
    100Ohm R = 271-152
    10KOhn R = 271-1126
    IR emmiter and detector: 276-0142

    Now, whats the watt tolerance mean on these resistors, which ones
    should I use and HOW can i make this work over a few feet distance?

    Also, I would like to have 0Vs when barrier on, 5V when its broke.
    I am able to control an led with a transistor 2N3904 (276-2016), but I
    don't really understand that.

    Thank you for lowering yourself to my level.
  2. Lord Garth

    Lord Garth Guest

    There is a lot more to this when you go for distance. The beam should be
    modulated so that the detector circuit can easily see the difference between
    your beam and background radiation. This is most easily done with an LM555
    integrated circuit operation in astable mode. A transmitter built around an
    analog oscillator will likely drift so the receiver needs to track the
    You can do that with an LM567 but you should know that designing the
    circuit is not for a beginner. There needs to be more power for the
    and maybe a lens to focus the invisible beam at the receiver.

    That being said, there are Ir receiver modules available, even at Rat Shack.
    You will need to build your 555 transmitter circuit to operate near 40kHz
    for the little detector to lock on. Don't forget the input voltage these
    need, you will need to read on the use of the 78xx series of voltage
    The LM7805 is one very common device. Note its input voltage requirements
    and its ability to dissipate the heat it will generate when operating.
    worry, they are rugged / abuse tolerant and easy to use.

    Have fun and enjoy the hobby. Most of the info you will need to read is
    on the National Semiconductor (and many other) web site.
  3. Guest

    Thank you, this is great stuff. I already bought one of those ir
    receivers at said store, but I will need a 555 now to get the emitter
    to pulse.

    About input voltage... I noticed that my amp readings came out right,
    but I always had more voltage than I expected. Should I build this:

    instead? Also, on the receiver side, I would like to use the power of
    the rs232 serial interface and send a signal to that very special pin.

    This is exciting. Another one: Why does using 20kOhm before the
    detector work better than using about (9-5)/0.05=80?

    Thanks for the help and the resources. I am currently checking out
    National Semiconductor.
  4. Chris

    Chris Guest

    How do, Manoo. Since you're at Radio Shack already, wander over to the
    counter and ask about the Electronic Sensor Lab, Catalog #: 28-278
    which costs $49.99. It's a good intro to sensors in general, and has
    several opto experiments, including the emitter-detector pair you're
    talking about. The best feature of the kit is the manual, written by
    Forrest M. Mims III. This kit is actually a pretty good intro to the
    subject of electronics in general for the complete newbie.

    You're going to have a bit of trouble getting a sensor range of a
    couple of feet with the emitter-detector pair you mentioned. Your
    light intensity is not sufficient at that range to cause much change in
    the output of the detector. Hence Lord Garth's suggestion of
    modulating the light output of the emitter at 40KHz, and then setting
    up a detector to select that frequency and amplify it. Even then, it's
    going to be a bit of a tough go. It may help to use additional optics
    to focus the light output of the emitter on the detector a couple of
    feet away.

    Either way, though, I would guess your goal is a little beyond your
    reach without quite a bit more technical knowledge. I'd suggest buying
    the Electronic Sensor Lab, and also getting a copy of Mr. Mims' book,
    "Getting Started in Electronics" (not available at Radio Shack, but you
    can get it from many libraries or ). He frequently
    references Radio Shack part numbers, which can be extremely helpful,

    With the level of electronics knowledge you have, it's actually amazing
    you got your emitter-detector pair to actually do something. An
    intelligent person like yourself shouldn't have much of a problem
    getting up to speed in electronics with a bit of help.

    If you're in a hurry, though, some of the infrared/opto circuits on
    this page may be of some help:

    Good luck
  5. Rich Webb

    Rich Webb Guest


    Depending on your intended use, you might find that you'll get better
    results using one a PIR (passive infrared) sensor

    Most of the heavy lifting has already been done; all you need to do,
    typically, is provide it power and ground and a line to monitor the

    Radio Shack has one, p/n 49-1327. However, where they were once famous
    for having great on-line documentation for their parts, they are now
    rather infamous for not having docs. Hard to tell what connector you
    need or what the output is like.

    There are other choices. MPJA has one, that looks like it would be easy
    to use, for $10 at
  6. Guest

    Thank you, Chris and Rich. I noticed that even though I had some
    classes on the topic back in College, I didn't retain the slightest bit
    of practical knowledge about the topic. Therefore, you can imagine how
    thrilled I was when my visible LED would actually turn on and off when
    I put my finger through the beam.

    Since I actually AM in a hurry, (and I like experimenting better than
    reading), please let me know what you all think about this circuit,
    generously published by David Cook:

    If I can make my Voltage lower to +5V DC steady, (with an LM7805),
    can't I just build above circuit and voila? Or what major problems
    would I face?

    And on the other end, I use the (276-640) 38kHz IR Receiver Module from
    Radio Shack, possibly in combination with another LM7805 for voltage
    regulation and maybe a transistor (as switch) for supplying the result
    to the rs232 pin? Or can I use the power of the rs232 directly? I
    remember having seen circuits on the topic, but I think there is some
    danger involved.

    The electronics kit sounds like a good idea, but on the other hand, you
    know how many parts I can burn out for 50 bucks? :)

    Thank you all again for your help. I'll keep you posted about my
    progress, if that's ok...
  7. Chris

    Chris Guest

    The circuit in the link looks like it would do the job for the emitter.
    There aren't any docs on the RS detector (usually they just have
    connections to Vcc and GND, with the third pin as output which may
    require a pullup resistor). There should be a basic hookup diagram on
    the back of the package. Line up your LED and detector, then start
    pulling the detector away until the signal just disappears, tweak the
    pot until the signal comes back on, then stretch it some more. Repeat
    until you can't get a signal. That should be the appropriate tuned
    frequency for the detector.

    Good luck
  8. Jasen Betts

    Jasen Betts Guest

    38Khz Crystals are easily had, what sort of circuit would be needed to use
    one of them?

  9. Guest

    Ok. I am still not sure I am doing it right, but it seems to work. I
    have a configuration where I use a 555 timer in astable mode, cranking
    its frequency to what I hope is somewhere between 38 and 40 kHz. On the
    other side, I have the Rat Shack IR receiver and a couple of
    transistors and resistors and when I wave my hand between the two, my
    LED on the receiver lights up.

    I noticed, that it mainly picks up on a signal change, rather than the
    emitter itsself, that's why I liked someone's suggestion (forgot who it
    was) to use a second 555 timer to turn the emitter circuit on and off
    in a range of about 1 to 10 khZ.

    Getting that LED to light up tho, tells me its time to move on to the
    next problem: How do I get that flash to translate to a signal to the
    serial interface on my computer? (rs232). The wiring should be real
    easy, but I can't find a simple circuit out there on the internet.

    Does anyone have a good resource?
    Thank you
  10. Guest

    I actually did it! Thank you for your contributions all.

    I built a sender emitting IR at good-know-what frequency (with a 555
    and a 38kHz receiver from radio-shack that responds to my hand moving
    at a certain minimal speed through the beam. The signal on the output
    goes into the receive-pin on the rs232, the handshake is looped-back
    ground goes to exactly there.

    When running a port diagnostics program, I receive a '0' when my hand
    breaks the beam.

    I already downloaded a library for reading the COM-Port in c#. I might
    up having to set an interrupt trap, but from here on out, I feel much

    Thanks again
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